Re: [PATCH v4 13/16] Convert colo main documentation to restructuredText

[Fabiano Rosas <farosas@suse.de>]

Lukas Straub <lukasstraub2@web.de> writes:

> Signed-off-by: Lukas Straub <lukasstraub2@web.de>
> ---
>  MAINTAINERS               |   2 +-
>  docs/COLO-FT.txt          | 334 ------------------------------------------
>  docs/system/index.rst     |   1 +
>  docs/system/qemu-colo.rst | 360 ++++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 362 insertions(+), 335 deletions(-)
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index ceb3ecb1d955a267018c70429de61e45abb7a7ba..b04a5fef47d26ef14842e8a2e2a674651e9215e3 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -3854,7 +3854,7 @@ F: migration/multifd-colo.*
>  F: include/migration/colo.h
>  F: include/migration/failover.h
>  F: tests/qtest/migration/colo-tests.c
> -F: docs/COLO-FT.txt
> +F: docs/system/qemu-colo.rst
>  
>  COLO Proxy
>  M: Zhang Chen <zhangckid@gmail.com>
> diff --git a/docs/COLO-FT.txt b/docs/COLO-FT.txt
> deleted file mode 100644
> index 2283a09c080b8996f9767eeb415e8d4fbdc940af..0000000000000000000000000000000000000000
> --- a/docs/COLO-FT.txt
> +++ /dev/null
> @@ -1,334 +0,0 @@
> -COarse-grained LOck-stepping Virtual Machines for Non-stop Service
> -----------------------------------------
> -Copyright (c) 2016 Intel Corporation
> -Copyright (c) 2016 HUAWEI TECHNOLOGIES CO., LTD.
> -Copyright (c) 2016 Fujitsu, Corp.
> -
> -This work is licensed under the terms of the GNU GPL, version 2 or later.
> -See the COPYING file in the top-level directory.
> -
> -This document gives an overview of COLO's design and how to use it.
> -
> -== Background ==
> -Virtual machine (VM) replication is a well known technique for providing
> -application-agnostic software-implemented hardware fault tolerance,
> -also known as "non-stop service".
> -
> -COLO (COarse-grained LOck-stepping) is a high availability solution.
> -Both primary VM (PVM) and secondary VM (SVM) run in parallel. They receive the
> -same request from client, and generate response in parallel too.
> -If the response packets from PVM and SVM are identical, they are released
> -immediately. Otherwise, a VM checkpoint (on demand) is conducted.
> -
> -== Architecture ==
> -
> -The architecture of COLO is shown in the diagram below.
> -It consists of a pair of networked physical nodes:
> -The primary node running the PVM, and the secondary node running the SVM
> -to maintain a valid replica of the PVM.
> -PVM and SVM execute in parallel and generate output of response packets for
> -client requests according to the application semantics.
> -
> -The incoming packets from the client or external network are received by the
> -primary node, and then forwarded to the secondary node, so that both the PVM
> -and the SVM are stimulated with the same requests.
> -
> -COLO receives the outbound packets from both the PVM and SVM and compares them
> -before allowing the output to be sent to clients.
> -
> -The SVM is qualified as a valid replica of the PVM, as long as it generates
> -identical responses to all client requests. Once the differences in the outputs
> -are detected between the PVM and SVM, COLO withholds transmission of the
> -outbound packets until it has successfully synchronized the PVM state to the SVM.
> -
> -  Primary Node                                                            Secondary Node
> -+------------+  +-----------------------+       +------------------------+  +------------+
> -|            |  |       HeartBeat       +<----->+       HeartBeat        |  |            |
> -| Primary VM |  +-----------+-----------+       +-----------+------------+  |Secondary VM|
> -|            |              |                               |               |            |
> -|            |  +-----------|-----------+       +-----------|------------+  |            |
> -|            |  |QEMU   +---v----+      |       |QEMU  +----v---+        |  |            |
> -|            |  |       |Failover|      |       |      |Failover|        |  |            |
> -|            |  |       +--------+      |       |      +--------+        |  |            |
> -|            |  |   +---------------+   |       |   +---------------+    |  |            |
> -|            |  |   | VM Checkpoint +-------------->+ VM Checkpoint |    |  |            |
> -|            |  |   +---------------+   |       |   +---------------+    |  |            |
> -|Requests<--------------------------\ /-----------------\ /--------------------->Requests|
> -|            |  |                   ^ ^ |       |       | |              |  |            |
> -|Responses+---------------------\ /-|-|------------\ /-------------------------+Responses|
> -|            |  |               | | | | |       |  | |  | |              |  |            |
> -|            |  | +-----------+ | | | | |       |  | |  | | +----------+ |  |            |
> -|            |  | | COLO disk | | | | | |       |  | |  | | | COLO disk| |  |            |
> -|            |  | |   Manager +---------------------------->| Manager  | |  |            |
> -|            |  | ++----------+ v v | | |       |  | v  v | +---------++ |  |            |
> -|            |  |  |+-----------+-+-+-++|       | ++-+--+-+---------+ |  |  |            |
> -|            |  |  ||   COLO Proxy     ||       | |   COLO Proxy    | |  |  |            |
> -|            |  |  || (compare packet  ||       | |(adjust sequence | |  |  |            |
> -|            |  |  ||and mirror packet)||       | |    and ACK)     | |  |  |            |
> -|            |  |  |+------------+---+-+|       | +-----------------+ |  |  |            |
> -+------------+  +-----------------------+       +------------------------+  +------------+
> -+------------+     |             |   |                                |     +------------+
> -| VM Monitor |     |             |   |                                |     | VM Monitor |
> -+------------+     |             |   |                                |     +------------+
> -+---------------------------------------+       +----------------------------------------+
> -|   Kernel         |             |   |  |       |   Kernel            |                  |
> -+---------------------------------------+       +----------------------------------------+
> -                   |             |   |                                |
> -    +--------------v+  +---------v---+--+       +------------------+ +v-------------+
> -    |   Storage     |  |External Network|       | External Network | |   Storage    |
> -    +---------------+  +----------------+       +------------------+ +--------------+
> -
> -
> -== Components introduction ==
> -
> -You can see there are several components in COLO's diagram of architecture.
> -Their functions are described below.
> -
> -HeartBeat:
> -Runs on both the primary and secondary nodes, to periodically check platform
> -availability. When the primary node suffers a hardware fail-stop failure,
> -the heartbeat stops responding, the secondary node will trigger a failover
> -as soon as it determines the absence.
> -
> -COLO disk Manager:
> -When primary VM writes data into image, the colo disk manager captures this data
> -and sends it to secondary VM's which makes sure the context of secondary VM's
> -image is consistent with the context of primary VM 's image.
> -For more details, please refer to docs/block-replication.txt.
> -
> -Checkpoint/Failover Controller:
> -Modifications of save/restore flow to realize continuous migration,
> -to make sure the state of VM in Secondary side is always consistent with VM in
> -Primary side.
> -
> -COLO Proxy:
> -Delivers packets to Primary and Secondary, and then compare the responses from
> -both side. Then decide whether to start a checkpoint according to some rules.
> -Please refer to docs/colo-proxy.txt for more information.
> -
> -Note:
> -HeartBeat has not been implemented yet, so you need to trigger failover process
> -by using 'x-colo-lost-heartbeat' command.
> -
> -== COLO operation status ==
> -
> -+-----------------+
> -|                 |
> -|    Start COLO   |
> -|                 |
> -+--------+--------+
> -         |
> -         |  Main qmp command:
> -         |  migrate-set-capabilities with x-colo
> -         |  migrate
> -         |
> -         v
> -+--------+--------+
> -|                 |
> -|  COLO running   |
> -|                 |
> -+--------+--------+
> -         |
> -         |  Main qmp command:
> -         |  x-colo-lost-heartbeat
> -         |  or
> -         |  some error happened
> -         v
> -+--------+--------+
> -|                 |  send qmp event:
> -|  COLO failover  |  COLO_EXIT
> -|                 |
> -+-----------------+
> -
> -COLO use the qmp command to switch and report operation status.
> -The diagram just shows the main qmp command, you can get the detail
> -in test procedure.
> -
> -== Test procedure ==
> -Note: Here we are running both instances on the same host for testing,
> -change the IP Addresses if you want to run it on two hosts. Initially
> -127.0.0.1 is the Primary Host and 127.0.0.2 is the Secondary Host.
> -
> -== Startup qemu ==
> -1. Primary:
> -Note: Initially, $imagefolder/primary.qcow2 needs to be copied to all hosts.
> -You don't need to change any IP's here, because 0.0.0.0 listens on any
> -interface. The chardev's with 127.0.0.1 IP's loopback to the local qemu
> -instance.
> -
> -# imagefolder="/mnt/vms/colo-test-primary"
> -
> -# qemu-system-x86_64 -enable-kvm -cpu qemu64,kvmclock=on -m 512 -smp 1 -qmp stdio \
> -   -device piix3-usb-uhci -device usb-tablet -name primary \
> -   -netdev tap,id=hn0,vhost=off,helper=/usr/lib/qemu/qemu-bridge-helper \
> -   -device rtl8139,id=e0,netdev=hn0 \
> -   -chardev socket,id=mirror0,host=0.0.0.0,port=9003,server=on,wait=off \
> -   -chardev socket,id=compare1,host=0.0.0.0,port=9004,server=on,wait=on \
> -   -chardev socket,id=compare0,host=127.0.0.1,port=9001,server=on,wait=off \
> -   -chardev socket,id=compare0-0,host=127.0.0.1,port=9001 \
> -   -chardev socket,id=compare_out,host=127.0.0.1,port=9005,server=on,wait=off \
> -   -chardev socket,id=compare_out0,host=127.0.0.1,port=9005 \
> -   -object filter-mirror,id=m0,netdev=hn0,queue=tx,outdev=mirror0 \
> -   -object filter-redirector,netdev=hn0,id=redire0,queue=rx,indev=compare_out \
> -   -object filter-redirector,netdev=hn0,id=redire1,queue=rx,outdev=compare0 \
> -   -object iothread,id=iothread1 \
> -   -object colo-compare,id=comp0,primary_in=compare0-0,secondary_in=compare1,\
> -outdev=compare_out0,iothread=iothread1 \
> -   -drive if=ide,id=colo-disk0,driver=quorum,read-pattern=fifo,vote-threshold=1,\
> -children.0.file.filename=$imagefolder/primary.qcow2,children.0.driver=qcow2 -S
> -
> -2. Secondary:
> -Note: Active and hidden images need to be created only once and the
> -size should be the same as primary.qcow2. Again, you don't need to change
> -any IP's here, except for the $primary_ip variable.
> -
> -# imagefolder="/mnt/vms/colo-test-secondary"
> -# primary_ip=127.0.0.1
> -
> -# qemu-img create -f qcow2 $imagefolder/secondary-active.qcow2 10G
> -
> -# qemu-img create -f qcow2 $imagefolder/secondary-hidden.qcow2 10G
> -
> -# qemu-system-x86_64 -enable-kvm -cpu qemu64,kvmclock=on -m 512 -smp 1 -qmp stdio \
> -   -device piix3-usb-uhci -device usb-tablet -name secondary \
> -   -netdev tap,id=hn0,vhost=off,helper=/usr/lib/qemu/qemu-bridge-helper \
> -   -device rtl8139,id=e0,netdev=hn0 \
> -   -chardev socket,id=red0,host=$primary_ip,port=9003,reconnect-ms=1000 \
> -   -chardev socket,id=red1,host=$primary_ip,port=9004,reconnect-ms=1000 \
> -   -object filter-redirector,id=f1,netdev=hn0,queue=tx,indev=red0 \
> -   -object filter-redirector,id=f2,netdev=hn0,queue=rx,outdev=red1 \
> -   -object filter-rewriter,id=rew0,netdev=hn0,queue=all \
> -   -drive if=none,id=parent0,file.filename=$imagefolder/primary.qcow2,driver=qcow2 \
> -   -drive if=none,id=childs0,driver=replication,mode=secondary,file.driver=qcow2,\
> -top-id=colo-disk0,file.file.filename=$imagefolder/secondary-active.qcow2,\
> -file.backing.driver=qcow2,file.backing.file.filename=$imagefolder/secondary-hidden.qcow2,\
> -file.backing.backing=parent0 \
> -   -drive if=ide,id=colo-disk0,driver=quorum,read-pattern=fifo,vote-threshold=1,\
> -children.0=childs0 \
> -   -incoming tcp:0.0.0.0:9998
> -
> -
> -3. On Secondary VM's QEMU monitor, issue command
> -{"execute":"qmp_capabilities"}
> -{"execute": "migrate-set-capabilities", "arguments": {"capabilities": [ {"capability": "x-colo", "state": true } ] } }
> -{"execute": "nbd-server-start", "arguments": {"addr": {"type": "inet", "data": {"host": "0.0.0.0", "port": "9999"} } } }
> -{"execute": "nbd-server-add", "arguments": {"device": "parent0", "writable": true } }
> -
> -Note:
> -  a. The qmp command nbd-server-start and nbd-server-add must be run
> -     before running the qmp command migrate on primary QEMU
> -  b. Active disk, hidden disk and nbd target's length should be the
> -     same.
> -  c. It is better to put active disk and hidden disk in ramdisk. They
> -     will be merged into the parent disk on failover.
> -
> -4. On Primary VM's QEMU monitor, issue command:
> -{"execute":"qmp_capabilities"}
> -{"execute": "human-monitor-command", "arguments": {"command-line": "drive_add -n buddy driver=replication,mode=primary,file.driver=nbd,file.host=127.0.0.2,file.port=9999,file.export=parent0,node-name=replication0"}}
> -{"execute": "x-blockdev-change", "arguments":{"parent": "colo-disk0", "node": "replication0" } }
> -{"execute": "migrate-set-capabilities", "arguments": {"capabilities": [ {"capability": "x-colo", "state": true } ] } }
> -{"execute": "migrate", "arguments": {"uri": "tcp:127.0.0.2:9998" } }
> -
> -  Note:
> -  a. There should be only one NBD Client for each primary disk.
> -  b. The qmp command line must be run after running qmp command line in
> -     secondary qemu.
> -
> -5. After the above steps, you will see, whenever you make changes to PVM, SVM will be synced.
> -You can issue command '{ "execute": "migrate-set-parameters" , "arguments":{ "x-checkpoint-delay": 2000 } }'
> -to change the idle checkpoint period time
> -
> -6. Failover test
> -You can kill one of the VMs and Failover on the surviving VM:
> -
> -If you killed the Secondary, then follow "Primary Failover". After that,
> -if you want to resume the replication, follow "Primary resume replication"
> -
> -If you killed the Primary, then follow "Secondary Failover". After that,
> -if you want to resume the replication, follow "Secondary resume replication"
> -
> -== Primary Failover ==
> -The Secondary died, resume on the Primary
> -
> -{"execute": "x-blockdev-change", "arguments":{ "parent": "colo-disk0", "child": "children.1"} }
> -{"execute": "human-monitor-command", "arguments":{ "command-line": "drive_del replication0" } }
> -{"execute": "object-del", "arguments":{ "id": "comp0" } }
> -{"execute": "object-del", "arguments":{ "id": "iothread1" } }
> -{"execute": "object-del", "arguments":{ "id": "m0" } }
> -{"execute": "object-del", "arguments":{ "id": "redire0" } }
> -{"execute": "object-del", "arguments":{ "id": "redire1" } }
> -{"execute": "x-colo-lost-heartbeat" }
> -
> -== Secondary Failover ==
> -The Primary died, resume on the Secondary and prepare to become the new Primary
> -
> -{"execute": "nbd-server-stop"}
> -{"execute": "x-colo-lost-heartbeat"}
> -
> -{"execute": "object-del", "arguments":{ "id": "f2" } }
> -{"execute": "object-del", "arguments":{ "id": "f1" } }
> -{"execute": "chardev-remove", "arguments":{ "id": "red1" } }
> -{"execute": "chardev-remove", "arguments":{ "id": "red0" } }
> -
> -{"execute": "chardev-add", "arguments":{ "id": "mirror0", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "0.0.0.0", "port": "9003" } }, "server": true } } } }
> -{"execute": "chardev-add", "arguments":{ "id": "compare1", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "0.0.0.0", "port": "9004" } }, "server": true } } } }
> -{"execute": "chardev-add", "arguments":{ "id": "compare0", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "127.0.0.1", "port": "9001" } }, "server": true } } } }
> -{"execute": "chardev-add", "arguments":{ "id": "compare0-0", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "127.0.0.1", "port": "9001" } }, "server": false } } } }
> -{"execute": "chardev-add", "arguments":{ "id": "compare_out", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "127.0.0.1", "port": "9005" } }, "server": true } } } }
> -{"execute": "chardev-add", "arguments":{ "id": "compare_out0", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "127.0.0.1", "port": "9005" } }, "server": false } } } }
> -
> -== Primary resume replication ==
> -Resume replication after new Secondary is up.
> -
> -Start the new Secondary (Steps 2 and 3 above), then on the Primary:
> -{"execute": "drive-mirror", "arguments":{ "device": "colo-disk0", "job-id": "resync", "target": "nbd://127.0.0.2:9999/parent0", "mode": "existing", "format": "raw", "sync": "full"} }
> -
> -Wait until disk is synced, then:
> -{"execute": "stop"}
> -{"execute": "block-job-cancel", "arguments":{ "device": "resync"} }
> -
> -{"execute": "human-monitor-command", "arguments":{ "command-line": "drive_add -n buddy driver=replication,mode=primary,file.driver=nbd,file.host=127.0.0.2,file.port=9999,file.export=parent0,node-name=replication0"}}
> -{"execute": "x-blockdev-change", "arguments":{ "parent": "colo-disk0", "node": "replication0" } }
> -
> -{"execute": "object-add", "arguments":{ "qom-type": "filter-mirror", "id": "m0", "netdev": "hn0", "queue": "tx", "outdev": "mirror0" } }
> -{"execute": "object-add", "arguments":{ "qom-type": "filter-redirector", "id": "redire0", "netdev": "hn0", "queue": "rx", "indev": "compare_out" } }
> -{"execute": "object-add", "arguments":{ "qom-type": "filter-redirector", "id": "redire1", "netdev": "hn0", "queue": "rx", "outdev": "compare0" } }
> -{"execute": "object-add", "arguments":{ "qom-type": "iothread", "id": "iothread1" } }
> -{"execute": "object-add", "arguments":{ "qom-type": "colo-compare", "id": "comp0", "primary_in": "compare0-0", "secondary_in": "compare1", "outdev": "compare_out0", "iothread": "iothread1" } }
> -
> -{"execute": "migrate-set-capabilities", "arguments":{ "capabilities": [ {"capability": "x-colo", "state": true } ] } }
> -{"execute": "migrate", "arguments":{ "uri": "tcp:127.0.0.2:9998" } }
> -
> -Note:
> -If this Primary previously was a Secondary, then we need to insert the
> -filters before the filter-rewriter by using the
> -""insert": "before", "position": "id=rew0"" Options. See below.
> -
> -== Secondary resume replication ==
> -Become Primary and resume replication after new Secondary is up. Note
> -that now 127.0.0.1 is the Secondary and 127.0.0.2 is the Primary.
> -
> -Start the new Secondary (Steps 2 and 3 above, but with primary_ip=127.0.0.2),
> -then on the old Secondary:
> -{"execute": "drive-mirror", "arguments":{ "device": "colo-disk0", "job-id": "resync", "target": "nbd://127.0.0.1:9999/parent0", "mode": "existing", "format": "raw", "sync": "full"} }
> -
> -Wait until disk is synced, then:
> -{"execute": "stop"}
> -{"execute": "block-job-cancel", "arguments":{ "device": "resync" } }
> -
> -{"execute": "human-monitor-command", "arguments":{ "command-line": "drive_add -n buddy driver=replication,mode=primary,file.driver=nbd,file.host=127.0.0.1,file.port=9999,file.export=parent0,node-name=replication0"}}
> -{"execute": "x-blockdev-change", "arguments":{ "parent": "colo-disk0", "node": "replication0" } }
> -
> -{"execute": "object-add", "arguments":{ "qom-type": "filter-mirror", "id": "m0", "insert": "before", "position": "id=rew0", "netdev": "hn0", "queue": "tx", "outdev": "mirror0" } }
> -{"execute": "object-add", "arguments":{ "qom-type": "filter-redirector", "id": "redire0", "insert": "before", "position": "id=rew0", "netdev": "hn0", "queue": "rx", "indev": "compare_out" } }
> -{"execute": "object-add", "arguments":{ "qom-type": "filter-redirector", "id": "redire1", "insert": "before", "position": "id=rew0", "netdev": "hn0", "queue": "rx", "outdev": "compare0" } }
> -{"execute": "object-add", "arguments":{ "qom-type": "iothread", "id": "iothread1" } }
> -{"execute": "object-add", "arguments":{ "qom-type": "colo-compare", "id": "comp0", "primary_in": "compare0-0", "secondary_in": "compare1", "outdev": "compare_out0", "iothread": "iothread1" } }
> -
> -{"execute": "migrate-set-capabilities", "arguments":{ "capabilities": [ {"capability": "x-colo", "state": true } ] } }
> -{"execute": "migrate", "arguments":{ "uri": "tcp:127.0.0.1:9998" } }
> -
> -== TODO ==
> -1. Support shared storage.
> -2. Develop the heartbeat part.
> -3. Reduce checkpoint VM’s downtime while doing checkpoint.
> diff --git a/docs/system/index.rst b/docs/system/index.rst
> index 427b020483104f6589878bbf255a367ae114c61b..6268c41aea9c74dc3e59d896b5ae082360bfbb1a 100644
> --- a/docs/system/index.rst
> +++ b/docs/system/index.rst
> @@ -41,3 +41,4 @@ or Hypervisor.Framework.
>     igvm
>     vm-templating
>     sriov
> +   qemu-colo
> diff --git a/docs/system/qemu-colo.rst b/docs/system/qemu-colo.rst
> new file mode 100644
> index 0000000000000000000000000000000000000000..4b5fbbf398f8a5c4ea6baad615bde94b2b4678d2
> --- /dev/null
> +++ b/docs/system/qemu-colo.rst
> @@ -0,0 +1,360 @@
> +Qemu COLO Fault Tolerance
> +=========================
> +
> +| Copyright (c) 2016 Intel Corporation
> +| Copyright (c) 2016 HUAWEI TECHNOLOGIES CO., LTD.
> +| Copyright (c) 2016 Fujitsu, Corp.
> +
> +This work is licensed under the terms of the GNU GPL, version 2 or later.
> +See the COPYING file in the top-level directory.
> +
> +This document gives an overview of COLO's design and how to use it.
> +
> +Background
> +----------
> +Virtual machine (VM) replication is a well known technique for providing
> +application-agnostic software-implemented hardware fault tolerance,
> +also known as "non-stop service".
> +
> +COLO (COarse-grained LOck-stepping) is a high availability solution.
> +Both primary VM (PVM) and secondary VM (SVM) run in parallel. They receive the
> +same request from client, and generate response in parallel too.
> +If the response packets from PVM and SVM are identical, they are released
> +immediately. Otherwise, a VM checkpoint (on demand) is conducted.
> +
> +Architecture
> +------------
> +The architecture of COLO is shown in the diagram below.
> +It consists of a pair of networked physical nodes:
> +The primary node running the PVM, and the secondary node running the SVM
> +to maintain a valid replica of the PVM.
> +PVM and SVM execute in parallel and generate output of response packets for
> +client requests according to the application semantics.
> +
> +The incoming packets from the client or external network are received by the
> +primary node, and then forwarded to the secondary node, so that both the PVM
> +and the SVM are stimulated with the same requests.
> +
> +COLO receives the outbound packets from both the PVM and SVM and compares them
> +before allowing the output to be sent to clients.
> +
> +The SVM is qualified as a valid replica of the PVM, as long as it generates
> +identical responses to all client requests. Once the differences in the outputs
> +are detected between the PVM and SVM, COLO withholds transmission of the
> +outbound packets until it has successfully synchronized the PVM state to the SVM.
> +
> +Overview::
> +
> +      Primary Node                                                            Secondary Node
> +    +------------+  +-----------------------+       +------------------------+  +------------+
> +    |            |  |       HeartBeat       +<----->+       HeartBeat        |  |            |
> +    | Primary VM |  +-----------+-----------+       +-----------+------------+  |Secondary VM|
> +    |            |              |                               |               |            |
> +    |            |  +-----------|-----------+       +-----------|------------+  |            |
> +    |            |  |QEMU   +---v----+      |       |QEMU  +----v---+        |  |            |
> +    |            |  |       |Failover|      |       |      |Failover|        |  |            |
> +    |            |  |       +--------+      |       |      +--------+        |  |            |
> +    |            |  |   +---------------+   |       |   +---------------+    |  |            |
> +    |            |  |   | VM Checkpoint +-------------->+ VM Checkpoint |    |  |            |
> +    |            |  |   +---------------+   |       |   +---------------+    |  |            |
> +    |Requests<--------------------------\ /-----------------\ /--------------------->Requests|
> +    |            |  |                   ^ ^ |       |       | |              |  |            |
> +    |Responses+---------------------\ /-|-|------------\ /-------------------------+Responses|
> +    |            |  |               | | | | |       |  | |  | |              |  |            |
> +    |            |  | +-----------+ | | | | |       |  | |  | | +----------+ |  |            |
> +    |            |  | | COLO disk | | | | | |       |  | |  | | | COLO disk| |  |            |
> +    |            |  | |   Manager +---------------------------->| Manager  | |  |            |
> +    |            |  | ++----------+ v v | | |       |  | v  v | +---------++ |  |            |
> +    |            |  |  |+-----------+-+-+-++|       | ++-+--+-+---------+ |  |  |            |
> +    |            |  |  ||   COLO Proxy     ||       | |   COLO Proxy    | |  |  |            |
> +    |            |  |  || (compare packet  ||       | |(adjust sequence | |  |  |            |
> +    |            |  |  ||and mirror packet)||       | |    and ACK)     | |  |  |            |
> +    |            |  |  |+------------+---+-+|       | +-----------------+ |  |  |            |
> +    +------------+  +-----------------------+       +------------------------+  +------------+
> +    +------------+     |             |   |                                |     +------------+
> +    | VM Monitor |     |             |   |                                |     | VM Monitor |
> +    +------------+     |             |   |                                |     +------------+
> +    +---------------------------------------+       +----------------------------------------+
> +    |   Kernel         |             |   |  |       |   Kernel            |                  |
> +    +---------------------------------------+       +----------------------------------------+
> +                       |             |   |                                |
> +        +--------------v+  +---------v---+--+       +------------------+ +v-------------+
> +        |   Storage     |  |External Network|       | External Network | |   Storage    |
> +        +---------------+  +----------------+       +------------------+ +--------------+
> +
> +Components introduction
> +^^^^^^^^^^^^^^^^^^^^^^^
> +You can see there are several components in COLO's diagram of architecture.
> +Their functions are described below.
> +
> +HeartBeat
> +~~~~~~~~~
> +Runs on both the primary and secondary nodes, to periodically check platform
> +availability. When the primary node suffers a hardware fail-stop failure,
> +the heartbeat stops responding, the secondary node will trigger a failover
> +as soon as it determines the absence.
> +
> +COLO disk Manager
> +~~~~~~~~~~~~~~~~~
> +When primary VM writes data into image, the colo disk manager captures this data
> +and sends it to secondary VM's which makes sure the context of secondary VM's
> +image is consistent with the context of primary VM 's image.
> +For more details, please refer to docs/block-replication.txt.
> +
> +Checkpoint/Failover Controller
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +Modifications of save/restore flow to realize continuous migration,
> +to make sure the state of VM in Secondary side is always consistent with VM in
> +Primary side.
> +
> +COLO Proxy
> +~~~~~~~~~~
> +Delivers packets to Primary and Secondary, and then compare the responses from
> +both side. Then decide whether to start a checkpoint according to some rules.
> +Please refer to docs/colo-proxy.txt for more information.
> +
> +Note:
> +HeartBeat has not been implemented yet, so you need to trigger failover process
> +by using 'x-colo-lost-heartbeat' command.
> +
> +COLO operation status
> +^^^^^^^^^^^^^^^^^^^^^
> +
> +Overview::
> +
> +    +-----------------+
> +    |                 |
> +    |    Start COLO   |
> +    |                 |
> +    +--------+--------+
> +             |
> +             |  Main qmp command:
> +             |  migrate-set-capabilities with x-colo
> +             |  migrate
> +             |
> +             v
> +    +--------+--------+
> +    |                 |
> +    |  COLO running   |
> +    |                 |
> +    +--------+--------+
> +             |
> +             |  Main qmp command:
> +             |  x-colo-lost-heartbeat
> +             |  or
> +             |  some error happened
> +             v
> +    +--------+--------+
> +    |                 |  send qmp event:
> +    |  COLO failover  |  COLO_EXIT
> +    |                 |
> +    +-----------------+
> +
> +
> +COLO use the qmp command to switch and report operation status.
> +The diagram just shows the main qmp command, you can get the detail
> +in test procedure.
> +
> +Test procedure
> +--------------
> +Note: Here we are running both instances on the same host for testing,
> +change the IP Addresses if you want to run it on two hosts. Initially
> +``127.0.0.1`` is the Primary Host and ``127.0.0.2`` is the Secondary Host.
> +
> +Startup qemu
> +^^^^^^^^^^^^
> +**1. Primary**:
> +Note: Initially, ``$imagefolder/primary.qcow2`` needs to be copied to all hosts.
