Re: [Qemu-devel] [PATCH COLO v4 01/15] docs: block replication's description

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Wen Congyang (wency@cn.fujitsu.com) wrote:
> Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
> Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
> Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
> Signed-off-by: Gonglei <arei.gonglei@huawei.com>
> ---
>  docs/block-replication.txt | 179 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 179 insertions(+)
>  create mode 100644 docs/block-replication.txt
> 
> diff --git a/docs/block-replication.txt b/docs/block-replication.txt
> new file mode 100644
> index 0000000..a29f51a
> --- /dev/null
> +++ b/docs/block-replication.txt
> @@ -0,0 +1,179 @@
> +Block replication
> +----------------------------------------
> +Copyright Fujitsu, Corp. 2015
> +Copyright (c) 2015 Intel Corporation
> +Copyright (c) 2015 HUAWEI TECHNOLOGIES CO., LTD.
> +
> +This work is licensed under the terms of the GNU GPL, version 2 or later.
> +See the COPYING file in the top-level directory.
> +
> +Block replication is used for continuous checkpoints. It is designed
> +for COLO (COurse-grain LOck-stepping) where the Secondary VM is running.
> +It can also be applied for FT/HA (Fault-tolerance/High Assurance) scenario,
> +where the Secondary VM is not running.
> +
> +This document gives an overview of block replication's design.
> +
> +== Background ==
> +High availability solutions such as micro checkpoint and COLO will do
> +consecutive checkpoints. The VM state of Primary VM and Secondary VM is
> +identical right after a VM checkpoint, but becomes different as the VM
> +executes till the next checkpoint. To support disk contents checkpoint,
> +the modified disk contents in the Secondary VM must be buffered, and are
> +only dropped at next checkpoint time. To reduce the network transportation
> +effort at the time of checkpoint, the disk modification operations of
> +Primary disk are asynchronously forwarded to the Secondary node.
> +
> +== Workflow ==
> +The following is the image of block replication workflow:
> +
> +        +----------------------+            +------------------------+
> +        |Primary Write Requests|            |Secondary Write Requests|
> +        +----------------------+            +------------------------+
> +                  |                                       |
> +                  |                                      (4)
> +                  |                                       V
> +                  |                              /-------------\
> +                  |      Copy and Forward        |             |
> +                  |---------(1)----------+       | Disk Buffer |
> +                  |                      |       |             |
> +                  |                     (3)      \-------------/
> +                  |                 speculative      ^
> +                  |                write through    (2)
> +                  |                      |           |
> +                  V                      V           |
> +           +--------------+           +----------------+
> +           | Primary Disk |           | Secondary Disk |
> +           +--------------+           +----------------+
> +
> +    1) Primary write requests will be copied and forwarded to Secondary
> +       QEMU.
> +    2) Before Primary write requests are written to Secondary disk, the
> +       original sector content will be read from Secondary disk and
> +       buffered in the Disk buffer, but it will not overwrite the existing
> +       sector content(it could be from either "Secondary Write Requests" or
> +       previous COW of "Primary Write Requests") in the Disk buffer.
> +    3) Primary write requests will be written to Secondary disk.
> +    4) Secondary write requests will be buffered in the Disk buffer and it
> +       will overwrite the existing sector content in the buffer.
> +
> +== Architecture ==
> +We are going to implement block replication from many basic
> +blocks that are already in QEMU.
> +
> +         virtio-blk       ||
> +             ^            ||                            .----------
> +             |            ||                            | Secondary
> +        1 Quorum          ||                            '----------
> +         /      \         ||
> +        /        \        ||
> +   Primary    2 filter
> +     disk         ^                                                             virtio-blk
> +                  |                                                                  ^
> +                3 NBD  ------->  3 NBD                                               |
> +                client    ||     server                                          2 filter
> +                          ||        ^                                                ^
> +--------.                 ||        |                                                |
> +Primary |                 ||  Secondary disk <--------- hidden-disk 5 <--------- active-disk 4
> +--------'                 ||        |          backing        ^       backing
> +                          ||        |                         |
> +                          ||        |                         |
> +                          ||        '-------------------------'
> +                          ||           drive-backup sync=none
> +
> +1) The disk on the primary is represented by a block device with two
> +children, providing replication between a primary disk and the host that
> +runs the secondary VM. The read pattern for quorum can be extended to
> +make the primary always read from the local disk instead of going through
> +NBD.
> +
> +2) The new block filter(the name is replication) will control the block
> +replication.
> +
> +3) The secondary disk receives writes from the primary VM through QEMU's
> +embedded NBD server (speculative write-through).
> +
> +4) The disk on the secondary is represented by a custom block device
> +(called active-disk). It should be an empty disk, and the format should
> +support bdrv_make_empty() and backing file.
> +
> +5) The hidden-disk is created automatically. It buffers the original content
> +that is modified by the primary VM. It should also be an empty disk, and
> +the driver supports bdrv_make_empty() and backing file.
> +
> +== Failure Handling ==
> +There are 6 internal errors when block replication is running:
> +1. I/O error on primary disk
> +2. Forwarding primay write requests failed
> +3. Bacup failed or writing to secondary disk failed

