* [Qemu-devel] [PATCH v5 0/2] Rewrite 'live-block-ops.txt'; convert 'bitmaps.md' to rST
@ 2017-07-06 14:36 Kashyap Chamarthy
2017-07-06 14:36 ` [Qemu-devel] [PATCH v5 1/2] bitmaps.md: Convert to rST; move it into 'interop' dir Kashyap Chamarthy
2017-07-06 14:36 ` [Qemu-devel] [PATCH v5 2/2] live-block-ops.txt: Rename, rewrite, and improve it Kashyap Chamarthy
0 siblings, 2 replies; 10+ messages in thread
From: Kashyap Chamarthy @ 2017-07-06 14:36 UTC (permalink / raw)
To: qemu-devel
Cc: eblake, qemu-block, kwolf, mreitz, jsnow, berto,
Kashyap Chamarthy
[v4: https://lists.nongnu.org/archive/html/qemu-devel/2017-06/msg06395.html]
Rewrite the 'live-block-ops.txt' document (after renaming it to
'live-block-operations.rst') in reStructuredText (rST) format. Given
upstream QEMU's desire[*] to take advantage of the Sphinx + rST
framework to gerate its documentation:
"Based on experience from the Linux kernel, QEMU's docs pipeline is
going to be based on Sphinx [...] Sphinx is extensible and it is
easy to add new input formats and input directives."
And as part of review, John Snow suggested[+] to link to the
'bitmaps.md' document. So while at it, convert the 'bitmaps.md'
document also into rST syntax.
Then, moved both the documents ('live-block-operations.rst', and
'bitmaps.rst') to 'qemu/docs/interop' directory. (Paolo explained the
term "interop" as: "management & interoperability with QEMU".)
That's the result of this series:
(1) Rewrite the 'live-block-ops.txt' document (for details, refer the
commit message of the patch) in rST.
Sphinx-generted HTML rendering:
https://kashyapc.fedorapeople.org/v5-QEMU-Docs/_build/html/docs/live-block-operations.html
(2) Convert the 'bitmaps.md' document to rST, as discussed on this[+]
thread.
Sphinx-generted HTML rendering:
https://kashyapc.fedorapeople.org/v5-QEMU-Docs/_build/html/docs/bitmaps.html
[*] http://wiki.qemu.org/Features/Documentation
[+] https://lists.nongnu.org/archive/html/qemu-devel/2017-06/msg05428.html
Kashyap Chamarthy (2):
bitmaps.md: Convert to rST; move it into 'interop' dir
live-block-ops.txt: Rename, rewrite, and improve it
docs/devel/bitmaps.md | 505 ---------------
docs/interop/bitmaps.rst | 555 +++++++++++++++++
docs/interop/live-block-operations.rst | 1072 ++++++++++++++++++++++++++++++++
docs/live-block-ops.txt | 72 ---
4 files changed, 1627 insertions(+), 577 deletions(-)
delete mode 100644 docs/devel/bitmaps.md
create mode 100644 docs/interop/bitmaps.rst
create mode 100644 docs/interop/live-block-operations.rst
delete mode 100644 docs/live-block-ops.txt
--
2.7.5
^ permalink raw reply [flat|nested] 10+ messages in thread* [Qemu-devel] [PATCH v5 1/2] bitmaps.md: Convert to rST; move it into 'interop' dir 2017-07-06 14:36 [Qemu-devel] [PATCH v5 0/2] Rewrite 'live-block-ops.txt'; convert 'bitmaps.md' to rST Kashyap Chamarthy @ 2017-07-06 14:36 ` Kashyap Chamarthy 2017-07-06 14:36 ` [Qemu-devel] [PATCH v5 2/2] live-block-ops.txt: Rename, rewrite, and improve it Kashyap Chamarthy 1 sibling, 0 replies; 10+ messages in thread From: Kashyap Chamarthy @ 2017-07-06 14:36 UTC (permalink / raw) To: qemu-devel Cc: eblake, qemu-block, kwolf, mreitz, jsnow, berto, Kashyap Chamarthy This is part of the on-going effort to convert QEMU upstream documentation syntax to reStructuredText (rST). The conversion to rST was done using: $ pandoc -f markdown -t rst bitmaps.md -o bitmaps.rst Then, make a couple of small syntactical adjustments. While at it, reword a statement to avoid ambiguity. Addressing the feedback from this thread: https://lists.nongnu.org/archive/html/qemu-devel/2017-06/msg05428.html Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com> Reviewed-by: John Snow <jsnow@redhat.com> --- * A Sphinx-rendered HTML version is here: https://kashyapc.fedorapeople.org/v5-QEMU-Docs/_build/html/docs/bitmaps.html * v5 -- No change; just carry the 'Reviewed-by' from John Snow. * For v4: the patch had "v4" subject prefix because I rolled this in along with the other document (live-block-operations.rst) that is in review, which was actually at v4. --- docs/devel/bitmaps.md | 505 ------------------------------------------ docs/interop/bitmaps.rst | 555 +++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 555 insertions(+), 505 deletions(-) delete mode 100644 docs/devel/bitmaps.md create mode 100644 docs/interop/bitmaps.rst diff --git a/docs/devel/bitmaps.md b/docs/devel/bitmaps.md deleted file mode 100644 index a2e8d51..0000000 --- a/docs/devel/bitmaps.md +++ /dev/null @@ -1,505 +0,0 @@ -<!-- -Copyright 2015 John Snow <jsnow@redhat.com> and Red Hat, Inc. -All rights reserved. - -This file is licensed via The FreeBSD Documentation License, the full text of -which is included at the end of this document. ---> - -# Dirty Bitmaps and Incremental Backup - -* Dirty Bitmaps are objects that track which data needs to be backed up for the - next incremental backup. - -* Dirty bitmaps can be created at any time and attached to any node - (not just complete drives.) - -## Dirty Bitmap Names - -* A dirty bitmap's name is unique to the node, but bitmaps attached to different - nodes can share the same name. - -* Dirty bitmaps created for internal use by QEMU may be anonymous and have no - name, but any user-created bitmaps may not be. There can be any number of - anonymous bitmaps per node. - -* The name of a user-created bitmap must not be empty (""). - -## Bitmap Modes - -* A Bitmap can be "frozen," which means that it is currently in-use by a backup - operation and cannot be deleted, renamed, written to, reset, - etc. - -* The normal operating mode for a bitmap is "active." - -## Basic QMP Usage - -### Supported Commands ### - -* block-dirty-bitmap-add -* block-dirty-bitmap-remove -* block-dirty-bitmap-clear - -### Creation - -* To create a new bitmap, enabled, on the drive with id=drive0: - -```json -{ "execute": "block-dirty-bitmap-add", - "arguments": { - "node": "drive0", - "name": "bitmap0" - } -} -``` - -* This bitmap will have a default granularity that matches the cluster size of - its associated drive, if available, clamped to between [4KiB, 64KiB]. - The current default for qcow2 is 64KiB. - -* To create a new bitmap that tracks changes in 32KiB segments: - -```json -{ "execute": "block-dirty-bitmap-add", - "arguments": { - "node": "drive0", - "name": "bitmap0", - "granularity": 32768 - } -} -``` - -### Deletion - -* Bitmaps that are frozen cannot be deleted. - -* Deleting the bitmap does not impact any other bitmaps attached to the same - node, nor does it affect any backups already created from this node. - -* Because bitmaps are only unique to the node to which they are attached, - you must specify the node/drive name here, too. - -```json -{ "execute": "block-dirty-bitmap-remove", - "arguments": { - "node": "drive0", - "name": "bitmap0" - } -} -``` - -### Resetting - -* Resetting a bitmap will clear all information it holds. - -* An incremental backup created from an empty bitmap will copy no data, - as if nothing has changed. - -```json -{ "execute": "block-dirty-bitmap-clear", - "arguments": { - "node": "drive0", - "name": "bitmap0" - } -} -``` - -## Transactions - -### Justification - -Bitmaps can be safely modified when the VM is paused or halted by using -the basic QMP commands. For instance, you might perform the following actions: - -1. Boot the VM in a paused state. -2. Create a full drive backup of drive0. -3. Create a new bitmap attached to drive0. -4. Resume execution of the VM. -5. Incremental backups are ready to be created. - -At this point, the bitmap and drive backup would be correctly in sync, -and incremental backups made from this point forward would be correctly aligned -to the full drive backup. - -This is not particularly useful if we decide we want to start incremental -backups after the VM has been running for a while, for which we will need to -perform actions such as the following: - -1. Boot the VM and begin execution. -2. Using a single transaction, perform the following operations: - * Create bitmap0. - * Create a full drive backup of drive0. -3. Incremental backups are now ready to be created. - -### Supported Bitmap Transactions - -* block-dirty-bitmap-add -* block-dirty-bitmap-clear - -The usages are identical to their respective QMP commands, but see below -for examples. - -### Example: New Incremental Backup - -As outlined in the justification, perhaps we want to create a new incremental -backup chain attached to a drive. - -```json -{ "execute": "transaction", - "arguments": { - "actions": [ - {"type": "block-dirty-bitmap-add", - "data": {"node": "drive0", "name": "bitmap0"} }, - {"type": "drive-backup", - "data": {"device": "drive0", "target": "/path/to/full_backup.img", - "sync": "full", "format": "qcow2"} } - ] - } -} -``` - -### Example: New Incremental Backup Anchor Point - -Maybe we just want to create a new full backup with an existing bitmap and -want to reset the bitmap to track the new chain. - -```json -{ "execute": "transaction", - "arguments": { - "actions": [ - {"type": "block-dirty-bitmap-clear", - "data": {"node": "drive0", "name": "bitmap0"} }, - {"type": "drive-backup", - "data": {"device": "drive0", "target": "/path/to/new_full_backup.img", - "sync": "full", "format": "qcow2"} } - ] - } -} -``` - -## Incremental Backups - -The star of the show. - -**Nota Bene!** Only incremental backups of entire drives are supported for now. -So despite the fact that you can attach a bitmap to any arbitrary node, they are -only currently useful when attached to the root node. This is because -drive-backup only supports drives/devices instead of arbitrary nodes. - -### Example: First Incremental Backup - -1. Create a full backup and sync it to the dirty bitmap, as in the transactional -examples above; or with the VM offline, manually create a full copy and then -create a new bitmap before the VM begins execution. - - * Let's assume the full backup is named 'full_backup.img'. - * Let's assume the bitmap you created is 'bitmap0' attached to 'drive0'. - -2. Create a destination image for the incremental backup that utilizes the -full backup as a backing image. - - * Let's assume it is named 'incremental.0.img'. - - ```sh - # qemu-img create -f qcow2 incremental.0.img -b full_backup.img -F qcow2 - ``` - -3. Issue the incremental backup command: - - ```json - { "execute": "drive-backup", - "arguments": { - "device": "drive0", - "bitmap": "bitmap0", - "target": "incremental.0.img", - "format": "qcow2", - "sync": "incremental", - "mode": "existing" - } - } - ``` - -### Example: Second Incremental Backup - -1. Create a new destination image for the incremental backup that points to the - previous one, e.g.: 'incremental.1.img' - - ```sh - # qemu-img create -f qcow2 incremental.1.img -b incremental.0.img -F qcow2 - ``` - -2. Issue a new incremental backup command. The only difference here is that we - have changed the target image below. - - ```json - { "execute": "drive-backup", - "arguments": { - "device": "drive0", - "bitmap": "bitmap0", - "target": "incremental.1.img", - "format": "qcow2", - "sync": "incremental", - "mode": "existing" - } - } - ``` - -## Errors - -* In the event of an error that occurs after a backup job is successfully - launched, either by a direct QMP command or a QMP transaction, the user - will receive a BLOCK_JOB_COMPLETE event with a failure message, accompanied - by a BLOCK_JOB_ERROR event. - -* In the case of an event being cancelled, the user will receive a - BLOCK_JOB_CANCELLED event instead of a pair of COMPLETE and ERROR events. - -* In either case, the incremental backup data contained within the bitmap is - safely rolled back, and the data within the bitmap is not lost. The image - file created for the failed attempt can be safely deleted. - -* Once the underlying problem is fixed (e.g. more storage space is freed up), - you can simply retry the incremental backup command with the same bitmap. - -### Example - -1. Create a target image: - - ```sh - # qemu-img create -f qcow2 incremental.0.img -b full_backup.img -F qcow2 - ``` - -2. Attempt to create an incremental backup via QMP: - - ```json - { "execute": "drive-backup", - "arguments": { - "device": "drive0", - "bitmap": "bitmap0", - "target": "incremental.0.img", - "format": "qcow2", - "sync": "incremental", - "mode": "existing" - } - } - ``` - -3. Receive an event notifying us of failure: - - ```json - { "timestamp": { "seconds": 1424709442, "microseconds": 844524 }, - "data": { "speed": 0, "offset": 0, "len": 67108864, - "error": "No space left on device", - "device": "drive1", "type": "backup" }, - "event": "BLOCK_JOB_COMPLETED" } - ``` - -4. Delete the failed incremental, and re-create the image. - - ```sh - # rm incremental.0.img - # qemu-img create -f qcow2 incremental.0.img -b full_backup.img -F qcow2 - ``` - -5. Retry the command after fixing the underlying problem, - such as freeing up space on the backup volume: - - ```json - { "execute": "drive-backup", - "arguments": { - "device": "drive0", - "bitmap": "bitmap0", - "target": "incremental.0.img", - "format": "qcow2", - "sync": "incremental", - "mode": "existing" - } - } - ``` - -6. Receive confirmation that the job completed successfully: - - ```json - { "timestamp": { "seconds": 1424709668, "microseconds": 526525 }, - "data": { "device": "drive1", "type": "backup", - "speed": 0, "len": 67108864, "offset": 67108864}, - "event": "BLOCK_JOB_COMPLETED" } - ``` - -### Partial Transactional Failures - -* Sometimes, a transaction will succeed in launching and return success, - but then later the backup jobs themselves may fail. It is possible that - a management application may have to deal with a partial backup failure - after a successful transaction. - -* If multiple backup jobs are specified in a single transaction, when one of - them fails, it will not interact with the other backup jobs in any way. - -* The job(s) that succeeded will clear the dirty bitmap associated with the - operation, but the job(s) that failed will not. It is not "safe" to delete - any incremental backups that were created successfully in this scenario, - even though others failed. - -#### Example - -* QMP example highlighting two backup jobs: - - ```json - { "execute": "transaction", - "arguments": { - "actions": [ - { "type": "drive-backup", - "data": { "device": "drive0", "bitmap": "bitmap0", - "format": "qcow2", "mode": "existing", - "sync": "incremental", "target": "d0-incr-1.qcow2" } }, - { "type": "drive-backup", - "data": { "device": "drive1", "bitmap": "bitmap1", - "format": "qcow2", "mode": "existing", - "sync": "incremental", "target": "d1-incr-1.qcow2" } }, - ] - } - } - ``` - -* QMP example response, highlighting one success and one failure: - * Acknowledgement that the Transaction was accepted and jobs were launched: - ```json - { "return": {} } - ``` - - * Later, QEMU sends notice that the first job was completed: - ```json - { "timestamp": { "seconds": 1447192343, "microseconds": 615698 }, - "data": { "device": "drive0", "type": "backup", - "speed": 0, "len": 67108864, "offset": 67108864 }, - "event": "BLOCK_JOB_COMPLETED" - } - ``` - - * Later yet, QEMU sends notice that the second job has failed: - ```json - { "timestamp": { "seconds": 1447192399, "microseconds": 683015 }, - "data": { "device": "drive1", "action": "report", - "operation": "read" }, - "event": "BLOCK_JOB_ERROR" } - ``` - - ```json - { "timestamp": { "seconds": 1447192399, "microseconds": 685853 }, - "data": { "speed": 0, "offset": 0, "len": 67108864, - "error": "Input/output error", - "device": "drive1", "type": "backup" }, - "event": "BLOCK_JOB_COMPLETED" } - -* In the above example, "d0-incr-1.qcow2" is valid and must be kept, - but "d1-incr-1.qcow2" is invalid and should be deleted. If a VM-wide - incremental backup of all drives at a point-in-time is to be made, - new backups for both drives will need to be made, taking into account - that a new incremental backup for drive0 needs to be based on top of - "d0-incr-1.qcow2." - -### Grouped Completion Mode - -* While jobs launched by transactions normally complete or fail on their own, - it is possible to instruct them to complete or fail together as a group. - -* QMP transactions take an optional properties structure that can affect - the semantics of the transaction. - -* The "completion-mode" transaction property can be either "individual" - which is the default, legacy behavior described above, or "grouped," - a new behavior detailed below. - -* Delayed Completion: In grouped completion mode, no jobs will report - success until all jobs are ready to report success. - -* Grouped failure: If any job fails in grouped completion mode, all remaining - jobs will be cancelled. Any incremental backups will restore their dirty - bitmap objects as if no backup command was ever issued. - - * Regardless of if QEMU reports a particular incremental backup job as - CANCELLED or as an ERROR, the in-memory bitmap will be restored. - -#### Example - -* Here's the same example scenario from above with the new property: - - ```json - { "execute": "transaction", - "arguments": { - "actions": [ - { "type": "drive-backup", - "data": { "device": "drive0", "bitmap": "bitmap0", - "format": "qcow2", "mode": "existing", - "sync": "incremental", "target": "d0-incr-1.qcow2" } }, - { "type": "drive-backup", - "data": { "device": "drive1", "bitmap": "bitmap1", - "format": "qcow2", "mode": "existing", - "sync": "incremental", "target": "d1-incr-1.qcow2" } }, - ], - "properties": { - "completion-mode": "grouped" - } - } - } - ``` - -* QMP example response, highlighting a failure for drive2: - * Acknowledgement that the Transaction was accepted and jobs were launched: - ```json - { "return": {} } - ``` - - * Later, QEMU sends notice that the second job has errored out, - but that the first job was also cancelled: - ```json - { "timestamp": { "seconds": 1447193702, "microseconds": 632377 }, - "data": { "device": "drive1", "action": "report", - "operation": "read" }, - "event": "BLOCK_JOB_ERROR" } - ``` - - ```json - { "timestamp": { "seconds": 1447193702, "microseconds": 640074 }, - "data": { "speed": 0, "offset": 0, "len": 67108864, - "error": "Input/output error", - "device": "drive1", "type": "backup" }, - "event": "BLOCK_JOB_COMPLETED" } - ``` - - ```json - { "timestamp": { "seconds": 1447193702, "microseconds": 640163 }, - "data": { "device": "drive0", "type": "backup", "speed": 0, - "len": 67108864, "offset": 16777216 }, - "event": "BLOCK_JOB_CANCELLED" } - ``` - -<!-- -The FreeBSD Documentation License - -Redistribution and use in source (Markdown) and 'compiled' forms (SGML, HTML, -PDF, PostScript, RTF and so forth) with or without modification, are permitted -provided that the following conditions are met: - -Redistributions of source code (Markdown) must retain the above copyright -notice, this list of conditions and the following disclaimer of this file -unmodified. - -Redistributions in compiled form (transformed to other DTDs, converted to PDF, -PostScript, RTF and other formats) must reproduce the above copyright notice, -this list of conditions and the following disclaimer in the documentation and/or -other materials provided with the distribution. - -THIS DOCUMENTATION IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" -AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE -IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE -DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE -FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL -DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR -SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER -CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, -OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF -THIS DOCUMENTATION, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. ---> diff --git a/docs/interop/bitmaps.rst b/docs/interop/bitmaps.rst new file mode 100644 index 0000000..b154d54 --- /dev/null +++ b/docs/interop/bitmaps.rst @@ -0,0 +1,555 @@ +.. + Copyright 2015 John Snow <jsnow@redhat.com> and Red Hat, Inc. + All rights reserved. + + This file is licensed via The FreeBSD Documentation License, the full + text of which is included at the end of this document. + +==================================== +Dirty Bitmaps and Incremental Backup +==================================== + +- Dirty Bitmaps are objects that track which data needs to be backed up + for the next incremental backup. + +- Dirty bitmaps can be created at any time and attached to any node + (not just complete drives.) + +.. contents:: + +Dirty Bitmap Names +------------------ + +- A dirty bitmap's name is unique to the node, but bitmaps attached to + different nodes can share the same name. + +- Dirty bitmaps created for internal use by QEMU may be anonymous and + have no name, but any user-created bitmaps may not be. There can be + any number of anonymous bitmaps per node. + +- The name of a user-created bitmap must not be empty (""). + +Bitmap Modes +------------ + +- A bitmap can be "frozen," which means that it is currently in-use by + a backup operation and cannot be deleted, renamed, written to, reset, + etc. + +- The normal operating mode for a bitmap is "active." + +Basic QMP Usage +--------------- + +Supported Commands +~~~~~~~~~~~~~~~~~~ + +- ``block-dirty-bitmap-add`` +- ``block-dirty-bitmap-remove`` +- ``block-dirty-bitmap-clear`` + +Creation +~~~~~~~~ + +- To create a new bitmap, enabled, on the drive with id=drive0: + +.. code:: json + + { "execute": "block-dirty-bitmap-add", + "arguments": { + "node": "drive0", + "name": "bitmap0" + } + } + +- This bitmap will have a default granularity that matches the cluster + size of its associated drive, if available, clamped to between [4KiB, + 64KiB]. The current default for qcow2 is 64KiB. + +- To create a new bitmap that tracks changes in 32KiB segments: + +.. code:: json + + { "execute": "block-dirty-bitmap-add", + "arguments": { + "node": "drive0", + "name": "bitmap0", + "granularity": 32768 + } + } + +Deletion +~~~~~~~~ + +- Bitmaps that are frozen cannot be deleted. + +- Deleting the bitmap does not impact any other bitmaps attached to the + same node, nor does it affect any backups already created from this + node. + +- Because bitmaps are only unique to the node to which they are + attached, you must specify the node/drive name here, too. + +.. code:: json + + { "execute": "block-dirty-bitmap-remove", + "arguments": { + "node": "drive0", + "name": "bitmap0" + } + } + +Resetting +~~~~~~~~~ + +- Resetting a bitmap will clear all information it holds. + +- An incremental backup created from an empty bitmap will copy no data, + as if nothing has changed. + +.. code:: json + + { "execute": "block-dirty-bitmap-clear", + "arguments": { + "node": "drive0", + "name": "bitmap0" + } + } + +Transactions +------------ + +Justification +~~~~~~~~~~~~~ + +Bitmaps can be safely modified when the VM is paused or halted by using +the basic QMP commands. For instance, you might perform the following +actions: + +1. Boot the VM in a paused state. +2. Create a full drive backup of drive0. +3. Create a new bitmap attached to drive0. +4. Resume execution of the VM. +5. Incremental backups are ready to be created. + +At this point, the bitmap and drive backup would be correctly in sync, +and incremental backups made from this point forward would be correctly +aligned to the full drive backup. + +This is not particularly useful if we decide we want to start +incremental backups after the VM has been running for a while, for which +we will need to perform actions such as the following: + +1. Boot the VM and begin execution. +2. Using a single transaction, perform the following operations: + + - Create ``bitmap0``. + - Create a full drive backup of ``drive0``. + +3. Incremental backups are now ready to be created. + +Supported Bitmap Transactions +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +- ``block-dirty-bitmap-add`` +- ``block-dirty-bitmap-clear`` + +The usages are identical to their respective QMP commands, but see below +for examples. + +Example: New Incremental Backup +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +As outlined in the justification, perhaps we want to create a new +incremental backup chain attached to a drive. + +.. code:: json + + { "execute": "transaction", + "arguments": { + "actions": [ + {"type": "block-dirty-bitmap-add", + "data": {"node": "drive0", "name": "bitmap0"} }, + {"type": "drive-backup", + "data": {"device": "drive0", "target": "/path/to/full_backup.img", + "sync": "full", "format": "qcow2"} } + ] + } + } + +Example: New Incremental Backup Anchor Point +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Maybe we just want to create a new full backup with an existing bitmap +and want to reset the bitmap to track the new chain. + +.. code:: json + + { "execute": "transaction", + "arguments": { + "actions": [ + {"type": "block-dirty-bitmap-clear", + "data": {"node": "drive0", "name": "bitmap0"} }, + {"type": "drive-backup", + "data": {"device": "drive0", "target": "/path/to/new_full_backup.img", + "sync": "full", "format": "qcow2"} } + ] + } + } + +Incremental Backups +------------------- + +The star of the show. + +**Nota Bene!** Only incremental backups of entire drives are supported +for now. So despite the fact that you can attach a bitmap to any +arbitrary node, they are only currently useful when attached to the root +node. This is because drive-backup only supports drives/devices instead +of arbitrary nodes. + +Example: First Incremental Backup +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +1. Create a full backup and sync it to the dirty bitmap, as in the + transactional examples above; or with the VM offline, manually create + a full copy and then create a new bitmap before the VM begins + execution. + + - Let's assume the full backup is named ``full_backup.img``. + - Let's assume the bitmap you created is ``bitmap0`` attached to + ``drive0``. + +2. Create a destination image for the incremental backup that utilizes + the full backup as a backing image. + + - Let's assume the new incremental image is named + ``incremental.0.img``. + + .. code:: bash + + $ qemu-img create -f qcow2 incremental.0.img -b full_backup.img -F qcow2 + +3. Issue the incremental backup command: + + .. code:: json + + { "execute": "drive-backup", + "arguments": { + "device": "drive0", + "bitmap": "bitmap0", + "target": "incremental.0.img", + "format": "qcow2", + "sync": "incremental", + "mode": "existing" + } + } + +Example: Second Incremental Backup +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +1. Create a new destination image for the incremental backup that points + to the previous one, e.g.: ``incremental.1.img`` + + .. code:: bash + + $ qemu-img create -f qcow2 incremental.1.img -b incremental.0.img -F qcow2 + +2. Issue a new incremental backup command. The only difference here is + that we have changed the target image below. + + .. code:: json + + { "execute": "drive-backup", + "arguments": { + "device": "drive0", + "bitmap": "bitmap0", + "target": "incremental.1.img", + "format": "qcow2", + "sync": "incremental", + "mode": "existing" + } + } + +Errors +------ + +- In the event of an error that occurs after a backup job is + successfully launched, either by a direct QMP command or a QMP + transaction, the user will receive a ``BLOCK_JOB_COMPLETE`` event with + a failure message, accompanied by a ``BLOCK_JOB_ERROR`` event. + +- In the case of an event being cancelled, the user will receive a + ``BLOCK_JOB_CANCELLED`` event instead of a pair of COMPLETE and ERROR + events. + +- In either case, the incremental backup data contained within the + bitmap is safely rolled back, and the data within the bitmap is not + lost. The image file created for the failed attempt can be safely + deleted. + +- Once the underlying problem is fixed (e.g. more storage space is + freed up), you can simply retry the incremental backup command with + the same bitmap. + +Example +~~~~~~~ + +1. Create a target image: + + .. code:: bash + + $ qemu-img create -f qcow2 incremental.0.img -b full_backup.img -F qcow2 + +2. Attempt to create an incremental backup via QMP: + + .. code:: json + + { "execute": "drive-backup", + "arguments": { + "device": "drive0", + "bitmap": "bitmap0", + "target": "incremental.0.img", + "format": "qcow2", + "sync": "incremental", + "mode": "existing" + } + } + +3. Receive an event notifying us of failure: + + .. code:: json + + { "timestamp": { "seconds": 1424709442, "microseconds": 844524 }, + "data": { "speed": 0, "offset": 0, "len": 67108864, + "error": "No space left on device", + "device": "drive1", "type": "backup" }, + "event": "BLOCK_JOB_COMPLETED" } + +4. Delete the failed incremental, and re-create the image. + + .. code:: bash + + $ rm incremental.0.img + $ qemu-img create -f qcow2 incremental.0.img -b full_backup.img -F qcow2 + +5. Retry the command after fixing the underlying problem, such as + freeing up space on the backup volume: + + .. code:: json + + { "execute": "drive-backup", + "arguments": { + "device": "drive0", + "bitmap": "bitmap0", + "target": "incremental.0.img", + "format": "qcow2", + "sync": "incremental", + "mode": "existing" + } + } + +6. Receive confirmation that the job completed successfully: + + .. code:: json + + { "timestamp": { "seconds": 1424709668, "microseconds": 526525 }, + "data": { "device": "drive1", "type": "backup", + "speed": 0, "len": 67108864, "offset": 67108864}, + "event": "BLOCK_JOB_COMPLETED" } + +Partial Transactional Failures +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +- Sometimes, a transaction will succeed in launching and return + success, but then later the backup jobs themselves may fail. It is + possible that a management application may have to deal with a + partial backup failure after a successful transaction. + +- If multiple backup jobs are specified in a single transaction, when + one of them fails, it will not interact with the other backup jobs in + any way. + +- The job(s) that succeeded will clear the dirty bitmap associated with + the operation, but the job(s) that failed will not. It is not "safe" + to delete any incremental backups that were created successfully in + this scenario, even though others failed. + +Example +^^^^^^^ + +- QMP example highlighting two backup jobs: + + .. code:: json + + { "execute": "transaction", + "arguments": { + "actions": [ + { "type": "drive-backup", + "data": { "device": "drive0", "bitmap": "bitmap0", + "format": "qcow2", "mode": "existing", + "sync": "incremental", "target": "d0-incr-1.qcow2" } }, + { "type": "drive-backup", + "data": { "device": "drive1", "bitmap": "bitmap1", + "format": "qcow2", "mode": "existing", + "sync": "incremental", "target": "d1-incr-1.qcow2" } }, + ] + } + } + +- QMP example response, highlighting one success and one failure: + + - Acknowledgement that the Transaction was accepted and jobs were + launched: + + .. code:: json + + { "return": {} } + + - Later, QEMU sends notice that the first job was completed: + + .. code:: json + + { "timestamp": { "seconds": 1447192343, "microseconds": 615698 }, + "data": { "device": "drive0", "type": "backup", + "speed": 0, "len": 67108864, "offset": 67108864 }, + "event": "BLOCK_JOB_COMPLETED" + } + + - Later yet, QEMU sends notice that the second job has failed: + + .. code:: json + + { "timestamp": { "seconds": 1447192399, "microseconds": 683015 }, + "data": { "device": "drive1", "action": "report", + "operation": "read" }, + "event": "BLOCK_JOB_ERROR" } + + .. code:: json + + { "timestamp": { "seconds": 1447192399, "microseconds": + 685853 }, "data": { "speed": 0, "offset": 0, "len": 67108864, + "error": "Input/output error", "device": "drive1", "type": + "backup" }, "event": "BLOCK_JOB_COMPLETED" } + +- In the above example, ``d0-incr-1.qcow2`` is valid and must be kept, + but ``d1-incr-1.qcow2`` is invalid and should be deleted. If a VM-wide + incremental backup of all drives at a point-in-time is to be made, + new backups for both drives will need to be made, taking into account + that a new incremental backup for drive0 needs to be based on top of + ``d0-incr-1.qcow2``. + +Grouped Completion Mode +~~~~~~~~~~~~~~~~~~~~~~~ + +- While jobs launched by transactions normally complete or fail on + their own, it is possible to instruct them to complete or fail + together as a group. + +- QMP transactions take an optional properties structure that can + affect the semantics of the transaction. + +- The "completion-mode" transaction property can be either "individual" + which is the default, legacy behavior described above, or "grouped," + a new behavior detailed below. + +- Delayed Completion: In grouped completion mode, no jobs will report + success until all jobs are ready to report success. + +- Grouped failure: If any job fails in grouped completion mode, all + remaining jobs will be cancelled. Any incremental backups will + restore their dirty bitmap objects as if no backup command was ever + issued. + + - Regardless of if QEMU reports a particular incremental backup job + as CANCELLED or as an ERROR, the in-memory bitmap will be + restored. + +Example +^^^^^^^ + +- Here's the same example scenario from above with the new property: + + .. code:: json + + { "execute": "transaction", + "arguments": { + "actions": [ + { "type": "drive-backup", + "data": { "device": "drive0", "bitmap": "bitmap0", + "format": "qcow2", "mode": "existing", + "sync": "incremental", "target": "d0-incr-1.qcow2" } }, + { "type": "drive-backup", + "data": { "device": "drive1", "bitmap": "bitmap1", + "format": "qcow2", "mode": "existing", + "sync": "incremental", "target": "d1-incr-1.qcow2" } }, + ], + "properties": { + "completion-mode": "grouped" + } + } + } + +- QMP example response, highlighting a failure for ``drive2``: + + - Acknowledgement that the Transaction was accepted and jobs were + launched: + + .. code:: json + + { "return": {} } + + - Later, QEMU sends notice that the second job has errored out, but + that the first job was also cancelled: + + .. code:: json + + { "timestamp": { "seconds": 1447193702, "microseconds": 632377 }, + "data": { "device": "drive1", "action": "report", + "operation": "read" }, + "event": "BLOCK_JOB_ERROR" } + + .. code:: json + + { "timestamp": { "seconds": 1447193702, "microseconds": 640074 }, + "data": { "speed": 0, "offset": 0, "len": 67108864, + "error": "Input/output error", + "device": "drive1", "type": "backup" }, + "event": "BLOCK_JOB_COMPLETED" } + + .. code:: json + + { "timestamp": { "seconds": 1447193702, "microseconds": 640163 }, + "data": { "device": "drive0", "type": "backup", "speed": 0, + "len": 67108864, "offset": 16777216 }, + "event": "BLOCK_JOB_CANCELLED" } + +.. raw:: html + + <!-- + The FreeBSD Documentation License + + Redistribution and use in source (Markdown) and 'compiled' forms (SGML, HTML, + PDF, PostScript, RTF and so forth) with or without modification, are permitted + provided that the following conditions are met: + + Redistributions of source code (Markdown) must retain the above copyright + notice, this list of conditions and the following disclaimer of this file + unmodified. + + Redistributions in compiled form (transformed to other DTDs, converted to PDF, + PostScript, RTF and other formats) must reproduce the above copyright notice, + this list of conditions and the following disclaimer in the documentation and/or + other materials provided with the distribution. + + THIS DOCUMENTATION IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" + AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE + IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE + DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE + FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL + DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR + SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER + CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, + OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF + THIS DOCUMENTATION, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. + --> -- 2.7.5 ^ permalink raw reply related [flat|nested] 10+ messages in thread
* [Qemu-devel] [PATCH v5 2/2] live-block-ops.txt: Rename, rewrite, and improve it 2017-07-06 14:36 [Qemu-devel] [PATCH v5 0/2] Rewrite 'live-block-ops.txt'; convert 'bitmaps.md' to rST Kashyap Chamarthy 2017-07-06 14:36 ` [Qemu-devel] [PATCH v5 1/2] bitmaps.md: Convert to rST; move it into 'interop' dir Kashyap Chamarthy @ 2017-07-06 14:36 ` Kashyap Chamarthy 2017-07-06 18:11 ` Kashyap Chamarthy ` (2 more replies) 1 sibling, 3 replies; 10+ messages in thread From: Kashyap Chamarthy @ 2017-07-06 14:36 UTC (permalink / raw) To: qemu-devel Cc: eblake, qemu-block, kwolf, mreitz, jsnow, berto, Kashyap Chamarthy This patch documents (including their QMP invocations) all the four major kinds of live block operations: - `block-stream` - `block-commit` - `drive-mirror` (& `blockdev-mirror`) - `drive-backup` (& `blockdev-backup`) Things considered while writing this document: - Use reStructuredText as markup language (with the goal of generating the HTML output using the Sphinx Documentation Generator). It is gentler on the eye, and can be trivially converted to different formats. (Another reason: upstream QEMU is considering to switch to Sphinx, which uses reStructuredText as its markup language.) - Raw QMP JSON output vs. 'qmp-shell'. I debated with myself whether to only show raw QMP JSON output (as that is the canonical representation), or use 'qmp-shell', which takes key-value pairs. I settled on the approach of: for the first occurence of a command, use raw JSON; for subsequent occurences, use 'qmp-shell', with an occasional exception. - Usage of `-blockdev` command-line. - Usage of 'node-name' vs. file path to refer to disks. While we have `blockdev-{mirror, backup}` as 'node-name'-alternatives for `drive-{mirror, backup}`, the `block-commit` command still operate on file names for parameters 'base' and 'top'. So I added a caveat at the beginning to that effect. Refer this related thread that I started (where I learnt `block-stream` was recently reworked to accept 'node-name' for 'top' and 'base' parameters): https://lists.nongnu.org/archive/html/qemu-devel/2017-05/msg06466.html "[RFC] Making 'block-stream', and 'block-commit' accept node-name" All commands showed in this document were tested while documenting. Thanks: Eric Blake for the section: "A note on points-in-time vs file names". This useful bit was originally articulated by Eric in his KVMForum 2015 presentation, so I included that specific bit in this document. Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com> --- * A Sphinx-rendered HTML version is here: https://kashyapc.fedorapeople.org/v5-QEMU-Docs/_build/html/docs/live-block-operations.html * Changes in v5: - Clarify that overlays can be any other format, but 'qcow2' is preferred [Berto, Eric] - Update description of `block-stream`: mention that QEMU rewrites the backing chain [Berto, Eric] - s/right-most/rightmost/ (Although, Oxford English Dictionary lists both spellings to be correct, the latter, 'rightmost' is more frequently used.) [Berto, Eric] - Add two explicit 'note' admonitions for `block-{stream, commit}` in the section "Brief overview of live block QMP primitives", spelling out the disction between valid and invalid images [Berto, Eric] - Clarify that a block device job can either be cancelled or completed only after the corresponding event is emitted in the section "Live disk synchronization --- `drive-mirror` and `blockdev-mirror`" [Kevin, on IRC] - s/backup.qcow2/e.qcow2/g in the section "Notes on `blockdev-mirror`" to maintain consistency with the section "QMP invocation for `blockdev-mirror`"). Do the same in the section "Notes on `blockdev-mirror`" [Berto] - Move the summary of the different 'sync' modes to the section "Live disk synchronization -- `drive-mirror` and `blockdev-mirror`" as that's where it was first mentioned. Also clarify the workings of "drive-backup sync=none" mode by mentioning a 'note' admonition about it [Berto, Kevin] - s/copy-drive-backup.qcow2/e.qcow2/ in the section "QMP invocation for `drive-backup`" [Kashyap] * Changes in v4 - As per Paolo's suggestion on IRC, move the document from docs/ to docs/interop/. (Where "interop" means: management and interoperability with QEMU) - Mention synchronization modes as part of the "Live disk backup --- ``drive-backup`` and `blockdev-backup``" section. - And add a note mentioning the docs/interop/bitmaps.rst for detailed workings of the 'incremental' synchronization mode. * Change in v3 [address feedback from SFinucane]: - Make the Copyright header only part of the source, but not the rendered version - Use the ".. note::", ".. important::" admonitions where appropriate - Add a TODO note to remove the ".. contents::" directive when Sphinx is integrated - Make effective use of "::", resulting in removing lots of needless blank lines - Use anchors where applicable - Reword content in some places; fix some spelling mistakes * Changes in v2 [address content feedback from Eric; styling changes from Stephen Finucane]: - [Styling] Remove the ToC, as the Sphinx, ".. contents::" will take auto-generate it as part of the rendered version - [Styling] Replace ".. code-block::" with "::" as it depends on the external 'pygments' library and the syntaxes available vary between different versions. [Thanks to Stephen Finucane, who this tip on IRC, from experience of doing Sphinx documentation for the Open vSwitch project] - [Styling] Remove all needless hyperlinks, since ToC will take care of them - Fix commit message typos - Add Copyright / License boilerplate text at the top - Reword sentences in "Disk image backing chain notation" section - Fix descriptions of `block-{stream, commit}` - Rework `block-stream` QMP invocations to take its 'node-name' parameter 'base-node' - Add 'file.node-name=file' to the '-blockdev' command-line - s/shall/will/g - Clarify throughout the document, where appropriate, that we're starting afresh with the original disk image chain - Address mistakes in "Live block commit (`block-commit`)" and "QMP invocation for `block-commit`" sections - Describe the case of "shallow mirroring" (synchronize only the contents of the *top*-most disk image -- "sync": "top") for `drive-mirror`, as it's part of an important use case: live storage migration without shared storage setup. (Add a new section: "QMP invocation for live storage migration with `drive-mirror` + NBD" as part of this) - Add QMP invocation example for `blockdev-{mirror, backup}` (cherry picked from commit aef3e734f2d30b97e29345943d9e1cab794d0846) --- docs/interop/live-block-operations.rst | 1072 ++++++++++++++++++++++++++++++++ docs/live-block-ops.txt | 72 --- 2 files changed, 1072 insertions(+), 72 deletions(-) create mode 100644 docs/interop/live-block-operations.rst delete mode 100644 docs/live-block-ops.txt diff --git a/docs/interop/live-block-operations.rst b/docs/interop/live-block-operations.rst new file mode 100644 index 0000000..ba02bca --- /dev/null +++ b/docs/interop/live-block-operations.rst @@ -0,0 +1,1072 @@ +.. + Copyright (C) 2017 Red Hat Inc. + + This work is licensed under the terms of the GNU GPL, version 2 or + later. See the COPYING file in the top-level directory. + +============================ +Live Block Device Operations +============================ + +QEMU Block Layer currently (as of QEMU 2.9) supports four major kinds of +live block device jobs -- stream, commit, mirror, and backup. These can +be used to manipulate disk image chains to accomplish certain tasks, +namely: live copy data from backing files into overlays; shorten long +disk image chains by merging data from overlays into backing files; live +synchronize data from a disk image chain (including current active disk) +to another target image; point-in-time (and incremental) backups of a +block device. Below is a description of the said block (QMP) +primitives, and some (non-exhaustive list of) examples to illustrate +their use. + +.. note:: + The file ``qapi/block-core.json`` in the QEMU source tree has the + canonical QEMU API (QAPI) schema documentation for the QMP + primitives discussed here. + +.. todo (kashyapc):: Remove the ".. contents::" directive when Sphinx is + integrated. + +.. contents:: + +Disk image backing chain notation +--------------------------------- + +A simple disk image chain. (This can be created live, using QMP +``blockdev-snapshot-sync``, or offline, via ``qemu-img``):: + + (Live QEMU) + | + . + V + + [A] <----- [B] + + (backing file) (overlay) + +The arrow can be read as: Image [A] is the backing file of disk image +[B]. And live QEMU is currently writing to image [B], consequently, it +is also referred to as the "active layer". + +There are two kinds of terminology that are common when referring to +files in a disk image backing chain: + +(1) Directional: 'base' and 'top'. Given the simple disk image chain + above, image [A] can be referred to as 'base', and image [B] as + 'top'. (This terminology can be seen in in QAPI schema file, + block-core.json.) + +(2) Relational: 'backing file' and 'overlay'. Again, taking the same + simple disk image chain from the above, disk image [A] is referred + to as the backing file, and image [B] as overlay. + + Throughout this document, we will use the relational terminology. + +.. important:: + The overlay files can generally be any format that supports a + backing file, although QCOW2 is the preferred format and the one + used in this document. + + +Brief overview of live block QMP primitives +------------------------------------------- + +The following are the four different kinds of live block operations that +QEMU block layer supports. + +(1) ``block-stream``: Live copy of data from backing files into overlay + files. + + .. note:: Once the ``stream`` operation has finished, three things + to note: (a) QEMU rewrites the backing chain to remove + reference to the now-streamed and redundant backing file; + (b) the streamed file *itself* won't be removed by QEMU, + and must be explicitly discarded by the user; (c) the + streamed file remains valid -- i.e. further overlays can + be created based on it. Refer the ``block-stream`` + section further below for more details. + +(2) ``block-commit``: Live merge of data from overlay files into backing + files (with the optional goal of removing the overlay file from the + chain). Since QEMU 2.0, this includes "active ``block-commit``" + (i.e. merge the current active layer into the base image). + + .. note:: Once the ``commit`` operation has finished, here, too, + three things to note: (a) QEMU rewrites the backing chain + to remove reference to now-redundant overlay images that + have been commited into a backing file; (b) the commited + file *itself* won't be removed by QEMU -- it ought to be + manually removed; (c) however, unlike in the case of + ``block-stream``, the intermediate images will be rendered + invalid -- i.e. no more further overlays can be created + based on them. Refer the ``block-commit`` section further + below for more details. + +(3) ``drive-mirror`` (and ``blockdev-mirror``): Synchronize running disk + to another image. + +(4) ``drive-backup`` (and ``blockdev-backup``): Point-in-time (live) copy + of a block device to a destination. + + +.. _`Interacting with a QEMU instance`: + +Interacting with a QEMU instance +-------------------------------- + +To show some example invocations of command-line, we will use the +following invocation of QEMU, with a QMP server running over UNIX +socket:: + + $ ./x86_64-softmmu/qemu-system-x86_64 -display none -nodefconfig \ + -M q35 -nodefaults -m 512 \ + -blockdev node-name=node-A,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./a.qcow2 \ + -device virtio-blk,drive=node-A,id=virtio0 \ + -monitor stdio -qmp unix:/tmp/qmp-sock,server,nowait + +The ``-blockdev`` command-line option, used above, is available from +QEMU 2.9 onwards. In the above invocation, notice the ``node-name`` +parameter that is used to refer to the disk image a.qcow2 ('node-A') -- +this is a cleaner way to refer to a disk image (as opposed to referring +to it by spelling out file paths). So, we will continue to designate a +``node-name`` to each further disk image created (either via +``blockdev-snapshot-sync``, or ``blockdev-add``) as part of the disk +image chain, and continue to refer to the disks using their +``node-name`` (where possible, because ``block-commit`` does not yet, as +of QEMU 2.9, accept ``node-name`` parameter) when performing various +block operations. + +To interact with the QEMU instance launched above, we will use the +``qmp-shell`` (located at: ``qemu/scripts/qmp``, as part of the QEMU +source directory) utility, which takes key-value pairs for QMP commands. +Invoke it as below (which will also print out the complete raw JSON +syntax for reference -- examples in the following sections):: + + $ ./qmp-shell -v -p /tmp/qmp-sock + (QEMU) + +.. note:: + In the event we have to repeat a certain QMP command, we will: for + the first occurrence of it, show the the ``qmp-shell`` invocation, + *and* the corresponding raw JSON QMP syntax; but for subsequent + invocations, present just the ``qmp-shell`` syntax, and omit the + equivalent JSON output. + + +Example disk image chain +------------------------ + +We will use the below disk image chain (and occasionally spelling it +out where appropriate) when discussing various primitives:: + + [A] <-- [B] <-- [C] <-- [D] + +Where [A] is the original base image; [B] and [C] are intermediate +overlay images; image [D] is the active layer -- i.e. live QEMU is +writing to it. (The rule of thumb is: live QEMU will always be pointing +to the rightmost image in a disk image chain.) + +The above image chain can be created by invoking +``blockdev-snapshot-sync`` command as following (which shows the +creation of overlay image [B]) using the ``qmp-shell`` (our invocation +also prints the raw JSON invocation of it):: + + (QEMU) blockdev-snapshot-sync node-name=node-A snapshot-file=b.qcow2 snapshot-node-name=node-B format=qcow2 + { + "execute": "blockdev-snapshot-sync", + "arguments": { + "node-name": "node-A", + "snapshot-file": "b.qcow2", + "format": "qcow2", + "snapshot-node-name": "node-B" + } + } + +Here, "node-A" is the name QEMU internally uses to refer to the base +image [A] -- it is the backing file, based on which the overlay image, +[B], is created. + +To create the rest of the two overlay images, [C], and [D] (omitted the +raw JSON output for brevity):: + + (QEMU) blockdev-snapshot-sync node-name=node-B snapshot-file=c.qcow2 snapshot-node-name=node-C format=qcow2 + (QEMU) blockdev-snapshot-sync node-name=node-C snapshot-file=d.qcow2 snapshot-node-name=node-D format=qcow2 + + +A note on points-in-time vs file names +-------------------------------------- + +In our disk image chain:: + + [A] <-- [B] <-- [C] <-- [D] + +We have *three* points in time and an active layer: + +- Point 1: Guest state when [B] was created is contained in file [A] +- Point 2: Guest state when [C] was created is contained in [A] + [B] +- Point 3: Guest state when [D] was created is contained in + [A] + [B] + [C] +- Active layer: Current guest state is contained in [A] + [B] + [C] + + [D] + +Therefore, be aware with naming choices: + +- Naming a file after the time it is created is misleading -- the + guest data for that point in time is *not* contained in that file + (as explained earlier) +- Rather, think of files as a *delta* from the backing file + + +Live block streaming --- ``block-stream`` +----------------------------------------- + +The ``block-stream`` command allows you to do live copy data from backing +files into overlay images. + +Given our original example disk image chain from earlier:: + + [A] <-- [B] <-- [C] <-- [D] + +The disk image chain can be shortened in one of the following different +ways (not an exhaustive list). + +.. _`Case-1`: + +(1) Merge everything into the active layer: I.e. copy all contents from + the base image, [A], and overlay images, [B] and [C], into [D], + _while_ the guest is running. The resulting chain will be a + standalone image, [D] -- with contents from [A], [B] and [C] merged + into it (where live QEMU writes go to):: + + [D] + +.. _`Case-2`: + +(2) Taking the same example disk image chain mentioned earlier, merge + only images [B] and [C] into [D], the active layer. The result will + be contents of images [B] and [C] will be copied into [D], and the + backing file pointer of image [D] will be adjusted to point to image + [A]. The resulting chain will be:: + + [A] <-- [D] + +.. _`Case-3`: + +(3) Intermediate streaming (available since QEMU 2.8): Starting afresh + with the original example disk image chain, with a total of four + images, it is possible to copy contents from image [B] into image + [C]. Once the copy is finished, image [B] can now be (optionally) + discarded; and the backing file pointer of image [C] will be + adjusted to point to [A]. I.e. after performing "intermediate + streaming" of [B] into [C], the resulting image chain will be (where + live QEMU is writing to [D]):: + + [A] <-- [C] <-- [D] + + +QMP invocation for ``block-stream`` +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +For `Case-1`_, to merge contents of all the backing files into the +active layer, where 'node-D' is the current active image (by default +``block-stream`` will flatten the entire chain); ``qmp-shell`` (and its +corresponding JSON output):: + + (QEMU) block-stream device=node-D job-id=job0 + { + "execute": "block-stream", + "arguments": { + "device": "node-D", + "job-id": "job0" + } + } + +For `Case-2`_, merge contents of the images [B] and [C] into [D], where +image [D] ends up referring to image [A] as its backing file:: + + (QEMU) block-stream device=node-D base-node=node-A job-id=job0 + +And for `Case-3`_, of "intermediate" streaming", merge contents of +images [B] into [C], where [C] ends up referring to [A] as its backing +image:: + + (QEMU) block-stream device=node-C base-node=node-A job-id=job0 + +Progress of a ``block-stream`` operation can be monitored via the QMP +command:: + + (QEMU) query-block-jobs + { + "execute": "query-block-jobs", + "arguments": {} + } + + +Once the ``block-stream`` operation has completed, QEMU will emit an +event, ``BLOCK_JOB_COMPLETED``. The intermediate overlays remain valid, +and can now be (optionally) discarded, or retained to create further +overlays based on them. Finally, the ``block-stream`` jobs can be +restarted at anytime. + + +Live block commit --- ``block-commit`` +-------------------------------------- + +The ``block-commit`` command lets you merge live data from overlay +images into backing file(s). Since QEMU 2.0, this includes "live active +commit" (i.e. it is possible to merge the "active layer", the right-most +image in a disk image chain where live QEMU will be writing to, into the +base image). This is analogous to ``block-stream``, but in opposite +direction. + +Again, starting afresh with our example disk image chain, where live +QEMU is writing to the right-most image in the chain, [D]:: + + [A] <-- [B] <-- [C] <-- [D] + +The disk image chain can be shortened in one of the following ways: + +.. _`block-commit_Case-1`: + +(1) Commit content from only image [B] into image [A]. The resulting + chain is the following, where image [C] is adjusted to point at [A] + as its new backing file:: + + [A] <-- [C] <-- [D] + +(2) Commit content from images [B] and [C] into image [A]. The + resulting chain, where image [D] is adjusted to point to image [A] + as its new backing file:: + + [A] <-- [D] + +.. _`block-commit_Case-3`: + +(3) Commit content from images [B], [C], and the active layer [D] into + image [A]. The resulting chain (in this case, a consolidated single + image):: + + [A] + +(4) Commit content from image only image [C] into image [B]. The + resulting chain:: + + [A] <-- [B] <-- [D] + +(5) Commit content from image [C] and the active layer [D] into image + [B]. The resulting chain:: + + [A] <-- [B] + + +QMP invocation for ``block-commit`` +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +For `block-commit_Case-1`_, to merge contents only from +image [B] into image [A], the invocation is as following:: + + (QEMU) block-commit device=node-D base=a.qcow2 top=b.qcow2 job-id=job0 + { + "execute": "block-commit", + "arguments": { + "device": "node-D", + "job-id": "job0", + "top": "b.qcow2", + "base": "a.qcow2" + } + } + +Once the above ``block-commit`` operation has completed, a +``BLOCK_JOB_COMPLETED`` event will be issued, and no further action is +required. The end result being, the backing file of image [C] is +adjusted to point to image [A], and the original 4-image chain will end +up being transformed to:: + + [A] <-- [C] <-- [D] + +..note:: + The intermediate image [B] is invalid (as in: no more further + overlays based on it can be created) and, therefore, should be + dropped. + + +However, `block-commit_Case-3`_ (also called: "active ``block-commit``") +is a *two-phase* operation: in the first phase, the content from the +active overlay, along with the intermediate overlays, is copied into the +backing file (also called, the base image); in the second phase, adjust +the said backing file as the current active image -- possible via +issuing the command ``block-job-complete``. [Optionally, the +``block-commit`` operation can be cancelled, by issuing the command +``block-job-cancel``, but be careful when doing this.] + +Once the 'commit' operation (started by ``block-commit``) has completed, +the event ``BLOCK_JOB_READY`` is emitted, signalling the synchronization +has finished, and the job can be gracefully completed, by issuing +``block-job-complete``. (Until such a command is issued, the 'commit' +operation remains active.) + +So, the following is the flow for `block-commit_Case-3`_ -- to convert a +disk image chain such as this:: + + [A] <-- [B] <-- [C] <-- [D] + +Into (where content from all the subsequent overlays, [B], and [C], +including the active layer, [D], is committed back to [A] -- which is +where live QEMU is performing all its current writes):: + + [A] + +Start the "active ``block-commit``" operation:: + + (QEMU) block-commit device=node-D base=a.qcow2 top=d.qcow2 job-id=job0 + { + "execute": "block-commit", + "arguments": { + "device": "node-D", + "job-id": "job0", + "top": "d.qcow2", + "base": "a.qcow2" + } + } + + +Once the synchronization has completed, the event ``BLOCK_JOB_READY`` will +be emitted. + +Then, (optionally) query for the status of the active block operations +(we can see the 'commit' job is now ready to be completed, as indicated +by the line *"ready": true*):: + + (QEMU) query-block-jobs + { + "execute": "query-block-jobs", + "arguments": {} + } + { + "return": [ + { + "busy": false, + "type": "commit", + "len": 1376256, + "paused": false, + "ready": true, + "io-status": "ok", + "offset": 1376256, + "device": "job0", + "speed": 0 + } + ] + } + +Gracefully complete the 'commit' block device job:: + + (QEMU) block-job-complete device=job0 + { + "execute": "block-job-complete", + "arguments": { + "device": "job0" + } + } + { + "return": {} + } + +Finally, once the above job is completed, an event +``BLOCK_JOB_COMPLETED`` will be emitted. + +.. note:: + The invocation for rest of the cases (2, 4, and 5), discussed in the + previous section, is omitted for brevity. + + +Live disk synchronization --- ``drive-mirror`` and ``blockdev-mirror`` +---------------------------------------------------------------------- + +Synchronize a running disk image chain (all or part of it) to a target +image. + +Again, given our familiar disk image chain:: + + [A] <-- [B] <-- [C] <-- [D] + +The ``drive-mirror`` (and its newer equivalent ``blockdev-mirror``) allows +you to copy data from the entire chain into a single target image (which +can be located on a different host). + +Once a 'mirror' job has started, there are two possible actions when a +``drive-mirror`` job is active: + +(1) Issuing the command ``block-job-cancel``, after it emits the event + ``BLOCK_JOB_CANCELLED``: will (after completing synchronization of + the content from the disk image chain to the target image, [E]) + create a point-in-time (which is at the time of *triggering* the + cancel command) copy, contained in image [E], of the the entire disk + image chain (or only the top-most image, depending on the ``sync`` + mode). + +(2) Issuing the command ``block-job-complete``, after it emits the event + ``BLOCK_JOB_COMPLETED``: will, after completing synchronization of + the content, adjust the guest device (i.e. live QEMU) to point to + the target image, and, causing all the new writes from this point on + to happen there. One use case for this is live storage migration. + +A note on synchronization modes -- determines *what* part of the disk +image chain must be copied to the target. Currently, there are four +different kinds: + +(1) ``full`` -- Synchronize the content of entire disk image chain to + the target + +(2) ``top`` -- Synchronize only the contents of the top-most disk image + in the chain to the target + +(3) ``none`` -- Synchronize only the new writes from this point on. + + .. note:: In the case of ``drive-backup`` (or ``blockdev-backup``), + the behavior of ``none`` sychronization mode is different. + Normally, a ``backup`` job consists of two parts: Anything + that is overwritten by the guest is first copied out to + the backup, and in the background the whole image is + copied from start to end. With ``sync=none``, it's only + the first part. + +(4) ``incremental`` -- Synchronize content that is described by the + dirty bitmap + +.. note:: + Refer to the :doc:`bitmaps` document in the QEMU source + tree to learn about the detailed workings of the ``incremental`` + synchronization mode. + + +QMP invocation for ``drive-mirror`` +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +To copy the contents of the entire disk image chain, from [A] all the +way to [D], to a new target (``drive-mirror`` will create the destination +file, if it doesn't already exist), call it [E]:: + + (QEMU) drive-mirror device=node-D target=e.qcow2 sync=full job-id=job0 + { + "execute": "drive-mirror", + "arguments": { + "device": "node-D", + "job-id": "job0", + "target": "e.qcow2", + "sync": "full" + } + } + +The ``"sync": "full"``, from the above, means: copy the *entire* chain +to the destination. + +Following the above, querying for active block jobs will show that a +'mirror' job is "ready" to be completed (and QEMU will also emit an +event, ``BLOCK_JOB_READY``):: + + (QEMU) query-block-jobs + { + "execute": "query-block-jobs", + "arguments": {} + } + { + "return": [ + { + "busy": false, + "type": "mirror", + "len": 21757952, + "paused": false, + "ready": true, + "io-status": "ok", + "offset": 21757952, + "device": "job0", + "speed": 0 + } + ] + } + +And, as noted in the previous section, there are two possible actions +at this point: + +(a) Create a point-in-time snapshot by ending the synchronization. The + point-in-time is at the time of *ending* the sync. (The result of + the following being: the target image, [E], will be populated with + content from the entire chain, [A] to [D]):: + + (QEMU) block-job-cancel device=job0 + { + "execute": "block-job-cancel", + "arguments": { + "device": "job0" + } + } + +(b) Or, complete the operation and pivot the live QEMU to the target + copy:: + + (QEMU) block-job-complete device=job0 + +In either of the above cases, if you once again run the +`query-block-jobs` command, there should not be any active block +operation. + +Comparing 'commit' and 'mirror': In both then cases, the overlay images +can be discarded. However, with 'commit', the *existing* base image +will be modified (by updating it with contents from overlays); while in +the case of 'mirror', a *new* target image is populated with the data +from the disk image chain. + + +QMP invocation for live storage migration with ``drive-mirror`` + NBD +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Live storage migration (without shared storage setup) is one of the most +common use-cases that takes advantage of the ``drive-mirror`` primitive +and QEMU's built-in Network Block Device (NBD) server. Here's a quick +walk-through of this setup. + +Given the disk image chain:: + + [A] <-- [B] <-- [C] <-- [D] + +Instead of copying content from the entire chain, synchronize *only* the +contents of the *top*-most disk image (i.e. the active layer), [D], to a +target, say, [TargetDisk]. + +.. important:: + The destination host must already have the contents of the backing + chain, involving images [A], [B], and [C], visible via other means + -- whether by ``cp``, ``rsync``, or by some storage array-specific + command.) + +Sometimes, this is also referred to as "shallow copy" -- because: only +the "active layer", and not the rest of the image chain, is copied to +the destination. + +.. note:: + In this example, for the sake of simplicity, we'll be using the same + ``localhost`` as both, source and destination. + +As noted earlier, on the destination host the contents of the backing +chain -- from images [A] to [C] -- are already expected to exist in some +form (e.g. in a file called, ``Contents-of-A-B-C.qcow2``). Now, on the +destination host, let's create a target overlay image (with the image +``Contents-of-A-B-C.qcow2`` as its backing file), to which the contents +of image [D] (from the source QEMU) will be mirrored to:: + + $ qemu-img create -f qcow2 -b ./Contents-of-A-B-C.qcow2 \ + -F qcow2 ./target-disk.qcow2 + +And start the destination QEMU (we already have the source QEMU running +-- discussed in the section: `Interacting with a QEMU instance`_) +instance, with the following invocation. (As noted earlier, for +simplicity's sake, the destination QEMU is started on the same host, but +it could be located elsewhere):: + + $ ./x86_64-softmmu/qemu-system-x86_64 -display none -nodefconfig \ + -M q35 -nodefaults -m 512 \ + -blockdev node-name=node-TargetDisk,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./target-disk.qcow2 \ + -device virtio-blk,drive=node-TargetDisk,id=virtio0 \ + -S -monitor stdio -qmp unix:./qmp-sock2,server,nowait \ + -incoming tcp:localhost:6666 + +Given the disk image chain on source QEMU:: + + [A] <-- [B] <-- [C] <-- [D] + +On the destination host, it is expected that the contents of the chain +``[A] <-- [B] <-- [C]`` are *already* present, and therefore copy *only* +the content of image [D]. + +(1) [On *destination* QEMU] As part of the first step, start the + built-in NBD server on a given host (local host, represented by + ``::``)and port:: + + (QEMU) nbd-server-start addr={"type":"inet","data":{"host":"::","port":"49153"}} + { + "execute": "nbd-server-start", + "arguments": { + "addr": { + "data": { + "host": "::", + "port": "49153" + }, + "type": "inet" + } + } + } + +(2) [On *destination* QEMU] And export the destination disk image using + QEMU's built-in NBD server:: + + (QEMU) nbd-server-add device=node-TargetDisk writable=true + { + "execute": "nbd-server-add", + "arguments": { + "device": "node-TargetDisk" + } + } + +(3) [On *source* QEMU] Then, invoke ``drive-mirror`` (NB: since we're + running ``drive-mirror`` with ``mode=existing`` (meaning: + synchronize to a pre-created file, therefore 'existing', file on the + target host), with the synchronization mode as 'top' (``"sync: + "top"``):: + + (QEMU) drive-mirror device=node-D target=nbd:localhost:49153:exportname=node-TargetDisk sync=top mode=existing job-id=job0 + { + "execute": "drive-mirror", + "arguments": { + "device": "node-D", + "mode": "existing", + "job-id": "job0", + "target": "nbd:localhost:49153:exportname=node-TargetDisk", + "sync": "top" + } + } + +(4) [On *source* QEMU] Once ``drive-mirror`` copies the entire data, and the + event ``BLOCK_JOB_READY`` is emitted, issue ``block-job-cancel`` to + gracefully end the synchronization, from source QEMU:: + + (QEMU) block-job-cancel device=job0 + { + "execute": "block-job-cancel", + "arguments": { + "device": "job0" + } + } + +(5) [On *destination* QEMU] Then, stop the NBD server:: + + (QEMU) nbd-server-stop + { + "execute": "nbd-server-stop", + "arguments": {} + } + +(6) [On *destination* QEMU] Finally, resume the guest vCPUs by issuing the + QMP command `cont`:: + + (QEMU) cont + { + "execute": "cont", + "arguments": {} + } + + +.. note:: + Higher-level libraries (e.g. libvirt) automate the entire above + process. + + +Notes on ``blockdev-mirror`` +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``blockdev-mirror`` command is equivalent in core functionality to +``drive-mirror``, except that it operates at node-level in a BDS graph. + +Also: for ``blockdev-mirror``, the 'target' image needs to be explicitly +created (using ``qemu-img``) and attach it to live QEMU via +``blockdev-add``, which assigns a name to the to-be created target node. + +E.g. the sequence of actions to create a point-in-time backup of an +entire disk image chain, to a target, using ``blockdev-mirror`` would be: + +(0) Create the QCOW2 overlays, to arrive at a backing chain of desired + depth + +(1) Create the target image (using ``qemu-img``), say, ``e.qcow2`` + +(2) Attach the above created file (``e.qcow2``), run-time, using + ``blockdev-add`` to QEMU + +(3) Perform ``blockdev-mirror`` (use ``"sync": "full"`` to copy the + entire chain to the target). And observe for the event + ``BLOCK_JOB_READY`` + +(4) Optionally, query for active block jobs, there should be a 'mirror' + job ready to be completed + +(5) Gracefully complete the 'mirror' block device job, and observe for + the event ``BLOCK_JOB_COMPLETED`` + +(6) Shutdown the guest, by issuing the QMP ``quit`` command, so that + caches are flushed + +(7) Then, finally, compare the contents of the disk image chain, and + the target copy with ``qemu-img compare``. You should notice: + "Images are identical" + + +QMP invocation for ``blockdev-mirror`` +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Given the disk image chain:: + + [A] <-- [B] <-- [C] <-- [D] + +To copy the contents of the entire disk image chain, from [A] all the +way to [D], to a new target, call it [E]. The following is the flow. + +Create the overlay images, [B], [C], and [D]:: + + (QEMU) blockdev-snapshot-sync node-name=node-A snapshot-file=b.qcow2 snapshot-node-name=node-B format=qcow2 + (QEMU) blockdev-snapshot-sync node-name=node-B snapshot-file=c.qcow2 snapshot-node-name=node-C format=qcow2 + (QEMU) blockdev-snapshot-sync node-name=node-C snapshot-file=d.qcow2 snapshot-node-name=node-D format=qcow2 + +Create the target image, [E]:: + + $ qemu-img create -f qcow2 e.qcow2 39M + +Add the above created target image to QEMU, via ``blockdev-add``:: + + (QEMU) blockdev-add driver=qcow2 node-name=node-E file={"driver":"file","filename":"e.qcow2"} + { + "execute": "blockdev-add", + "arguments": { + "node-name": "node-E", + "driver": "qcow2", + "file": { + "driver": "file", + "filename": "e.qcow2" + } + } + } + +Perform ``blockdev-mirror``, and observe for the event +``BLOCK_JOB_READY``:: + + (QEMU) blockdev-mirror device=node-B target=node-E sync=full job-id=job0 + { + "execute": "blockdev-mirror", + "arguments": { + "device": "node-D", + "job-id": "job0", + "target": "node-E", + "sync": "full" + } + } + +Query for active block jobs, there should be a 'mirror' job ready:: + + (QEMU) query-block-jobs + { + "execute": "query-block-jobs", + "arguments": {} + } + { + "return": [ + { + "busy": false, + "type": "mirror", + "len": 21561344, + "paused": false, + "ready": true, + "io-status": "ok", + "offset": 21561344, + "device": "job0", + "speed": 0 + } + ] + } + +Gracefully complete the block device job operation, and observe for the +event ``BLOCK_JOB_COMPLETED``:: + + (QEMU) block-job-complete device=job0 + { + "execute": "block-job-complete", + "arguments": { + "device": "job0" + } + } + { + "return": {} + } + +Shutdown the guest, by issuing the ``quit`` QMP command:: + + (QEMU) quit + { + "execute": "quit", + "arguments": {} + } + + +Live disk backup --- ``drive-backup`` and ``blockdev-backup`` +------------------------------------------------------------- + +The ``drive-backup`` (and its newer equivalent ``blockdev-backup``) allows +you to create a point-in-time snapshot. + +In this case, the point-in-time is when you *start* the ``drive-backup`` +(or its newer equivalent ``blockdev-backup``) command. + + +QMP invocation for ``drive-backup`` +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Yet again, starting afresh with our example disk image chain:: + + [A] <-- [B] <-- [C] <-- [D] + +To create a target image [E], with content populated from image [A] to +[D], from the above chain, the following is the syntax. (If the target +image does not exist, ``drive-backup`` will create it):: + + (QEMU) drive-backup device=node-D sync=full target=e.qcow2 job-id=job0 + { + "execute": "drive-backup", + "arguments": { + "device": "node-D", + "job-id": "job0", + "sync": "full", + "target": "e.qcow2" + } + } + +Once the above ``drive-backup`` has completed, a ``BLOCK_JOB_COMPLETED`` event +will be issued, indicating the live block device job operation has +completed, and no further action is required. + + +Notes on ``blockdev-backup`` +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``blockdev-backup`` command is equivalent in functionality to +``drive-backup``, except that it operates at node-level in a Block Driver +State (BDS) graph. + +E.g. the sequence of actions to create a point-in-time backup +of an entire disk image chain, to a target, using ``blockdev-backup`` +would be: + +(0) Create the QCOW2 overlays, to arrive at a backing chain of desired + depth + +(1) Create the target image (using ``qemu-img``), say, ``e.qcow2`` + +(2) Attach the above created file (``e.qcow2``), run-time, using + ``blockdev-add`` to QEMU + +(3) Perform ``blockdev-backup`` (use ``"sync": "full"`` to copy the + entire chain to the target). And observe for the event + ``BLOCK_JOB_COMPLETED`` + +(4) Shutdown the guest, by issuing the QMP ``quit`` command, so that + caches are flushed + +(5) Then, finally, compare the contents of the disk image chain, and + the target copy with ``qemu-img compare``. You should notice: + "Images are identical" + +The following section shows an example QMP invocation for +``blockdev-backup``. + +QMP invocation for ``blockdev-backup`` +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Given, a disk image chain of depth 1, where image [B] is the active +overlay (live QEMU is writing to it):: + + [A] <-- [B] + +The following is the procedure to copy the content from the entire chain +to a target image (say, [E]), which has the full content from [A] and +[B]. + +Create the overlay, [B]:: + + (QEMU) blockdev-snapshot-sync node-name=node-A snapshot-file=b.qcow2 snapshot-node-name=node-B format=qcow2 + { + "execute": "blockdev-snapshot-sync", + "arguments": { + "node-name": "node-A", + "snapshot-file": "b.qcow2", + "format": "qcow2", + "snapshot-node-name": "node-B" + } + } + + +Create a target image, that will contain the copy:: + + $ qemu-img create -f qcow2 e.qcow2 39M + +Then, add it to QEMU via ``blockdev-add``:: + + (QEMU) blockdev-add driver=qcow2 node-name=node-E file={"driver":"file","filename":"e.qcow2"} + { + "execute": "blockdev-add", + "arguments": { + "node-name": "node-E", + "driver": "qcow2", + "file": { + "driver": "file", + "filename": "e.qcow2" + } + } + } + +Then, invoke ``blockdev-backup``, to copy the contents from the entire +image chain, consisting of images [A], and [B], to the target image +'e.qcow2':: + + (QEMU) blockdev-backup device=node-B target=node-E sync=full job-id=job0 + { + "execute": "blockdev-backup", + "arguments": { + "device": "node-B", + "job-id": "job0", + "target": "node-E", + "sync": "full" + } + } + +Once the above 'backup' operation has completed, an event, +``BLOCK_JOB_COMPLETED``, will be emitted, signalling successful +completion. + +Next, query for any active block device jobs (there should be none):: + + (QEMU) query-block-jobs + { + "execute": "query-block-jobs", + "arguments": {} + } + +Shutdown the guest:: + + (QEMU) quit + { + "execute": "quit", + "arguments": {} + } + "return": {} + } + +.. note:: + The above step is really important; if forgotten, an error, "Failed + to get shared "write" lock on e.qcow2", will be thrown when you do + ``qemu-img compare`` to verify the integrity of the disk image + with the backup content. + + +The end result will be, the image 'e.qcow2' containing a +point-in-time backup of the disk image chain -- i.e. contents from +images [A], and [B] at the time the ``blockdev-backup`` command was +initiated. + +One way to confirm the backup disk image contains the identical content +with the disk image chain is to compare the backup, and the contents of +the chain, you should see "Images are identical". (NB: this is assuming +QEMU was launched with `-S` option, which will not start the CPUs at +guest boot up):: + + $ qemu-img compare b.qcow2 e.qcow2 + Warning: Image size mismatch! + Images are identical. + +NOTE: The "Warning: Image size mismatch!" is expected, as we created the +target image (e.qcow2) with 39M size. diff --git a/docs/live-block-ops.txt b/docs/live-block-ops.txt deleted file mode 100644 index 2211d14..0000000 --- a/docs/live-block-ops.txt +++ /dev/null @@ -1,72 +0,0 @@ -LIVE BLOCK OPERATIONS -===================== - -High level description of live block operations. Note these are not -supported for use with the raw format at the moment. - -Note also that this document is incomplete and it currently only -covers the 'stream' operation. Other operations supported by QEMU such -as 'commit', 'mirror' and 'backup' are not described here yet. Please -refer to the qapi/block-core.json file for an overview of those. - -Snapshot live merge -=================== - -Given a snapshot chain, described in this document in the following -format: - -[A] <- [B] <- [C] <- [D] <- [E] - -Where the rightmost object ([E] in the example) described is the current -image which the guest OS has write access to. To the left of it is its base -image, and so on accordingly until the leftmost image, which has no -base. - -The snapshot live merge operation transforms such a chain into a -smaller one with fewer elements, such as this transformation relative -to the first example: - -[A] <- [E] - -Data is copied in the right direction with destination being the -rightmost image, but any other intermediate image can be specified -instead. In this example data is copied from [C] into [D], so [D] can -be backed by [B]: - -[A] <- [B] <- [D] <- [E] - -The operation is implemented in QEMU through image streaming facilities. - -The basic idea is to execute 'block_stream virtio0' while the guest is -running. Progress can be monitored using 'info block-jobs'. When the -streaming operation completes it raises a QMP event. 'block_stream' -copies data from the backing file(s) into the active image. When finished, -it adjusts the backing file pointer. - -The 'base' parameter specifies an image which data need not be -streamed from. This image will be used as the backing file for the -destination image when the operation is finished. - -In the first example above, the command would be: - -(qemu) block_stream virtio0 file-A.img - -In order to specify a destination image different from the active -(rightmost) one we can use its node name instead. - -In the second example above, the command would be: - -(qemu) block_stream node-D file-B.img - -Live block copy -=============== - -To copy an in use image to another destination in the filesystem, one -should create a live snapshot in the desired destination, then stream -into that image. Example: - -(qemu) snapshot_blkdev ide0-hd0 /new-path/disk.img qcow2 - -(qemu) block_stream ide0-hd0 - - -- 2.7.5 ^ permalink raw reply related [flat|nested] 10+ messages in thread
* Re: [Qemu-devel] [PATCH v5 2/2] live-block-ops.txt: Rename, rewrite, and improve it 2017-07-06 14:36 ` [Qemu-devel] [PATCH v5 2/2] live-block-ops.txt: Rename, rewrite, and improve it Kashyap Chamarthy @ 2017-07-06 18:11 ` Kashyap Chamarthy 2017-07-06 22:41 ` John Snow 2017-07-07 22:25 ` John Snow 2 siblings, 0 replies; 10+ messages in thread From: Kashyap Chamarthy @ 2017-07-06 18:11 UTC (permalink / raw) To: qemu-devel; +Cc: kwolf, berto, qemu-block, mreitz, jsnow On Thu, Jul 06, 2017 at 04:36:46PM +0200, Kashyap Chamarthy wrote: > This patch documents (including their QMP invocations) all the four > major kinds of live block operations: [...] > (cherry picked from commit aef3e734f2d30b97e29345943d9e1cab794d0846) Oops, the above "cherry picked" line is accidental -- as I was doing some local branch wrangling. It can be removed when applying the patch. If the maintainer asks, I can respin. But I'll first wait for any other feedback, if there is. > --- > docs/interop/live-block-operations.rst | 1072 ++++++++++++++++++++++++++++++++ > docs/live-block-ops.txt | 72 --- > 2 files changed, 1072 insertions(+), 72 deletions(-) > create mode 100644 docs/interop/live-block-operations.rst > delete mode 100644 docs/live-block-ops.txt [...] -- /kashyap ^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [Qemu-devel] [PATCH v5 2/2] live-block-ops.txt: Rename, rewrite, and improve it 2017-07-06 14:36 ` [Qemu-devel] [PATCH v5 2/2] live-block-ops.txt: Rename, rewrite, and improve it Kashyap Chamarthy 2017-07-06 18:11 ` Kashyap Chamarthy @ 2017-07-06 22:41 ` John Snow 2017-07-07 10:46 ` Kashyap Chamarthy 2017-07-07 22:25 ` John Snow 2 siblings, 1 reply; 10+ messages in thread From: John Snow @ 2017-07-06 22:41 UTC (permalink / raw) To: Kashyap Chamarthy, qemu-devel; +Cc: kwolf, berto, qemu-block, mreitz On 07/06/2017 10:36 AM, Kashyap Chamarthy wrote: > This patch documents (including their QMP invocations) all the four > major kinds of live block operations: > > - `block-stream` > - `block-commit` > - `drive-mirror` (& `blockdev-mirror`) > - `drive-backup` (& `blockdev-backup`) > > Things considered while writing this document: > > - Use reStructuredText as markup language (with the goal of generating > the HTML output using the Sphinx Documentation Generator). It is > gentler on the eye, and can be trivially converted to different > formats. (Another reason: upstream QEMU is considering to switch to > Sphinx, which uses reStructuredText as its markup language.) > > - Raw QMP JSON output vs. 'qmp-shell'. I debated with myself whether > to only show raw QMP JSON output (as that is the canonical > representation), or use 'qmp-shell', which takes key-value pairs. I > settled on the approach of: for the first occurence of a command, > use raw JSON; for subsequent occurences, use 'qmp-shell', with an > occasional exception. > > - Usage of `-blockdev` command-line. > > - Usage of 'node-name' vs. file path to refer to disks. While we have > `blockdev-{mirror, backup}` as 'node-name'-alternatives for > `drive-{mirror, backup}`, the `block-commit` command still operate > on file names for parameters 'base' and 'top'. So I added a caveat > at the beginning to that effect. > > Refer this related thread that I started (where I learnt > `block-stream` was recently reworked to accept 'node-name' for 'top' > and 'base' parameters): > https://lists.nongnu.org/archive/html/qemu-devel/2017-05/msg06466.html > "[RFC] Making 'block-stream', and 'block-commit' accept node-name" > > All commands showed in this document were tested while documenting. > > Thanks: Eric Blake for the section: "A note on points-in-time vs file > names". This useful bit was originally articulated by Eric in his > KVMForum 2015 presentation, so I included that specific bit in this > document. > > Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com> > --- > * A Sphinx-rendered HTML version is here: > https://kashyapc.fedorapeople.org/v5-QEMU-Docs/_build/html/docs/live-block-operations.html > > * Changes in v5: > - Clarify that overlays can be any other format, but 'qcow2' is > preferred [Berto, Eric] > - Update description of `block-stream`: mention that QEMU rewrites > the backing chain [Berto, Eric] > - s/right-most/rightmost/ (Although, Oxford English Dictionary lists > both spellings to be correct, the latter, 'rightmost' is more > frequently used.) [Berto, Eric] > - Add two explicit 'note' admonitions for `block-{stream, commit}` in > the section "Brief overview of live block QMP primitives", spelling > out the disction between valid and invalid images [Berto, Eric] > - Clarify that a block device job can either be cancelled or > completed only after the corresponding event is emitted in the > section "Live disk synchronization --- `drive-mirror` and > `blockdev-mirror`" [Kevin, on IRC] > - s/backup.qcow2/e.qcow2/g in the section "Notes on > `blockdev-mirror`" to maintain consistency with the section "QMP > invocation for `blockdev-mirror`"). Do the same in the section > "Notes on `blockdev-mirror`" [Berto] > - Move the summary of the different 'sync' modes to the section "Live > disk synchronization -- `drive-mirror` and `blockdev-mirror`" as > that's where it was first mentioned. Also clarify the workings of > "drive-backup sync=none" mode by mentioning a 'note' admonition > about it [Berto, Kevin] > - s/copy-drive-backup.qcow2/e.qcow2/ in the section "QMP invocation > for `drive-backup`" [Kashyap] > > * Changes in v4 > - As per Paolo's suggestion on IRC, move the document from docs/ to > docs/interop/. (Where "interop" means: management and > interoperability with QEMU) > - Mention synchronization modes as part of the "Live disk backup --- > ``drive-backup`` and `blockdev-backup``" section. > - And add a note mentioning the docs/interop/bitmaps.rst for detailed > workings of the 'incremental' synchronization mode. > > * Change in v3 [address feedback from SFinucane]: > - Make the Copyright header only part of the source, but not the > rendered version > - Use the ".. note::", ".. important::" admonitions where appropriate > - Add a TODO note to remove the ".. contents::" directive when Sphinx > is integrated > - Make effective use of "::", resulting in removing lots of needless > blank lines > - Use anchors where applicable > - Reword content in some places; fix some spelling mistakes > > * Changes in v2 [address content feedback from Eric; styling changes > from Stephen Finucane]: > - [Styling] Remove the ToC, as the Sphinx, ".. contents::" will take > auto-generate it as part of the rendered version > - [Styling] Replace ".. code-block::" with "::" as it depends on the > external 'pygments' library and the syntaxes available vary between > different versions. [Thanks to Stephen Finucane, who this tip on > IRC, from experience of doing Sphinx documentation for the Open > vSwitch project] > - [Styling] Remove all needless hyperlinks, since ToC will take care > of them > - Fix commit message typos > - Add Copyright / License boilerplate text at the top > - Reword sentences in "Disk image backing chain notation" section > - Fix descriptions of `block-{stream, commit}` > - Rework `block-stream` QMP invocations to take its 'node-name' > parameter 'base-node' > - Add 'file.node-name=file' to the '-blockdev' command-line > - s/shall/will/g > - Clarify throughout the document, where appropriate, > that we're starting afresh with the original disk image chain > - Address mistakes in "Live block commit (`block-commit`)" and > "QMP invocation for `block-commit`" sections > - Describe the case of "shallow mirroring" (synchronize only the > contents of the *top*-most disk image -- "sync": "top") for > `drive-mirror`, as it's part of an important use case: live storage > migration without shared storage setup. (Add a new section: "QMP > invocation for live storage migration with `drive-mirror` + NBD" as > part of this) > - Add QMP invocation example for `blockdev-{mirror, backup}` > > (cherry picked from commit aef3e734f2d30b97e29345943d9e1cab794d0846) > --- > docs/interop/live-block-operations.rst | 1072 ++++++++++++++++++++++++++++++++ > docs/live-block-ops.txt | 72 --- > 2 files changed, 1072 insertions(+), 72 deletions(-) > create mode 100644 docs/interop/live-block-operations.rst > delete mode 100644 docs/live-block-ops.txt > > diff --git a/docs/interop/live-block-operations.rst b/docs/interop/live-block-operations.rst > new file mode 100644 > index 0000000..ba02bca > --- /dev/null > +++ b/docs/interop/live-block-operations.rst > @@ -0,0 +1,1072 @@ > +.. > + Copyright (C) 2017 Red Hat Inc. > + > + This work is licensed under the terms of the GNU GPL, version 2 or > + later. See the COPYING file in the top-level directory. > + > +============================ > +Live Block Device Operations > +============================ > + > +QEMU Block Layer currently (as of QEMU 2.9) supports four major kinds of > +live block device jobs -- stream, commit, mirror, and backup. These can > +be used to manipulate disk image chains to accomplish certain tasks, > +namely: live copy data from backing files into overlays; shorten long > +disk image chains by merging data from overlays into backing files; live > +synchronize data from a disk image chain (including current active disk) > +to another target image; point-in-time (and incremental) backups of a "and point-in-time" to indicate the conclusion of a list? 1, 2, 3, and 4. > +block device. Below is a description of the said block (QMP) > +primitives, and some (non-exhaustive list of) examples to illustrate > +their use. > + > +.. note:: > + The file ``qapi/block-core.json`` in the QEMU source tree has the > + canonical QEMU API (QAPI) schema documentation for the QMP > + primitives discussed here. > + > +.. todo (kashyapc):: Remove the ".. contents::" directive when Sphinx is > + integrated. > + > +.. contents:: > + > +Disk image backing chain notation > +--------------------------------- > + > +A simple disk image chain. (This can be created live, using QMP > +``blockdev-snapshot-sync``, or offline, via ``qemu-img``):: "can be created live using QMP ``foo``, or offline via ``bar``" I, tend, to, use, too, many, commas, in, my, own, writing, to, mimic, a, conversational, tone, but, admittedly, I, go, overboard. Take with a grain of salt. Eric is our ultimate copy-editor... > + > + (Live QEMU) > + | > + . > + V > + > + [A] <----- [B] > + > + (backing file) (overlay) > + > +The arrow can be read as: Image [A] is the backing file of disk image > +[B]. And live QEMU is currently writing to image [B], consequently, it > +is also referred to as the "active layer". > + > +There are two kinds of terminology that are common when referring to > +files in a disk image backing chain: > + > +(1) Directional: 'base' and 'top'. Given the simple disk image chain > + above, image [A] can be referred to as 'base', and image [B] as > + 'top'. (This terminology can be seen in in QAPI schema file, > + block-core.json.) > + > +(2) Relational: 'backing file' and 'overlay'. Again, taking the same > + simple disk image chain from the above, disk image [A] is referred > + to as the backing file, and image [B] as overlay. > + > + Throughout this document, we will use the relational terminology. > + > +.. important:: > + The overlay files can generally be any format that supports a > + backing file, although QCOW2 is the preferred format and the one > + used in this document. > + > + > +Brief overview of live block QMP primitives > +------------------------------------------- > + > +The following are the four different kinds of live block operations that > +QEMU block layer supports. > + > +(1) ``block-stream``: Live copy of data from backing files into overlay > + files. > + > + .. note:: Once the ``stream`` operation has finished, three things > + to note: (a) QEMU rewrites the backing chain to remove > + reference to the now-streamed and redundant backing file; > + (b) the streamed file *itself* won't be removed by QEMU, > + and must be explicitly discarded by the user; (c) the > + streamed file remains valid -- i.e. further overlays can > + be created based on it. Refer the ``block-stream`` > + section further below for more details. Is it possible to listify these three points? > + > +(2) ``block-commit``: Live merge of data from overlay files into backing > + files (with the optional goal of removing the overlay file from the > + chain). Since QEMU 2.0, this includes "active ``block-commit``" > + (i.e. merge the current active layer into the base image). > + > + .. note:: Once the ``commit`` operation has finished, here, too, You've gone comma-wild! "Once the ``commit`` operation has finished, there are three things to note here <too/as well>:" > + three things to note: (a) QEMU rewrites the backing chain > + to remove reference to now-redundant overlay images that > + have been commited into a backing file; (b) the commited > + file *itself* won't be removed by QEMU -- it ought to be > + manually removed; (c) however, unlike in the case of > + ``block-stream``, the intermediate images will be rendered > + invalid -- i.e. no more further overlays can be created > + based on them. Refer the ``block-commit`` section further > + below for more details. > + > +(3) ``drive-mirror`` (and ``blockdev-mirror``): Synchronize running disk > + to another image. > + > +(4) ``drive-backup`` (and ``blockdev-backup``): Point-in-time (live) copy > + of a block device to a destination. > + > + > +.. _`Interacting with a QEMU instance`: > + > +Interacting with a QEMU instance > +-------------------------------- > + > +To show some example invocations of command-line, we will use the > +following invocation of QEMU, with a QMP server running over UNIX > +socket:: > + > + $ ./x86_64-softmmu/qemu-system-x86_64 -display none -nodefconfig \ > + -M q35 -nodefaults -m 512 \ > + -blockdev node-name=node-A,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./a.qcow2 \ > + -device virtio-blk,drive=node-A,id=virtio0 \ > + -monitor stdio -qmp unix:/tmp/qmp-sock,server,nowait > + > +The ``-blockdev`` command-line option, used above, is available from > +QEMU 2.9 onwards. In the above invocation, notice the ``node-name`` > +parameter that is used to refer to the disk image a.qcow2 ('node-A') -- > +this is a cleaner way to refer to a disk image (as opposed to referring > +to it by spelling out file paths). So, we will continue to designate a > +``node-name`` to each further disk image created (either via > +``blockdev-snapshot-sync``, or ``blockdev-add``) as part of the disk > +image chain, and continue to refer to the disks using their > +``node-name`` (where possible, because ``block-commit`` does not yet, as > +of QEMU 2.9, accept ``node-name`` parameter) when performing various > +block operations. > + > +To interact with the QEMU instance launched above, we will use the > +``qmp-shell`` (located at: ``qemu/scripts/qmp``, as part of the QEMU > +source directory) utility, which takes key-value pairs for QMP commands. > +Invoke it as below (which will also print out the complete raw JSON > +syntax for reference -- examples in the following sections):: > + > + $ ./qmp-shell -v -p /tmp/qmp-sock > + (QEMU) > + > +.. note:: > + In the event we have to repeat a certain QMP command, we will: for > + the first occurrence of it, show the the ``qmp-shell`` invocation, > + *and* the corresponding raw JSON QMP syntax; but for subsequent > + invocations, present just the ``qmp-shell`` syntax, and omit the > + equivalent JSON output. > + It's like I'm reading a university textbook. Very nice! > + > +Example disk image chain > +------------------------ > + > +We will use the below disk image chain (and occasionally spelling it > +out where appropriate) when discussing various primitives:: > + > + [A] <-- [B] <-- [C] <-- [D] > + > +Where [A] is the original base image; [B] and [C] are intermediate > +overlay images; image [D] is the active layer -- i.e. live QEMU is > +writing to it. (The rule of thumb is: live QEMU will always be pointing > +to the rightmost image in a disk image chain.) > + > +The above image chain can be created by invoking > +``blockdev-snapshot-sync`` command as following (which shows the command_s_ as following? We're starting from a single image and we need to create several snapshots. > +creation of overlay image [B]) using the ``qmp-shell`` (our invocation > +also prints the raw JSON invocation of it):: > + > + (QEMU) blockdev-snapshot-sync node-name=node-A snapshot-file=b.qcow2 snapshot-node-name=node-B format=qcow2 > + { > + "execute": "blockdev-snapshot-sync", > + "arguments": { > + "node-name": "node-A", > + "snapshot-file": "b.qcow2", > + "format": "qcow2", > + "snapshot-node-name": "node-B" > + } > + } > + > +Here, "node-A" is the name QEMU internally uses to refer to the base > +image [A] -- it is the backing file, based on which the overlay image, > +[B], is created. > + > +To create the rest of the two overlay images, [C], and [D] (omitted the "rest of the two" strikes me as peculiar. Either "rest of the images" or "The last/final/other two" > +raw JSON output for brevity):: > + > + (QEMU) blockdev-snapshot-sync node-name=node-B snapshot-file=c.qcow2 snapshot-node-name=node-C format=qcow2 > + (QEMU) blockdev-snapshot-sync node-name=node-C snapshot-file=d.qcow2 snapshot-node-name=node-D format=qcow2 > + > + > +A note on points-in-time vs file names > +-------------------------------------- > + > +In our disk image chain:: > + > + [A] <-- [B] <-- [C] <-- [D] > + > +We have *three* points in time and an active layer: > + > +- Point 1: Guest state when [B] was created is contained in file [A] > +- Point 2: Guest state when [C] was created is contained in [A] + [B] > +- Point 3: Guest state when [D] was created is contained in > + [A] + [B] + [C] > +- Active layer: Current guest state is contained in [A] + [B] + [C] + > + [D] > + > +Therefore, be aware with naming choices: > + > +- Naming a file after the time it is created is misleading -- the > + guest data for that point in time is *not* contained in that file > + (as explained earlier) > +- Rather, think of files as a *delta* from the backing file > + > + > +Live block streaming --- ``block-stream`` > +----------------------------------------- > + > +The ``block-stream`` command allows you to do live copy data from backing > +files into overlay images. > + > +Given our original example disk image chain from earlier:: > + > + [A] <-- [B] <-- [C] <-- [D] > + > +The disk image chain can be shortened in one of the following different > +ways (not an exhaustive list). > + > +.. _`Case-1`: > + > +(1) Merge everything into the active layer: I.e. copy all contents from > + the base image, [A], and overlay images, [B] and [C], into [D], > + _while_ the guest is running. The resulting chain will be a is _while_ meant to apply formatting in this case? > + standalone image, [D] -- with contents from [A], [B] and [C] merged > + into it (where live QEMU writes go to):: > + > + [D] > + > +.. _`Case-2`: > + > +(2) Taking the same example disk image chain mentioned earlier, merge > + only images [B] and [C] into [D], the active layer. The result will > + be contents of images [B] and [C] will be copied into [D], and the > + backing file pointer of image [D] will be adjusted to point to image > + [A]. The resulting chain will be:: > + > + [A] <-- [D] > + > +.. _`Case-3`: > + > +(3) Intermediate streaming (available since QEMU 2.8): Starting afresh > + with the original example disk image chain, with a total of four > + images, it is possible to copy contents from image [B] into image > + [C]. Once the copy is finished, image [B] can now be (optionally) > + discarded; and the backing file pointer of image [C] will be > + adjusted to point to [A]. I.e. after performing "intermediate > + streaming" of [B] into [C], the resulting image chain will be (where > + live QEMU is writing to [D]):: > + > + [A] <-- [C] <-- [D] > + > + > +QMP invocation for ``block-stream`` > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +For `Case-1`_, to merge contents of all the backing files into the > +active layer, where 'node-D' is the current active image (by default > +``block-stream`` will flatten the entire chain); ``qmp-shell`` (and its > +corresponding JSON output):: > + > + (QEMU) block-stream device=node-D job-id=job0 > + { > + "execute": "block-stream", > + "arguments": { > + "device": "node-D", > + "job-id": "job0" > + } > + } > + > +For `Case-2`_, merge contents of the images [B] and [C] into [D], where > +image [D] ends up referring to image [A] as its backing file:: > + > + (QEMU) block-stream device=node-D base-node=node-A job-id=job0 > + > +And for `Case-3`_, of "intermediate" streaming", merge contents of > +images [B] into [C], where [C] ends up referring to [A] as its backing > +image:: > + > + (QEMU) block-stream device=node-C base-node=node-A job-id=job0 > + > +Progress of a ``block-stream`` operation can be monitored via the QMP > +command:: > + > + (QEMU) query-block-jobs > + { > + "execute": "query-block-jobs", > + "arguments": {} > + } > + > + > +Once the ``block-stream`` operation has completed, QEMU will emit an > +event, ``BLOCK_JOB_COMPLETED``. The intermediate overlays remain valid, > +and can now be (optionally) discarded, or retained to create further > +overlays based on them. Finally, the ``block-stream`` jobs can be > +restarted at anytime. > + > + > +Live block commit --- ``block-commit`` > +-------------------------------------- > + > +The ``block-commit`` command lets you merge live data from overlay > +images into backing file(s). Since QEMU 2.0, this includes "live active > +commit" (i.e. it is possible to merge the "active layer", the right-most > +image in a disk image chain where live QEMU will be writing to, into the > +base image). This is analogous to ``block-stream``, but in opposite > +direction. "in the opposite direction" > + > +Again, starting afresh with our example disk image chain, where live > +QEMU is writing to the right-most image in the chain, [D]:: > + > + [A] <-- [B] <-- [C] <-- [D] > + > +The disk image chain can be shortened in one of the following ways: > + > +.. _`block-commit_Case-1`: > + > +(1) Commit content from only image [B] into image [A]. The resulting > + chain is the following, where image [C] is adjusted to point at [A] > + as its new backing file:: > + > + [A] <-- [C] <-- [D] > + > +(2) Commit content from images [B] and [C] into image [A]. The > + resulting chain, where image [D] is adjusted to point to image [A] > + as its new backing file:: > + > + [A] <-- [D] > + > +.. _`block-commit_Case-3`: > + > +(3) Commit content from images [B], [C], and the active layer [D] into > + image [A]. The resulting chain (in this case, a consolidated single > + image):: > + > + [A] > + > +(4) Commit content from image only image [C] into image [B]. The > + resulting chain:: > + > + [A] <-- [B] <-- [D] > + > +(5) Commit content from image [C] and the active layer [D] into image > + [B]. The resulting chain:: > + > + [A] <-- [B] > + > + > +QMP invocation for ``block-commit`` > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +For `block-commit_Case-1`_, to merge contents only from This sort of bleeds the anchor names into the rendered document, is that intentional? It looks awkward in the final product. > +image [B] into image [A], the invocation is as following:: > + > + (QEMU) block-commit device=node-D base=a.qcow2 top=b.qcow2 job-id=job0 code block highlighting in the rendered document does weird things to job-id, which may be beyond your control. > + { > + "execute": "block-commit", > + "arguments": { > + "device": "node-D", > + "job-id": "job0", > + "top": "b.qcow2", > + "base": "a.qcow2" > + } > + } > + > +Once the above ``block-commit`` operation has completed, a > +``BLOCK_JOB_COMPLETED`` event will be issued, and no further action is > +required. The end result being, the backing file of image [C] is > +adjusted to point to image [A], and the original 4-image chain will end > +up being transformed to:: > + > + [A] <-- [C] <-- [D] > + > +..note:: > + The intermediate image [B] is invalid (as in: no more further > + overlays based on it can be created) and, therefore, should be > + dropped. > + Note syntax is malformed. "And therefore should be dropped." reads just as nicely. It may be worth explaining precisely in what ways that 'stream' does not invalidate intermediate images, but 'commit' does. It's not necessarily immediately intuitive. The core concept: - An intermediate image after 'stream' still represents that old point-in-time, and may be valid in that context. - An intermediate image after 'commit' no longer represents any point in time, and is invalid in any context. > + > +However, `block-commit_Case-3`_ (also called: "active ``block-commit``") Anchor name bleeding again. Can the link be re-titled "Case 3" or something more human readable? > +is a *two-phase* operation: in the first phase, the content from the > +active overlay, along with the intermediate overlays, is copied into the > +backing file (also called, the base image); in the second phase, adjust "also called the base image" reads just as well. In general I think you've got too many commas. > +the said backing file as the current active image -- possible via > +issuing the command ``block-job-complete``. [Optionally, the > +``block-commit`` operation can be cancelled, by issuing the command > +``block-job-cancel``, but be careful when doing this.] Stick to one set of parentheses, use (). Or maybe rewrite this paragraph. It's got an em dash, three parentheticals, seven commas, a colon and a semicolon. It's a busy paragraph syntactically! > + > +Once the 'commit' operation (started by ``block-commit``) has completed, You could omit the parentheses here and it reads just as well. > +the event ``BLOCK_JOB_READY`` is emitted, signalling the synchronization > +has finished, and the job can be gracefully completed, by issuing > +``block-job-complete``. (Until such a command is issued, the 'commit' > +operation remains active.) Bit of a run-on. I guess I should stop reviewing this from a style perspective for now. I'll come back and review the rest later, sorry! > + > +So, the following is the flow for `block-commit_Case-3`_ -- to convert a > +disk image chain such as this:: > + > + [A] <-- [B] <-- [C] <-- [D] > + > +Into (where content from all the subsequent overlays, [B], and [C], > +including the active layer, [D], is committed back to [A] -- which is > +where live QEMU is performing all its current writes):: > + > + [A] > + > +Start the "active ``block-commit``" operation:: > + > + (QEMU) block-commit device=node-D base=a.qcow2 top=d.qcow2 job-id=job0 > + { > + "execute": "block-commit", > + "arguments": { > + "device": "node-D", > + "job-id": "job0", > + "top": "d.qcow2", > + "base": "a.qcow2" > + } > + } > + > + > +Once the synchronization has completed, the event ``BLOCK_JOB_READY`` will > +be emitted. > + > +Then, (optionally) query for the status of the active block operations > +(we can see the 'commit' job is now ready to be completed, as indicated > +by the line *"ready": true*):: > + > + (QEMU) query-block-jobs > + { > + "execute": "query-block-jobs", > + "arguments": {} > + } > + { > + "return": [ > + { > + "busy": false, > + "type": "commit", > + "len": 1376256, > + "paused": false, > + "ready": true, > + "io-status": "ok", > + "offset": 1376256, > + "device": "job0", > + "speed": 0 > + } > + ] > + } > + > +Gracefully complete the 'commit' block device job:: > + > + (QEMU) block-job-complete device=job0 > + { > + "execute": "block-job-complete", > + "arguments": { > + "device": "job0" > + } > + } > + { > + "return": {} > + } > + > +Finally, once the above job is completed, an event > +``BLOCK_JOB_COMPLETED`` will be emitted. > + > +.. note:: > + The invocation for rest of the cases (2, 4, and 5), discussed in the > + previous section, is omitted for brevity. > + > + > +Live disk synchronization --- ``drive-mirror`` and ``blockdev-mirror`` > +---------------------------------------------------------------------- > + > +Synchronize a running disk image chain (all or part of it) to a target > +image. > + > +Again, given our familiar disk image chain:: > + > + [A] <-- [B] <-- [C] <-- [D] > + > +The ``drive-mirror`` (and its newer equivalent ``blockdev-mirror``) allows > +you to copy data from the entire chain into a single target image (which > +can be located on a different host). > + > +Once a 'mirror' job has started, there are two possible actions when a > +``drive-mirror`` job is active: > + > +(1) Issuing the command ``block-job-cancel``, after it emits the event > + ``BLOCK_JOB_CANCELLED``: will (after completing synchronization of > + the content from the disk image chain to the target image, [E]) > + create a point-in-time (which is at the time of *triggering* the > + cancel command) copy, contained in image [E], of the the entire disk > + image chain (or only the top-most image, depending on the ``sync`` > + mode). > + > +(2) Issuing the command ``block-job-complete``, after it emits the event > + ``BLOCK_JOB_COMPLETED``: will, after completing synchronization of > + the content, adjust the guest device (i.e. live QEMU) to point to > + the target image, and, causing all the new writes from this point on > + to happen there. One use case for this is live storage migration. > + > +A note on synchronization modes -- determines *what* part of the disk > +image chain must be copied to the target. Currently, there are four > +different kinds: > + > +(1) ``full`` -- Synchronize the content of entire disk image chain to > + the target > + > +(2) ``top`` -- Synchronize only the contents of the top-most disk image > + in the chain to the target > + > +(3) ``none`` -- Synchronize only the new writes from this point on. > + > + .. note:: In the case of ``drive-backup`` (or ``blockdev-backup``), > + the behavior of ``none`` sychronization mode is different. > + Normally, a ``backup`` job consists of two parts: Anything > + that is overwritten by the guest is first copied out to > + the backup, and in the background the whole image is > + copied from start to end. With ``sync=none``, it's only > + the first part. > + > +(4) ``incremental`` -- Synchronize content that is described by the > + dirty bitmap > + > +.. note:: > + Refer to the :doc:`bitmaps` document in the QEMU source > + tree to learn about the detailed workings of the ``incremental`` > + synchronization mode. > + > + > +QMP invocation for ``drive-mirror`` > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +To copy the contents of the entire disk image chain, from [A] all the > +way to [D], to a new target (``drive-mirror`` will create the destination > +file, if it doesn't already exist), call it [E]:: > + > + (QEMU) drive-mirror device=node-D target=e.qcow2 sync=full job-id=job0 > + { > + "execute": "drive-mirror", > + "arguments": { > + "device": "node-D", > + "job-id": "job0", > + "target": "e.qcow2", > + "sync": "full" > + } > + } > + > +The ``"sync": "full"``, from the above, means: copy the *entire* chain > +to the destination. > + > +Following the above, querying for active block jobs will show that a > +'mirror' job is "ready" to be completed (and QEMU will also emit an > +event, ``BLOCK_JOB_READY``):: > + > + (QEMU) query-block-jobs > + { > + "execute": "query-block-jobs", > + "arguments": {} > + } > + { > + "return": [ > + { > + "busy": false, > + "type": "mirror", > + "len": 21757952, > + "paused": false, > + "ready": true, > + "io-status": "ok", > + "offset": 21757952, > + "device": "job0", > + "speed": 0 > + } > + ] > + } > + > +And, as noted in the previous section, there are two possible actions > +at this point: > + > +(a) Create a point-in-time snapshot by ending the synchronization. The > + point-in-time is at the time of *ending* the sync. (The result of > + the following being: the target image, [E], will be populated with > + content from the entire chain, [A] to [D]):: > + > + (QEMU) block-job-cancel device=job0 > + { > + "execute": "block-job-cancel", > + "arguments": { > + "device": "job0" > + } > + } > + > +(b) Or, complete the operation and pivot the live QEMU to the target > + copy:: > + > + (QEMU) block-job-complete device=job0 > + > +In either of the above cases, if you once again run the > +`query-block-jobs` command, there should not be any active block > +operation. > + > +Comparing 'commit' and 'mirror': In both then cases, the overlay images > +can be discarded. However, with 'commit', the *existing* base image > +will be modified (by updating it with contents from overlays); while in > +the case of 'mirror', a *new* target image is populated with the data > +from the disk image chain. > + > + > +QMP invocation for live storage migration with ``drive-mirror`` + NBD > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +Live storage migration (without shared storage setup) is one of the most > +common use-cases that takes advantage of the ``drive-mirror`` primitive > +and QEMU's built-in Network Block Device (NBD) server. Here's a quick > +walk-through of this setup. > + > +Given the disk image chain:: > + > + [A] <-- [B] <-- [C] <-- [D] > + > +Instead of copying content from the entire chain, synchronize *only* the > +contents of the *top*-most disk image (i.e. the active layer), [D], to a > +target, say, [TargetDisk]. > + > +.. important:: > + The destination host must already have the contents of the backing > + chain, involving images [A], [B], and [C], visible via other means > + -- whether by ``cp``, ``rsync``, or by some storage array-specific > + command.) > + > +Sometimes, this is also referred to as "shallow copy" -- because: only > +the "active layer", and not the rest of the image chain, is copied to > +the destination. > + > +.. note:: > + In this example, for the sake of simplicity, we'll be using the same > + ``localhost`` as both, source and destination. > + > +As noted earlier, on the destination host the contents of the backing > +chain -- from images [A] to [C] -- are already expected to exist in some > +form (e.g. in a file called, ``Contents-of-A-B-C.qcow2``). Now, on the > +destination host, let's create a target overlay image (with the image > +``Contents-of-A-B-C.qcow2`` as its backing file), to which the contents > +of image [D] (from the source QEMU) will be mirrored to:: > + > + $ qemu-img create -f qcow2 -b ./Contents-of-A-B-C.qcow2 \ > + -F qcow2 ./target-disk.qcow2 > + > +And start the destination QEMU (we already have the source QEMU running > +-- discussed in the section: `Interacting with a QEMU instance`_) > +instance, with the following invocation. (As noted earlier, for > +simplicity's sake, the destination QEMU is started on the same host, but > +it could be located elsewhere):: > + > + $ ./x86_64-softmmu/qemu-system-x86_64 -display none -nodefconfig \ > + -M q35 -nodefaults -m 512 \ > + -blockdev node-name=node-TargetDisk,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./target-disk.qcow2 \ > + -device virtio-blk,drive=node-TargetDisk,id=virtio0 \ > + -S -monitor stdio -qmp unix:./qmp-sock2,server,nowait \ > + -incoming tcp:localhost:6666 > + > +Given the disk image chain on source QEMU:: > + > + [A] <-- [B] <-- [C] <-- [D] > + > +On the destination host, it is expected that the contents of the chain > +``[A] <-- [B] <-- [C]`` are *already* present, and therefore copy *only* > +the content of image [D]. > + > +(1) [On *destination* QEMU] As part of the first step, start the > + built-in NBD server on a given host (local host, represented by > + ``::``)and port:: > + > + (QEMU) nbd-server-start addr={"type":"inet","data":{"host":"::","port":"49153"}} > + { > + "execute": "nbd-server-start", > + "arguments": { > + "addr": { > + "data": { > + "host": "::", > + "port": "49153" > + }, > + "type": "inet" > + } > + } > + } > + > +(2) [On *destination* QEMU] And export the destination disk image using > + QEMU's built-in NBD server:: > + > + (QEMU) nbd-server-add device=node-TargetDisk writable=true > + { > + "execute": "nbd-server-add", > + "arguments": { > + "device": "node-TargetDisk" > + } > + } > + > +(3) [On *source* QEMU] Then, invoke ``drive-mirror`` (NB: since we're > + running ``drive-mirror`` with ``mode=existing`` (meaning: > + synchronize to a pre-created file, therefore 'existing', file on the > + target host), with the synchronization mode as 'top' (``"sync: > + "top"``):: > + > + (QEMU) drive-mirror device=node-D target=nbd:localhost:49153:exportname=node-TargetDisk sync=top mode=existing job-id=job0 > + { > + "execute": "drive-mirror", > + "arguments": { > + "device": "node-D", > + "mode": "existing", > + "job-id": "job0", > + "target": "nbd:localhost:49153:exportname=node-TargetDisk", > + "sync": "top" > + } > + } > + > +(4) [On *source* QEMU] Once ``drive-mirror`` copies the entire data, and the > + event ``BLOCK_JOB_READY`` is emitted, issue ``block-job-cancel`` to > + gracefully end the synchronization, from source QEMU:: > + > + (QEMU) block-job-cancel device=job0 > + { > + "execute": "block-job-cancel", > + "arguments": { > + "device": "job0" > + } > + } > + > +(5) [On *destination* QEMU] Then, stop the NBD server:: > + > + (QEMU) nbd-server-stop > + { > + "execute": "nbd-server-stop", > + "arguments": {} > + } > + > +(6) [On *destination* QEMU] Finally, resume the guest vCPUs by issuing the > + QMP command `cont`:: > + > + (QEMU) cont > + { > + "execute": "cont", > + "arguments": {} > + } > + > + > +.. note:: > + Higher-level libraries (e.g. libvirt) automate the entire above > + process. > + > + > +Notes on ``blockdev-mirror`` > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +The ``blockdev-mirror`` command is equivalent in core functionality to > +``drive-mirror``, except that it operates at node-level in a BDS graph. > + > +Also: for ``blockdev-mirror``, the 'target' image needs to be explicitly > +created (using ``qemu-img``) and attach it to live QEMU via > +``blockdev-add``, which assigns a name to the to-be created target node. > + > +E.g. the sequence of actions to create a point-in-time backup of an > +entire disk image chain, to a target, using ``blockdev-mirror`` would be: > + > +(0) Create the QCOW2 overlays, to arrive at a backing chain of desired > + depth > + > +(1) Create the target image (using ``qemu-img``), say, ``e.qcow2`` > + > +(2) Attach the above created file (``e.qcow2``), run-time, using > + ``blockdev-add`` to QEMU > + > +(3) Perform ``blockdev-mirror`` (use ``"sync": "full"`` to copy the > + entire chain to the target). And observe for the event > + ``BLOCK_JOB_READY`` > + > +(4) Optionally, query for active block jobs, there should be a 'mirror' > + job ready to be completed > + > +(5) Gracefully complete the 'mirror' block device job, and observe for > + the event ``BLOCK_JOB_COMPLETED`` > + > +(6) Shutdown the guest, by issuing the QMP ``quit`` command, so that > + caches are flushed > + > +(7) Then, finally, compare the contents of the disk image chain, and > + the target copy with ``qemu-img compare``. You should notice: > + "Images are identical" > + > + > +QMP invocation for ``blockdev-mirror`` > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +Given the disk image chain:: > + > + [A] <-- [B] <-- [C] <-- [D] > + > +To copy the contents of the entire disk image chain, from [A] all the > +way to [D], to a new target, call it [E]. The following is the flow. > + > +Create the overlay images, [B], [C], and [D]:: > + > + (QEMU) blockdev-snapshot-sync node-name=node-A snapshot-file=b.qcow2 snapshot-node-name=node-B format=qcow2 > + (QEMU) blockdev-snapshot-sync node-name=node-B snapshot-file=c.qcow2 snapshot-node-name=node-C format=qcow2 > + (QEMU) blockdev-snapshot-sync node-name=node-C snapshot-file=d.qcow2 snapshot-node-name=node-D format=qcow2 > + > +Create the target image, [E]:: > + > + $ qemu-img create -f qcow2 e.qcow2 39M > + > +Add the above created target image to QEMU, via ``blockdev-add``:: > + > + (QEMU) blockdev-add driver=qcow2 node-name=node-E file={"driver":"file","filename":"e.qcow2"} > + { > + "execute": "blockdev-add", > + "arguments": { > + "node-name": "node-E", > + "driver": "qcow2", > + "file": { > + "driver": "file", > + "filename": "e.qcow2" > + } > + } > + } > + > +Perform ``blockdev-mirror``, and observe for the event > +``BLOCK_JOB_READY``:: > + > + (QEMU) blockdev-mirror device=node-B target=node-E sync=full job-id=job0 > + { > + "execute": "blockdev-mirror", > + "arguments": { > + "device": "node-D", > + "job-id": "job0", > + "target": "node-E", > + "sync": "full" > + } > + } > + > +Query for active block jobs, there should be a 'mirror' job ready:: > + > + (QEMU) query-block-jobs > + { > + "execute": "query-block-jobs", > + "arguments": {} > + } > + { > + "return": [ > + { > + "busy": false, > + "type": "mirror", > + "len": 21561344, > + "paused": false, > + "ready": true, > + "io-status": "ok", > + "offset": 21561344, > + "device": "job0", > + "speed": 0 > + } > + ] > + } > + > +Gracefully complete the block device job operation, and observe for the > +event ``BLOCK_JOB_COMPLETED``:: > + > + (QEMU) block-job-complete device=job0 > + { > + "execute": "block-job-complete", > + "arguments": { > + "device": "job0" > + } > + } > + { > + "return": {} > + } > + > +Shutdown the guest, by issuing the ``quit`` QMP command:: > + > + (QEMU) quit > + { > + "execute": "quit", > + "arguments": {} > + } > + > + > +Live disk backup --- ``drive-backup`` and ``blockdev-backup`` > +------------------------------------------------------------- > + > +The ``drive-backup`` (and its newer equivalent ``blockdev-backup``) allows > +you to create a point-in-time snapshot. > + > +In this case, the point-in-time is when you *start* the ``drive-backup`` > +(or its newer equivalent ``blockdev-backup``) command. > + > + > +QMP invocation for ``drive-backup`` > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +Yet again, starting afresh with our example disk image chain:: > + > + [A] <-- [B] <-- [C] <-- [D] > + > +To create a target image [E], with content populated from image [A] to > +[D], from the above chain, the following is the syntax. (If the target > +image does not exist, ``drive-backup`` will create it):: > + > + (QEMU) drive-backup device=node-D sync=full target=e.qcow2 job-id=job0 > + { > + "execute": "drive-backup", > + "arguments": { > + "device": "node-D", > + "job-id": "job0", > + "sync": "full", > + "target": "e.qcow2" > + } > + } > + > +Once the above ``drive-backup`` has completed, a ``BLOCK_JOB_COMPLETED`` event > +will be issued, indicating the live block device job operation has > +completed, and no further action is required. > + > + > +Notes on ``blockdev-backup`` > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +The ``blockdev-backup`` command is equivalent in functionality to > +``drive-backup``, except that it operates at node-level in a Block Driver > +State (BDS) graph. > + > +E.g. the sequence of actions to create a point-in-time backup > +of an entire disk image chain, to a target, using ``blockdev-backup`` > +would be: > + > +(0) Create the QCOW2 overlays, to arrive at a backing chain of desired > + depth > + > +(1) Create the target image (using ``qemu-img``), say, ``e.qcow2`` > + > +(2) Attach the above created file (``e.qcow2``), run-time, using > + ``blockdev-add`` to QEMU > + > +(3) Perform ``blockdev-backup`` (use ``"sync": "full"`` to copy the > + entire chain to the target). And observe for the event > + ``BLOCK_JOB_COMPLETED`` > + > +(4) Shutdown the guest, by issuing the QMP ``quit`` command, so that > + caches are flushed > + > +(5) Then, finally, compare the contents of the disk image chain, and > + the target copy with ``qemu-img compare``. You should notice: > + "Images are identical" > + > +The following section shows an example QMP invocation for > +``blockdev-backup``. > + > +QMP invocation for ``blockdev-backup`` > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +Given, a disk image chain of depth 1, where image [B] is the active > +overlay (live QEMU is writing to it):: > + > + [A] <-- [B] > + > +The following is the procedure to copy the content from the entire chain > +to a target image (say, [E]), which has the full content from [A] and > +[B]. > + > +Create the overlay, [B]:: > + > + (QEMU) blockdev-snapshot-sync node-name=node-A snapshot-file=b.qcow2 snapshot-node-name=node-B format=qcow2 > + { > + "execute": "blockdev-snapshot-sync", > + "arguments": { > + "node-name": "node-A", > + "snapshot-file": "b.qcow2", > + "format": "qcow2", > + "snapshot-node-name": "node-B" > + } > + } > + > + > +Create a target image, that will contain the copy:: > + > + $ qemu-img create -f qcow2 e.qcow2 39M > + > +Then, add it to QEMU via ``blockdev-add``:: > + > + (QEMU) blockdev-add driver=qcow2 node-name=node-E file={"driver":"file","filename":"e.qcow2"} > + { > + "execute": "blockdev-add", > + "arguments": { > + "node-name": "node-E", > + "driver": "qcow2", > + "file": { > + "driver": "file", > + "filename": "e.qcow2" > + } > + } > + } > + > +Then, invoke ``blockdev-backup``, to copy the contents from the entire > +image chain, consisting of images [A], and [B], to the target image > +'e.qcow2':: > + > + (QEMU) blockdev-backup device=node-B target=node-E sync=full job-id=job0 > + { > + "execute": "blockdev-backup", > + "arguments": { > + "device": "node-B", > + "job-id": "job0", > + "target": "node-E", > + "sync": "full" > + } > + } > + > +Once the above 'backup' operation has completed, an event, > +``BLOCK_JOB_COMPLETED``, will be emitted, signalling successful > +completion. > + > +Next, query for any active block device jobs (there should be none):: > + > + (QEMU) query-block-jobs > + { > + "execute": "query-block-jobs", > + "arguments": {} > + } > + > +Shutdown the guest:: > + > + (QEMU) quit > + { > + "execute": "quit", > + "arguments": {} > + } > + "return": {} > + } > + > +.. note:: > + The above step is really important; if forgotten, an error, "Failed > + to get shared "write" lock on e.qcow2", will be thrown when you do > + ``qemu-img compare`` to verify the integrity of the disk image > + with the backup content. > + > + > +The end result will be, the image 'e.qcow2' containing a > +point-in-time backup of the disk image chain -- i.e. contents from > +images [A], and [B] at the time the ``blockdev-backup`` command was > +initiated. > + > +One way to confirm the backup disk image contains the identical content > +with the disk image chain is to compare the backup, and the contents of > +the chain, you should see "Images are identical". (NB: this is assuming > +QEMU was launched with `-S` option, which will not start the CPUs at > +guest boot up):: > + > + $ qemu-img compare b.qcow2 e.qcow2 > + Warning: Image size mismatch! > + Images are identical. > + > +NOTE: The "Warning: Image size mismatch!" is expected, as we created the > +target image (e.qcow2) with 39M size. > diff --git a/docs/live-block-ops.txt b/docs/live-block-ops.txt > deleted file mode 100644 > index 2211d14..0000000 > --- a/docs/live-block-ops.txt > +++ /dev/null > @@ -1,72 +0,0 @@ > -LIVE BLOCK OPERATIONS > -===================== > - > -High level description of live block operations. Note these are not > -supported for use with the raw format at the moment. > - > -Note also that this document is incomplete and it currently only > -covers the 'stream' operation. Other operations supported by QEMU such > -as 'commit', 'mirror' and 'backup' are not described here yet. Please > -refer to the qapi/block-core.json file for an overview of those. > - > -Snapshot live merge > -=================== > - > -Given a snapshot chain, described in this document in the following > -format: > - > -[A] <- [B] <- [C] <- [D] <- [E] > - > -Where the rightmost object ([E] in the example) described is the current > -image which the guest OS has write access to. To the left of it is its base > -image, and so on accordingly until the leftmost image, which has no > -base. > - > -The snapshot live merge operation transforms such a chain into a > -smaller one with fewer elements, such as this transformation relative > -to the first example: > - > -[A] <- [E] > - > -Data is copied in the right direction with destination being the > -rightmost image, but any other intermediate image can be specified > -instead. In this example data is copied from [C] into [D], so [D] can > -be backed by [B]: > - > -[A] <- [B] <- [D] <- [E] > - > -The operation is implemented in QEMU through image streaming facilities. > - > -The basic idea is to execute 'block_stream virtio0' while the guest is > -running. Progress can be monitored using 'info block-jobs'. When the > -streaming operation completes it raises a QMP event. 'block_stream' > -copies data from the backing file(s) into the active image. When finished, > -it adjusts the backing file pointer. > - > -The 'base' parameter specifies an image which data need not be > -streamed from. This image will be used as the backing file for the > -destination image when the operation is finished. > - > -In the first example above, the command would be: > - > -(qemu) block_stream virtio0 file-A.img > - > -In order to specify a destination image different from the active > -(rightmost) one we can use its node name instead. > - > -In the second example above, the command would be: > - > -(qemu) block_stream node-D file-B.img > - > -Live block copy > -=============== > - > -To copy an in use image to another destination in the filesystem, one > -should create a live snapshot in the desired destination, then stream > -into that image. Example: > - > -(qemu) snapshot_blkdev ide0-hd0 /new-path/disk.img qcow2 > - > -(qemu) block_stream ide0-hd0 > - > - > ^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [Qemu-devel] [PATCH v5 2/2] live-block-ops.txt: Rename, rewrite, and improve it 2017-07-06 22:41 ` John Snow @ 2017-07-07 10:46 ` Kashyap Chamarthy 2017-07-07 21:08 ` John Snow 0 siblings, 1 reply; 10+ messages in thread From: Kashyap Chamarthy @ 2017-07-07 10:46 UTC (permalink / raw) To: John Snow; +Cc: qemu-devel, kwolf, berto, qemu-block, mreitz, stephen On Thu, Jul 06, 2017 at 06:41:07PM -0400, John Snow wrote: > On 07/06/2017 10:36 AM, Kashyap Chamarthy wrote: [...] > > +disk image chains by merging data from overlays into backing files; live > > +synchronize data from a disk image chain (including current active disk) > > +to another target image; point-in-time (and incremental) backups of a > > "and point-in-time" to indicate the conclusion of a list? 1, 2, 3, and 4. Yes. Fixed in local v6. (Must not forget Oxford comma!) [...] > > +Disk image backing chain notation > > +--------------------------------- > > + > > +A simple disk image chain. (This can be created live, using QMP > > +``blockdev-snapshot-sync``, or offline, via ``qemu-img``):: > > "can be created live using QMP ``foo``, or offline via ``bar``" > > I, tend, to, use, too, many, commas, in, my, own, writing, to, mimic, a, > conversational, tone, but, admittedly, I, go, overboard. Hehe, I'm painfully aware of my "comma problem". Sometimes I catch myself and fix it, other times I miss. But I promise, I'm working on it. :-) > Take with a grain of salt. Eric is our ultimate copy-editor... Noted. About Eric, noted (in part jest): "Strunk, White, and Blake" [...] > > + .. note:: Once the ``stream`` operation has finished, three things > > + to note: (a) QEMU rewrites the backing chain to remove > > + reference to the now-streamed and redundant backing file; > > + (b) the streamed file *itself* won't be removed by QEMU, > > + and must be explicitly discarded by the user; (c) the > > + streamed file remains valid -- i.e. further overlays can > > + be created based on it. Refer the ``block-stream`` > > + section further below for more details. > > Is it possible to listify these three points? Yes, it's possible (I just tested) -- Sphinx renders the content as rST itself. (I briefly considered listifying it while writing, but didn't bother.) Listfied in local v6. > > + > > +(2) ``block-commit``: Live merge of data from overlay files into backing > > + files (with the optional goal of removing the overlay file from the > > + chain). Since QEMU 2.0, this includes "active ``block-commit``" > > + (i.e. merge the current active layer into the base image). > > + > > + .. note:: Once the ``commit`` operation has finished, here, too, > > You've gone comma-wild! Indeed, only now I notice the absurdity. But I'm trying to be conscious of this comma-madness. > "Once the ``commit`` operation has finished, there are three things to > note here <too/as well>:" Fixed in local v6. > > + three things to note: (a) QEMU rewrites the backing chain > > + to remove reference to now-redundant overlay images that > > + have been commited into a backing file; (b) the commited > > + file *itself* won't be removed by QEMU -- it ought to be > > + manually removed; (c) however, unlike in the case of > > + ``block-stream``, the intermediate images will be rendered > > + invalid -- i.e. no more further overlays can be created > > + based on them. Refer the ``block-commit`` section further > > + below for more details. Also listified the above. Fixed in local v6. [...] > > +.. note:: > > + In the event we have to repeat a certain QMP command, we will: for > > + the first occurrence of it, show the the ``qmp-shell`` invocation, > > + *and* the corresponding raw JSON QMP syntax; but for subsequent > > + invocations, present just the ``qmp-shell`` syntax, and omit the > > + equivalent JSON output. > > + > > It's like I'm reading a university textbook. Very nice! :-) Also fixed locally the double "the" in the above note. [...] > > +Where [A] is the original base image; [B] and [C] are intermediate > > +overlay images; image [D] is the active layer -- i.e. live QEMU is > > +writing to it. (The rule of thumb is: live QEMU will always be pointing > > +to the rightmost image in a disk image chain.) > > + > > +The above image chain can be created by invoking > > +``blockdev-snapshot-sync`` command as following (which shows the > > command_s_ as following? We're starting from a single image and we need > to create several snapshots. I see you're suggesting plural to indicate the multiple invocations. I thought using singular for "command" is correct because it's multiple invocations of a _single_ command. And using plural can give the impression that there are different kinds of commands, but probably it's just my brain. But I'll take your word as a native speaker and a fellow relisher (?) of language. Fixed it in local v6. > > +Here, "node-A" is the name QEMU internally uses to refer to the base > > +image [A] -- it is the backing file, based on which the overlay image, > > +[B], is created. > > + > > +To create the rest of the two overlay images, [C], and [D] (omitted the > > "rest of the two" strikes me as peculiar. Either "rest of the images" or > "The last/final/other two" Fixed in local v6 as "rest of the overlay images". (Didn't notice the peculiarity. A quick look up of the phrase "rest of the two" got barely 6 results.) > > +(1) Merge everything into the active layer: I.e. copy all contents from > > + the base image, [A], and overlay images, [B] and [C], into [D], > > + _while_ the guest is running. The resulting chain will be a > > is _while_ meant to apply formatting in this case? Oops, good catch. It's what I use on e-mail / plain text, so it just bled into rST. To get italics in rST, it is either: *foo*, or `foo`. Fixed in local v6. [...] > > +commit" (i.e. it is possible to merge the "active layer", the right-most > > +image in a disk image chain where live QEMU will be writing to, into the > > +base image). This is analogous to ``block-stream``, but in opposite > > +direction. > > "in the opposite direction" Fixed in local v6. [...] > > +QMP invocation for ``block-commit`` > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > + > > +For `block-commit_Case-1`_, to merge contents only from > > This sort of bleeds the anchor names into the rendered document, is that > intentional? It looks awkward in the final product. I see what you mean, I find it ugly, too. I reluctantly created these anchors, as I didn't think of a cleaner solution. But I've got good news (thanks: Stephen Finucane on IRC for reminding me of the use of ":ref:")! So the `diff` will be: - For `block-commit_Case-1`_ [...] + For :ref:`Case-1 <block-commit-case-1>` [...] And the rendered document will just see the following (where "Case-1" will hyperlink to appropriate location): "For Case-1, to merge contents only from image [...]" Fixed in local v6. > > +image [B] into image [A], the invocation is as following:: > > + > > + (QEMU) block-commit device=node-D base=a.qcow2 top=b.qcow2 job-id=job0 > > code block highlighting in the rendered document does weird things to > job-id, which may be beyond your control. You're right, I noticed it. But I can't see a way to fix it. [...] > > +..note:: > > + The intermediate image [B] is invalid (as in: no more further > > + overlays based on it can be created) and, therefore, should be > > + dropped. > > + > > Note syntax is malformed. Eagle eyes. Fixed in local v6. > "And therefore should be dropped." reads just as nicely. Yes, dropped it. > It may be worth explaining precisely in what ways that 'stream' does not > invalidate intermediate images, but 'commit' does. It's not necessarily > immediately intuitive. Agreed. Fixed in local v6. > The core concept: > > - An intermediate image after 'stream' still represents that old > point-in-time, and may be valid in that context. > - An intermediate image after 'commit' no longer represents any point in > time, and is invalid in any context. Yes, I incorported the above text as-is, and the ".. note::" admonition is now: .. note:: The intermediate image [B] is invalid (as in: no more further overlays based on it can be created). Reasoning: An intermediate image after a ``stream`` operation still represents that old point-in-time, and may be valid in that context. However, an intermediate image after a ``commit`` operation no longer represents any point-in-time, and is invalid in any context. Fixed in local v6. > > +However, `block-commit_Case-3`_ (also called: "active ``block-commit``") > > Anchor name bleeding again. Fixed in local v6 by using the ":ref:", the end result will just be "Case-3" in the rendered document. > Can the link be re-titled "Case 3" or something more human readable? Probably you missed to notice that "Case-{1,2,3}" anchors were already taken for the `block-stream` section. But now, thanks to the ":ref:" usage, we can use the more human-readable "Case-3", and link to the _correct_ section. > > +is a *two-phase* operation: in the first phase, the content from the > > +active overlay, along with the intermediate overlays, is copied into the > > +backing file (also called, the base image); in the second phase, adjust > > "also called the base image" reads just as well. In general I think > you've got too many commas. Fixed in local v6. Thanks for patiently pointing them out. > > +the said backing file as the current active image -- possible via > > +issuing the command ``block-job-complete``. [Optionally, the > > +``block-commit`` operation can be cancelled, by issuing the command > > +``block-job-cancel``, but be careful when doing this.] > > Stick to one set of parentheses, use (). Done. > Or maybe rewrite this paragraph. It's got an em dash, three > parentheticals, seven commas, a colon and a semicolon. It's a busy > paragraph syntactically! Noted. Slightly better? It's now reduced to one em dash, two parentheticals, five commas, and a colon: However, :ref:`Case-3 <block-commit_Case-3>` (also called: "active ``block-commit``") is a *two-phase* operation: In the first phase, the content from the active overlay, along with the intermediate overlays, is copied into the backing file (also called the base image). In the second phase, adjust the said backing file as the current active image -- possible via issuing the command ``block-job-complete``. Optionally, the ``block-commit`` operation can be cancelled by issuing the command ``block-job-cancel``, but be careful when doing this. Rephrasings welcome. > > +Once the 'commit' operation (started by ``block-commit``) has completed, > > You could omit the parentheses here and it reads just as well. Fixed in local v6. > > +the event ``BLOCK_JOB_READY`` is emitted, signalling the synchronization > > +has finished, and the job can be gracefully completed, by issuing > > +``block-job-complete``. (Until such a command is issued, the 'commit' > > +operation remains active.) > > Bit of a run-on. Oops, indeed. Rewrote it as following: Once the ``block-commit`` operation has completed, the event ``BLOCK_JOB_READY`` will be emitted, signalling that the synchronization has finished. Now the job can be gracefully completed by issuing the command ``block-job-complete`` -- until such a command is issued, the 'commit' operation remains active. > I guess I should stop reviewing this from a style perspective for now. > I'll come back and review the rest later, sorry! Thanks for your patience in review. Much appreciated (including the grammar corrections)! I'll wait for the rest of your comments before I spin a v6. [...] -- /kashyap ^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [Qemu-devel] [PATCH v5 2/2] live-block-ops.txt: Rename, rewrite, and improve it 2017-07-07 10:46 ` Kashyap Chamarthy @ 2017-07-07 21:08 ` John Snow 0 siblings, 0 replies; 10+ messages in thread From: John Snow @ 2017-07-07 21:08 UTC (permalink / raw) To: Kashyap Chamarthy; +Cc: qemu-devel, kwolf, berto, qemu-block, mreitz, stephen On 07/07/2017 06:46 AM, Kashyap Chamarthy wrote: > Thanks for your patience in review. Much appreciated (including the > grammar corrections)! > > I'll wait for the rest of your comments before I spin a v6. > > [...] Thank *you* for exercising the necessary patience to make incredibly nice documents on the block layer internals. ^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [Qemu-devel] [PATCH v5 2/2] live-block-ops.txt: Rename, rewrite, and improve it 2017-07-06 14:36 ` [Qemu-devel] [PATCH v5 2/2] live-block-ops.txt: Rename, rewrite, and improve it Kashyap Chamarthy 2017-07-06 18:11 ` Kashyap Chamarthy 2017-07-06 22:41 ` John Snow @ 2017-07-07 22:25 ` John Snow 2017-07-10 7:53 ` Kashyap Chamarthy 2 siblings, 1 reply; 10+ messages in thread From: John Snow @ 2017-07-07 22:25 UTC (permalink / raw) To: Kashyap Chamarthy, qemu-devel; +Cc: kwolf, berto, qemu-block, mreitz Last time, on DRAGON BALL Q: On 07/06/2017 10:36 AM, Kashyap Chamarthy wrote: [A lot of good documentation] > + > +So, the following is the flow for `block-commit_Case-3`_ -- to convert a > +disk image chain such as this:: > + Maybe drop the "So, " "The following is the flow for 'Block Commit Case 3' to convert a disk image chain such as this:" > + [A] <-- [B] <-- [C] <-- [D] > + > +Into (where content from all the subsequent overlays, [B], and [C], > +including the active layer, [D], is committed back to [A] -- which is > +where live QEMU is performing all its current writes):: > + "Into: " > + [A] > + "Where content from all [...] its current writes." > +Start the "active ``block-commit``" operation:: > + > + (QEMU) block-commit device=node-D base=a.qcow2 top=d.qcow2 job-id=job0 > + { > + "execute": "block-commit", > + "arguments": { > + "device": "node-D", > + "job-id": "job0", > + "top": "d.qcow2", > + "base": "a.qcow2" > + } > + } > + > + > +Once the synchronization has completed, the event ``BLOCK_JOB_READY`` will > +be emitted. > + > +Then, (optionally) query for the status of the active block operations > +(we can see the 'commit' job is now ready to be completed, as indicated > +by the line *"ready": true*):: > + Then, optionally query for the status of the active block operations. We can see the 'commit' job is now ready to be completed, as indicated by the line *"ready": true*. > + (QEMU) query-block-jobs > + { > + "execute": "query-block-jobs", > + "arguments": {} > + } > + { > + "return": [ > + { > + "busy": false, > + "type": "commit", > + "len": 1376256, > + "paused": false, > + "ready": true, > + "io-status": "ok", > + "offset": 1376256, > + "device": "job0", > + "speed": 0 > + } > + ] > + } > + > +Gracefully complete the 'commit' block device job:: > + > + (QEMU) block-job-complete device=job0 > + { > + "execute": "block-job-complete", > + "arguments": { > + "device": "job0" > + } > + } > + { > + "return": {} > + } > + > +Finally, once the above job is completed, an event > +``BLOCK_JOB_COMPLETED`` will be emitted. > + > +.. note:: > + The invocation for rest of the cases (2, 4, and 5), discussed in the > + previous section, is omitted for brevity. > + As a joke I almost want to write "The invocation for cases 2, 4, and 5 are left as an exercise to the reader." > + > +Live disk synchronization --- ``drive-mirror`` and ``blockdev-mirror`` > +---------------------------------------------------------------------- > + > +Synchronize a running disk image chain (all or part of it) to a target > +image. > + > +Again, given our familiar disk image chain:: > + > + [A] <-- [B] <-- [C] <-- [D] > + > +The ``drive-mirror`` (and its newer equivalent ``blockdev-mirror``) allows > +you to copy data from the entire chain into a single target image (which > +can be located on a different host). > + > +Once a 'mirror' job has started, there are two possible actions when a > +``drive-mirror`` job is active: > + > +(1) Issuing the command ``block-job-cancel``, after it emits the event nix the comma > + ``BLOCK_JOB_CANCELLED``: will (after completing synchronization of > + the content from the disk image chain to the target image, [E]) > + create a point-in-time (which is at the time of *triggering* the > + cancel command) copy, contained in image [E], of the the entire disk > + image chain (or only the top-most image, depending on the ``sync`` > + mode). > + > +(2) Issuing the command ``block-job-complete``, after it emits the event nix the comma > + ``BLOCK_JOB_COMPLETED``: will, after completing synchronization of > + the content, adjust the guest device (i.e. live QEMU) to point to > + the target image, and, causing all the new writes from this point on > + to happen there. One use case for this is live storage migration. > + > +A note on synchronization modes -- determines *what* part of the disk > +image chain must be copied to the target. Currently, there are four > +different kinds: > + You've got a note here, followed by a note box. Maybe just: "About synchronization modes: The synchronization mode determines *which* part of the disk image chain will be copied to the target. Currently, there are four different kinds:" > +(1) ``full`` -- Synchronize the content of entire disk image chain to > + the target > + > +(2) ``top`` -- Synchronize only the contents of the top-most disk image > + in the chain to the target > + > +(3) ``none`` -- Synchronize only the new writes from this point on. > + > + .. note:: In the case of ``drive-backup`` (or ``blockdev-backup``), > + the behavior of ``none`` sychronization mode is different. > + Normally, a ``backup`` job consists of two parts: Anything > + that is overwritten by the guest is first copied out to > + the backup, and in the background the whole image is > + copied from start to end. With ``sync=none``, it's only > + the first part. > + > +(4) ``incremental`` -- Synchronize content that is described by the > + dirty bitmap > + > +.. note:: > + Refer to the :doc:`bitmaps` document in the QEMU source > + tree to learn about the detailed workings of the ``incremental`` > + synchronization mode. > + > + > +QMP invocation for ``drive-mirror`` > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +To copy the contents of the entire disk image chain, from [A] all the > +way to [D], to a new target (``drive-mirror`` will create the destination > +file, if it doesn't already exist), call it [E]:: > + > + (QEMU) drive-mirror device=node-D target=e.qcow2 sync=full job-id=job0 > + { > + "execute": "drive-mirror", > + "arguments": { > + "device": "node-D", > + "job-id": "job0", > + "target": "e.qcow2", > + "sync": "full" > + } > + } > + > +The ``"sync": "full"``, from the above, means: copy the *entire* chain > +to the destination. > + > +Following the above, querying for active block jobs will show that a > +'mirror' job is "ready" to be completed (and QEMU will also emit an > +event, ``BLOCK_JOB_READY``):: > + > + (QEMU) query-block-jobs > + { > + "execute": "query-block-jobs", > + "arguments": {} > + } > + { > + "return": [ > + { > + "busy": false, > + "type": "mirror", > + "len": 21757952, > + "paused": false, > + "ready": true, > + "io-status": "ok", > + "offset": 21757952, > + "device": "job0", > + "speed": 0 > + } > + ] > + } > + > +And, as noted in the previous section, there are two possible actions > +at this point: > + > +(a) Create a point-in-time snapshot by ending the synchronization. The > + point-in-time is at the time of *ending* the sync. (The result of > + the following being: the target image, [E], will be populated with > + content from the entire chain, [A] to [D]):: > + > + (QEMU) block-job-cancel device=job0 > + { > + "execute": "block-job-cancel", > + "arguments": { > + "device": "job0" > + } > + } > + > +(b) Or, complete the operation and pivot the live QEMU to the target > + copy:: > + > + (QEMU) block-job-complete device=job0 > + > +In either of the above cases, if you once again run the > +`query-block-jobs` command, there should not be any active block > +operation. > + > +Comparing 'commit' and 'mirror': In both then cases, the overlay images > +can be discarded. However, with 'commit', the *existing* base image > +will be modified (by updating it with contents from overlays); while in > +the case of 'mirror', a *new* target image is populated with the data > +from the disk image chain. > + > + > +QMP invocation for live storage migration with ``drive-mirror`` + NBD > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +Live storage migration (without shared storage setup) is one of the most > +common use-cases that takes advantage of the ``drive-mirror`` primitive > +and QEMU's built-in Network Block Device (NBD) server. Here's a quick > +walk-through of this setup. > + > +Given the disk image chain:: > + > + [A] <-- [B] <-- [C] <-- [D] > + > +Instead of copying content from the entire chain, synchronize *only* the > +contents of the *top*-most disk image (i.e. the active layer), [D], to a > +target, say, [TargetDisk]. > + > +.. important:: > + The destination host must already have the contents of the backing > + chain, involving images [A], [B], and [C], visible via other means > + -- whether by ``cp``, ``rsync``, or by some storage array-specific > + command.) > + > +Sometimes, this is also referred to as "shallow copy" -- because: only > +the "active layer", and not the rest of the image chain, is copied to > +the destination. > + > +.. note:: > + In this example, for the sake of simplicity, we'll be using the same > + ``localhost`` as both, source and destination. > + > +As noted earlier, on the destination host the contents of the backing > +chain -- from images [A] to [C] -- are already expected to exist in some > +form (e.g. in a file called, ``Contents-of-A-B-C.qcow2``). Now, on the > +destination host, let's create a target overlay image (with the image > +``Contents-of-A-B-C.qcow2`` as its backing file), to which the contents > +of image [D] (from the source QEMU) will be mirrored to:: > + > + $ qemu-img create -f qcow2 -b ./Contents-of-A-B-C.qcow2 \ > + -F qcow2 ./target-disk.qcow2 > + > +And start the destination QEMU (we already have the source QEMU running > +-- discussed in the section: `Interacting with a QEMU instance`_) > +instance, with the following invocation. (As noted earlier, for > +simplicity's sake, the destination QEMU is started on the same host, but > +it could be located elsewhere):: > + > + $ ./x86_64-softmmu/qemu-system-x86_64 -display none -nodefconfig \ > + -M q35 -nodefaults -m 512 \ > + -blockdev node-name=node-TargetDisk,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./target-disk.qcow2 \ > + -device virtio-blk,drive=node-TargetDisk,id=virtio0 \ > + -S -monitor stdio -qmp unix:./qmp-sock2,server,nowait \ > + -incoming tcp:localhost:6666 > + > +Given the disk image chain on source QEMU:: > + > + [A] <-- [B] <-- [C] <-- [D] > + > +On the destination host, it is expected that the contents of the chain > +``[A] <-- [B] <-- [C]`` are *already* present, and therefore copy *only* > +the content of image [D]. > + > +(1) [On *destination* QEMU] As part of the first step, start the > + built-in NBD server on a given host (local host, represented by > + ``::``)and port:: > + > + (QEMU) nbd-server-start addr={"type":"inet","data":{"host":"::","port":"49153"}} > + { > + "execute": "nbd-server-start", > + "arguments": { > + "addr": { > + "data": { > + "host": "::", > + "port": "49153" > + }, > + "type": "inet" > + } > + } > + } > + > +(2) [On *destination* QEMU] And export the destination disk image using > + QEMU's built-in NBD server:: > + > + (QEMU) nbd-server-add device=node-TargetDisk writable=true > + { > + "execute": "nbd-server-add", > + "arguments": { > + "device": "node-TargetDisk" > + } > + } > + > +(3) [On *source* QEMU] Then, invoke ``drive-mirror`` (NB: since we're > + running ``drive-mirror`` with ``mode=existing`` (meaning: > + synchronize to a pre-created file, therefore 'existing', file on the > + target host), with the synchronization mode as 'top' (``"sync: > + "top"``):: > + > + (QEMU) drive-mirror device=node-D target=nbd:localhost:49153:exportname=node-TargetDisk sync=top mode=existing job-id=job0 > + { > + "execute": "drive-mirror", > + "arguments": { > + "device": "node-D", > + "mode": "existing", > + "job-id": "job0", > + "target": "nbd:localhost:49153:exportname=node-TargetDisk", > + "sync": "top" > + } > + } > + > +(4) [On *source* QEMU] Once ``drive-mirror`` copies the entire data, and the > + event ``BLOCK_JOB_READY`` is emitted, issue ``block-job-cancel`` to > + gracefully end the synchronization, from source QEMU:: > + > + (QEMU) block-job-cancel device=job0 > + { > + "execute": "block-job-cancel", > + "arguments": { > + "device": "job0" > + } > + } > + > +(5) [On *destination* QEMU] Then, stop the NBD server:: > + > + (QEMU) nbd-server-stop > + { > + "execute": "nbd-server-stop", > + "arguments": {} > + } > + > +(6) [On *destination* QEMU] Finally, resume the guest vCPUs by issuing the > + QMP command `cont`:: > + > + (QEMU) cont > + { > + "execute": "cont", > + "arguments": {} > + } > + > + > +.. note:: > + Higher-level libraries (e.g. libvirt) automate the entire above > + process. > + > + > +Notes on ``blockdev-mirror`` > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +The ``blockdev-mirror`` command is equivalent in core functionality to > +``drive-mirror``, except that it operates at node-level in a BDS graph. > + > +Also: for ``blockdev-mirror``, the 'target' image needs to be explicitly > +created (using ``qemu-img``) and attach it to live QEMU via > +``blockdev-add``, which assigns a name to the to-be created target node. > + > +E.g. the sequence of actions to create a point-in-time backup of an > +entire disk image chain, to a target, using ``blockdev-mirror`` would be: > + > +(0) Create the QCOW2 overlays, to arrive at a backing chain of desired > + depth > + > +(1) Create the target image (using ``qemu-img``), say, ``e.qcow2`` > + > +(2) Attach the above created file (``e.qcow2``), run-time, using > + ``blockdev-add`` to QEMU > + > +(3) Perform ``blockdev-mirror`` (use ``"sync": "full"`` to copy the > + entire chain to the target). And observe for the event > + ``BLOCK_JOB_READY`` > + > +(4) Optionally, query for active block jobs, there should be a 'mirror' > + job ready to be completed > + > +(5) Gracefully complete the 'mirror' block device job, and observe for > + the event ``BLOCK_JOB_COMPLETED`` > + "observe for the event" is slightly awkward > +(6) Shutdown the guest, by issuing the QMP ``quit`` command, so that > + caches are flushed > + nix both commas > +(7) Then, finally, compare the contents of the disk image chain, and > + the target copy with ``qemu-img compare``. You should notice: > + "Images are identical" > + > + > +QMP invocation for ``blockdev-mirror`` > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +Given the disk image chain:: > + > + [A] <-- [B] <-- [C] <-- [D] > + > +To copy the contents of the entire disk image chain, from [A] all the > +way to [D], to a new target, call it [E]. The following is the flow. > + > +Create the overlay images, [B], [C], and [D]:: > + > + (QEMU) blockdev-snapshot-sync node-name=node-A snapshot-file=b.qcow2 snapshot-node-name=node-B format=qcow2 > + (QEMU) blockdev-snapshot-sync node-name=node-B snapshot-file=c.qcow2 snapshot-node-name=node-C format=qcow2 > + (QEMU) blockdev-snapshot-sync node-name=node-C snapshot-file=d.qcow2 snapshot-node-name=node-D format=qcow2 > + > +Create the target image, [E]:: > + > + $ qemu-img create -f qcow2 e.qcow2 39M > + > +Add the above created target image to QEMU, via ``blockdev-add``:: > + > + (QEMU) blockdev-add driver=qcow2 node-name=node-E file={"driver":"file","filename":"e.qcow2"} > + { > + "execute": "blockdev-add", > + "arguments": { > + "node-name": "node-E", > + "driver": "qcow2", > + "file": { > + "driver": "file", > + "filename": "e.qcow2" > + } > + } > + } > + > +Perform ``blockdev-mirror``, and observe for the event > +``BLOCK_JOB_READY``:: > + > + (QEMU) blockdev-mirror device=node-B target=node-E sync=full job-id=job0 > + { > + "execute": "blockdev-mirror", > + "arguments": { > + "device": "node-D", > + "job-id": "job0", > + "target": "node-E", > + "sync": "full" > + } > + } > + > +Query for active block jobs, there should be a 'mirror' job ready:: > + > + (QEMU) query-block-jobs > + { > + "execute": "query-block-jobs", > + "arguments": {} > + } > + { > + "return": [ > + { > + "busy": false, > + "type": "mirror", > + "len": 21561344, > + "paused": false, > + "ready": true, > + "io-status": "ok", > + "offset": 21561344, > + "device": "job0", > + "speed": 0 > + } > + ] > + } > + > +Gracefully complete the block device job operation, and observe for the > +event ``BLOCK_JOB_COMPLETED``:: > + > + (QEMU) block-job-complete device=job0 > + { > + "execute": "block-job-complete", > + "arguments": { > + "device": "job0" > + } > + } > + { > + "return": {} > + } > + > +Shutdown the guest, by issuing the ``quit`` QMP command:: > + > + (QEMU) quit > + { > + "execute": "quit", > + "arguments": {} > + } > + > + > +Live disk backup --- ``drive-backup`` and ``blockdev-backup`` > +------------------------------------------------------------- > + > +The ``drive-backup`` (and its newer equivalent ``blockdev-backup``) allows > +you to create a point-in-time snapshot. > + > +In this case, the point-in-time is when you *start* the ``drive-backup`` > +(or its newer equivalent ``blockdev-backup``) command. > + > + > +QMP invocation for ``drive-backup`` > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +Yet again, starting afresh with our example disk image chain:: > + > + [A] <-- [B] <-- [C] <-- [D] > + > +To create a target image [E], with content populated from image [A] to > +[D], from the above chain, the following is the syntax. (If the target > +image does not exist, ``drive-backup`` will create it):: > + > + (QEMU) drive-backup device=node-D sync=full target=e.qcow2 job-id=job0 > + { > + "execute": "drive-backup", > + "arguments": { > + "device": "node-D", > + "job-id": "job0", > + "sync": "full", > + "target": "e.qcow2" > + } > + } > + > +Once the above ``drive-backup`` has completed, a ``BLOCK_JOB_COMPLETED`` event > +will be issued, indicating the live block device job operation has > +completed, and no further action is required. > + > + > +Notes on ``blockdev-backup`` > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +The ``blockdev-backup`` command is equivalent in functionality to > +``drive-backup``, except that it operates at node-level in a Block Driver > +State (BDS) graph. > + > +E.g. the sequence of actions to create a point-in-time backup > +of an entire disk image chain, to a target, using ``blockdev-backup`` > +would be: > + > +(0) Create the QCOW2 overlays, to arrive at a backing chain of desired > + depth > + > +(1) Create the target image (using ``qemu-img``), say, ``e.qcow2`` > + > +(2) Attach the above created file (``e.qcow2``), run-time, using > + ``blockdev-add`` to QEMU > + > +(3) Perform ``blockdev-backup`` (use ``"sync": "full"`` to copy the > + entire chain to the target). And observe for the event > + ``BLOCK_JOB_COMPLETED`` > + > +(4) Shutdown the guest, by issuing the QMP ``quit`` command, so that > + caches are flushed > + > +(5) Then, finally, compare the contents of the disk image chain, and > + the target copy with ``qemu-img compare``. You should notice: > + "Images are identical" > + > +The following section shows an example QMP invocation for > +``blockdev-backup``. > + > +QMP invocation for ``blockdev-backup`` > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +Given, a disk image chain of depth 1, where image [B] is the active > +overlay (live QEMU is writing to it):: > + Given a disk image chain of depth 1 where image [B] is the active overlay (live QEMU is writing it) > + [A] <-- [B] > + > +The following is the procedure to copy the content from the entire chain > +to a target image (say, [E]), which has the full content from [A] and > +[B]. > + > +Create the overlay, [B]:: the overlay [B] > + > + (QEMU) blockdev-snapshot-sync node-name=node-A snapshot-file=b.qcow2 snapshot-node-name=node-B format=qcow2 > + { > + "execute": "blockdev-snapshot-sync", > + "arguments": { > + "node-name": "node-A", > + "snapshot-file": "b.qcow2", > + "format": "qcow2", > + "snapshot-node-name": "node-B" > + } > + } > + > + > +Create a target image, that will contain the copy:: sans comma > + > + $ qemu-img create -f qcow2 e.qcow2 39M > + > +Then, add it to QEMU via ``blockdev-add``:: > + and one more time > + (QEMU) blockdev-add driver=qcow2 node-name=node-E file={"driver":"file","filename":"e.qcow2"} > + { > + "execute": "blockdev-add", > + "arguments": { > + "node-name": "node-E", > + "driver": "qcow2", > + "file": { > + "driver": "file", > + "filename": "e.qcow2" > + } > + } > + } > + > +Then, invoke ``blockdev-backup``, to copy the contents from the entire > +image chain, consisting of images [A], and [B], to the target image > +'e.qcow2':: > + Then invoke bd-b to copy the contents from the entire image chain, consisting of images [A] and [B] to the target image e.qcow2: > + (QEMU) blockdev-backup device=node-B target=node-E sync=full job-id=job0 > + { > + "execute": "blockdev-backup", > + "arguments": { > + "device": "node-B", > + "job-id": "job0", > + "target": "node-E", > + "sync": "full" > + } > + } > + > +Once the above 'backup' operation has completed, an event, > +``BLOCK_JOB_COMPLETED``, will be emitted, signalling successful > +completion. > + "the event BLOCK_JOB_COMPLETED will be emitted, signalling" > +Next, query for any active block device jobs (there should be none):: > + > + (QEMU) query-block-jobs > + { > + "execute": "query-block-jobs", > + "arguments": {} > + } > + > +Shutdown the guest:: > + > + (QEMU) quit > + { > + "execute": "quit", > + "arguments": {} > + } > + "return": {} > + } > + > +.. note:: > + The above step is really important; if forgotten, an error, "Failed > + to get shared "write" lock on e.qcow2", will be thrown when you do > + ``qemu-img compare`` to verify the integrity of the disk image > + with the backup content. > + > + > +The end result will be, the image 'e.qcow2' containing a Goodbye sweet comma, savor the void > +point-in-time backup of the disk image chain -- i.e. contents from > +images [A], and [B] at the time the ``blockdev-backup`` command was > +initiated. > + images [A] and [B] > +One way to confirm the backup disk image contains the identical content > +with the disk image chain is to compare the backup, and the contents of Could kill this comma too. > +the chain, you should see "Images are identical". (NB: this is assuming > +QEMU was launched with `-S` option, which will not start the CPUs at > +guest boot up):: > + > + $ qemu-img compare b.qcow2 e.qcow2 > + Warning: Image size mismatch! > + Images are identical. > + > +NOTE: The "Warning: Image size mismatch!" is expected, as we created the > +target image (e.qcow2) with 39M size. That's all I got for now. It's looking very good, thank you for your time. Please consider all of my suggestions are just that. There is a reason I am not an English professor :) --js ^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [Qemu-devel] [PATCH v5 2/2] live-block-ops.txt: Rename, rewrite, and improve it 2017-07-07 22:25 ` John Snow @ 2017-07-10 7:53 ` Kashyap Chamarthy 2017-07-10 16:23 ` John Snow 0 siblings, 1 reply; 10+ messages in thread From: Kashyap Chamarthy @ 2017-07-10 7:53 UTC (permalink / raw) To: John Snow; +Cc: qemu-devel, kwolf, berto, qemu-block, mreitz On Fri, Jul 07, 2017 at 06:25:24PM -0400, John Snow wrote: > Last time, on DRAGON BALL Q: Didn't know of this before; just learnt the reference. > On 07/06/2017 10:36 AM, Kashyap Chamarthy wrote: > > [A lot of good documentation] > > > + > > +So, the following is the flow for `block-commit_Case-3`_ -- to convert a > > +disk image chain such as this:: > > + > > Maybe drop the "So, " > > "The following is the flow for 'Block Commit Case 3' to convert a disk > image chain such as this:" Fixed in v6. > > + [A] <-- [B] <-- [C] <-- [D] > > + > > +Into (where content from all the subsequent overlays, [B], and [C], > > +including the active layer, [D], is committed back to [A] -- which is > > +where live QEMU is performing all its current writes):: > > + > > "Into: " > > > + [A] > > + > > "Where content from all [...] its current writes." Fixed in v6. [...] > > +Then, (optionally) query for the status of the active block operations > > +(we can see the 'commit' job is now ready to be completed, as indicated > > +by the line *"ready": true*):: > > + > > Then, optionally query for the status of the active block operations. We > can see the 'commit' job is now ready to be completed, as indicated by > the line *"ready": true*. Fixed in v6. [...] > > +.. note:: > > + The invocation for rest of the cases (2, 4, and 5), discussed in the > > + previous section, is omitted for brevity. > > + > > As a joke I almost want to write "The invocation for cases 2, 4, and 5 > are left as an exercise to the reader." Heh, I considered that, too. Maybe I should do it. But somehow I can't bring myself to. :-) > > +(1) Issuing the command ``block-job-cancel``, after it emits the event > > nix the comma Nixed in v6. [...] > > +(2) Issuing the command ``block-job-complete``, after it emits the event > > nix the comma In v6. [...] > > +A note on synchronization modes -- determines *what* part of the disk > > +image chain must be copied to the target. Currently, there are four > > +different kinds: > > + > > You've got a note here, followed by a note box. Maybe just: > > "About synchronization modes: The synchronization mode determines > *which* part of the disk image chain will be copied to the target. > Currently, there are four different kinds:" Yep, fixed as such in v6. (And I also see that two occurences of "synchronization mode" is OK in this instance.) [...] > > + .. note:: In the case of ``drive-backup`` (or ``blockdev-backup``), > > + the behavior of ``none`` sychronization mode is different. > > + Normally, a ``backup`` job consists of two parts: Anything > > + that is overwritten by the guest is first copied out to > > + the backup, and in the background the whole image is > > + copied from start to end. With ``sync=none``, it's only > > + the first part. [...] > > +(5) Gracefully complete the 'mirror' block device job, and observe for > > + the event ``BLOCK_JOB_COMPLETED`` > > + > > "observe for the event" is slightly awkward Replaced it with "notice the event" (fixed 5 occurences throughout the doc). > > +(6) Shutdown the guest, by issuing the QMP ``quit`` command, so that > > + caches are flushed > > + > > nix both commas Nixed in v6. [...] > > +QMP invocation for ``blockdev-backup`` > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > + > > +Given, a disk image chain of depth 1, where image [B] is the active > > +overlay (live QEMU is writing to it):: > > + > > Given a disk image chain of depth 1 where image [B] is the active > overlay (live QEMU is writing it) Fixed in v6. > > + [A] <-- [B] > > + > > +The following is the procedure to copy the content from the entire chain > > +to a target image (say, [E]), which has the full content from [A] and > > +[B]. > > + > > +Create the overlay, [B]:: > > the overlay [B] Likewise :-) > > +Create a target image, that will contain the copy:: > > sans comma Yep. > > + > > + $ qemu-img create -f qcow2 e.qcow2 39M > > + > > +Then, add it to QEMU via ``blockdev-add``:: > > + > > and one more time Sure. </embarassed> > > + > > +Then, invoke ``blockdev-backup``, to copy the contents from the entire > > +image chain, consisting of images [A], and [B], to the target image > > +'e.qcow2':: > > + > > Then invoke bd-b to copy the contents from the entire image chain, > consisting of images [A] and [B] to the target image e.qcow2: Comma-wildness again. Fixed in v6. > > +Once the above 'backup' operation has completed, an event, > > +``BLOCK_JOB_COMPLETED``, will be emitted, signalling successful > > +completion. > > + > > "the event BLOCK_JOB_COMPLETED will be emitted, signalling" Fixed in v6. [...] > > +The end result will be, the image 'e.qcow2' containing a > > Goodbye sweet comma, savor the void :-) Yep, v6 savors a lot of void. > > +point-in-time backup of the disk image chain -- i.e. contents from > > +images [A], and [B] at the time the ``blockdev-backup`` command was > > +initiated. > > + > > images [A] and [B] Yep. > > +One way to confirm the backup disk image contains the identical content > > +with the disk image chain is to compare the backup, and the contents of > > Could kill this comma too. Killed in v6. [...] > That's all I got for now. It's looking very good, thank you for your > time. Please consider all of my suggestions are just that. There is a > reason I am not an English professor :) They are useful suggestions. :-) I'm a stickler for English, and I like to get it right. Appreciate the patience! -- /kashyap ^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [Qemu-devel] [PATCH v5 2/2] live-block-ops.txt: Rename, rewrite, and improve it 2017-07-10 7:53 ` Kashyap Chamarthy @ 2017-07-10 16:23 ` John Snow 0 siblings, 0 replies; 10+ messages in thread From: John Snow @ 2017-07-10 16:23 UTC (permalink / raw) To: Kashyap Chamarthy; +Cc: qemu-devel, kwolf, berto, qemu-block, mreitz On 07/10/2017 03:53 AM, Kashyap Chamarthy wrote: > > They are useful suggestions. :-) I'm a stickler for English, and I like > to get it right. Appreciate the patience! > Hardly any such thing as "right". It's all very subjective. ^ permalink raw reply [flat|nested] 10+ messages in thread
end of thread, other threads:[~2017-07-10 16:23 UTC | newest] Thread overview: 10+ messages (download: mbox.gz follow: Atom feed -- links below jump to the message on this page -- 2017-07-06 14:36 [Qemu-devel] [PATCH v5 0/2] Rewrite 'live-block-ops.txt'; convert 'bitmaps.md' to rST Kashyap Chamarthy 2017-07-06 14:36 ` [Qemu-devel] [PATCH v5 1/2] bitmaps.md: Convert to rST; move it into 'interop' dir Kashyap Chamarthy 2017-07-06 14:36 ` [Qemu-devel] [PATCH v5 2/2] live-block-ops.txt: Rename, rewrite, and improve it Kashyap Chamarthy 2017-07-06 18:11 ` Kashyap Chamarthy 2017-07-06 22:41 ` John Snow 2017-07-07 10:46 ` Kashyap Chamarthy 2017-07-07 21:08 ` John Snow 2017-07-07 22:25 ` John Snow 2017-07-10 7:53 ` Kashyap Chamarthy 2017-07-10 16:23 ` John Snow
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