> +You don't need to change any IP's here, because ``0.0.0.0`` listens on any
> +interface. The chardev's with ``127.0.0.1`` IP's loopback to the local qemu
> +instance::
> +
> +    # imagefolder="/mnt/vms/colo-test-primary"
> +
> +    # qemu-system-x86_64 -enable-kvm -cpu qemu64,kvmclock=on -m 512 -smp 1 -qmp stdio \
> +       -device piix3-usb-uhci -device usb-tablet -name primary \
> +       -netdev tap,id=hn0,vhost=off,helper=/usr/lib/qemu/qemu-bridge-helper \
> +       -device rtl8139,id=e0,netdev=hn0 \
> +       -chardev socket,id=mirror0,host=0.0.0.0,port=9003,server=on,wait=off \
> +       -chardev socket,id=compare1,host=0.0.0.0,port=9004,server=on,wait=on \
> +       -chardev socket,id=compare0,host=127.0.0.1,port=9001,server=on,wait=off \
> +       -chardev socket,id=compare0-0,host=127.0.0.1,port=9001 \
> +       -chardev socket,id=compare_out,host=127.0.0.1,port=9005,server=on,wait=off \
> +       -chardev socket,id=compare_out0,host=127.0.0.1,port=9005 \
> +       -object filter-mirror,id=m0,netdev=hn0,queue=tx,outdev=mirror0 \
> +       -object filter-redirector,netdev=hn0,id=redire0,queue=rx,indev=compare_out \
> +       -object filter-redirector,netdev=hn0,id=redire1,queue=rx,outdev=compare0 \
> +       -object iothread,id=iothread1 \
> +       -object colo-compare,id=comp0,primary_in=compare0-0,secondary_in=compare1,\
> +    outdev=compare_out0,iothread=iothread1 \
> +       -drive if=ide,id=colo-disk0,driver=quorum,read-pattern=fifo,vote-threshold=1,\
> +    children.0.file.filename=$imagefolder/primary.qcow2,children.0.driver=qcow2 -S
> +
> +
> +**2. Secondary**:
> +Note: Active and hidden images need to be created only once and the
> +size should be the same as ``primary.qcow2``. Again, you don't need to change
> +any IP's here, except for the ``$primary_ip`` variable::
> +
> +    # imagefolder="/mnt/vms/colo-test-secondary"
> +    # primary_ip=127.0.0.1
> +
> +    # qemu-img create -f qcow2 $imagefolder/secondary-active.qcow2 10G
> +
> +    # qemu-img create -f qcow2 $imagefolder/secondary-hidden.qcow2 10G
> +
> +    # qemu-system-x86_64 -enable-kvm -cpu qemu64,kvmclock=on -m 512 -smp 1 -qmp stdio \
> +       -device piix3-usb-uhci -device usb-tablet -name secondary \
> +       -netdev tap,id=hn0,vhost=off,helper=/usr/lib/qemu/qemu-bridge-helper \
> +       -device rtl8139,id=e0,netdev=hn0 \
> +       -chardev socket,id=red0,host=$primary_ip,port=9003,reconnect-ms=1000 \
> +       -chardev socket,id=red1,host=$primary_ip,port=9004,reconnect-ms=1000 \
> +       -object filter-redirector,id=f1,netdev=hn0,queue=tx,indev=red0 \
> +       -object filter-redirector,id=f2,netdev=hn0,queue=rx,outdev=red1 \
> +       -object filter-rewriter,id=rew0,netdev=hn0,queue=all \
> +       -drive if=none,id=parent0,file.filename=$imagefolder/primary.qcow2,driver=qcow2 \
> +       -drive if=none,id=childs0,driver=replication,mode=secondary,file.driver=qcow2,\
> +    top-id=colo-disk0,file.file.filename=$imagefolder/secondary-active.qcow2,\
> +    file.backing.driver=qcow2,file.backing.file.filename=$imagefolder/secondary-hidden.qcow2,\
> +    file.backing.backing=parent0 \
> +       -drive if=ide,id=colo-disk0,driver=quorum,read-pattern=fifo,vote-threshold=1,\
> +    children.0=childs0 \
> +       -incoming tcp:0.0.0.0:9998
> +
> +
> +**3.** On Secondary VM's QEMU monitor, issue command::
> +
> +    {"execute":"qmp_capabilities"}
> +    {"execute": "migrate-set-capabilities", "arguments": {"capabilities": [ {"capability": "x-colo", "state": true } ] } }
> +    {"execute": "nbd-server-start", "arguments": {"addr": {"type": "inet", "data": {"host": "0.0.0.0", "port": "9999"} } } }
> +    {"execute": "nbd-server-add", "arguments": {"device": "parent0", "writable": true } }
> +
> +Note:
> +  a. The qmp command ``nbd-server-start`` and ``nbd-server-add`` must be run
> +     before running the qmp command migrate on primary QEMU
> +  b. Active disk, hidden disk and nbd target's length should be the
> +     same.
> +  c. It is better to put active disk and hidden disk in ramdisk. They
> +     will be