Typo: Bacup -> Backup

> +4. I/O error on secondary disk
> +5. I/O error on active disk
> +6. Making active disk or hidden disk empty failed
> +In case 1 and 5, we just report the error to the disk layer. In case 2, 3,
> +4 and 6, we just report block replication's error to FT/HA manager(which
> +decides when to do a new checkpoint, when to do failover).
> +There is one internal error when doing failover:
> +1. Commiting the data in active disk/hidden disk to secondary disk failed
> +We just to report this error to FT/HA manager.


One thing I wanted to check I understand;  how much RAM do the active and hidden
disks use;  lets say during the 1st checkpoint   10MB of disk is written,
and during hte 2nd checkpoint a different 10MB of the disk is written
(to different locations on the disk); how big are the active and hidden
disks in RAM - are they 10MB or 20MB?

Dave

> +
> +== New block driver interface ==
> +We add three block driver interfaces to control block replication:
> +a. bdrv_start_replication()
> +   Start block replication, called in migration/checkpoint thread.
> +   We must call bdrv_start_replication() in secondary QEMU before
> +   calling bdrv_start_replication() in primary QEMU. The caller
> +   must hold the I/O mutex lock if it is in migration/checkpoint
> +   thread.
> +b. bdrv_do_checkpoint()
> +   This interface is called after all VM state is transferred to
> +   Secondary QEMU. The Disk buffer will be dropped in this interface.
> +   The caller must hold the I/O mutex lock if it is in migration/checkpoint
> +   thread.
> +c. bdrv_stop_replication()
> +   It is called on failover. We will flush the Disk buffer into
> +   Secondary Disk and stop block replication. The vm should be stopped
> +   before calling it. The caller must hold the I/O mutex lock if it is
> +   in migration/checkpoint thread.
> +
> +== Usage ==
> +Primary:
> +  -drive if=xxx,driver=quorum,read-pattern=fifo,no-connect=on,\
> +         children.0.file.filename=1.raw,\
> +         children.0.driver=raw,\
> +         children.1.file.driver=nbd,\
> +         children.1.file.host=xxx,\
> +         children.1.file.port=xxx,\
> +         children.1.file.export=xxx,\
> +         children.1.driver=replication,\
> +         children.1.mode=primary,\
> +         children.1.ignore-errors=on
> +  Note:
> +  1. NBD Client should not be the first child of quorum.
> +  2. There should be only one NBD Client.
> +  3. host is the secondary physical machine's hostname or IP
> +  4. Each disk must have its own export name.
> +  5. It is all a single argument to -drive, and you should ignore
> +     the leading whitespace.
> +
> +Secondary:
> +  -drive if=none,driver=raw,file=1.raw,id=nbd_target1 \
> +  -drive if=xxx,driver=replication,mode=secondary,export=xxx,\
> +         file.file.filename=active_disk.qcow2,\
> +         file.driver=qcow2,\
> +         file.backing_reference.drive_id=nbd_target1,\
> +         file.backing_reference.hidden-disk.file.filename=hidden_disk.qcow2,\
> +         file.backing_reference.hidden-disk.driver=qcow2,\
> +         file.backing_reference.hidden-disk.allow-write-backing-file=on
> +  Then run qmp command:
> +    nbd-server-start host:port
> +  Note:
> +  1. The export name for the same disk must be the same in primary
> +     and secondary QEMU command line
> +  2. The qmp command nbd-server-start must be run before running the
> +     qmp command migrate on primary QEMU
> +  3. Don't use nbd-server-start's other options
> +  4. Active disk, hidden disk and nbd target's length should be the
> +     same.
> +  5. It is better to put active disk and hidden disk in ramdisk.
> +  6. It is all a single argument to -drive, and you should ignore
> +     the leading whitespace.
> -- 
> 2.1.0
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK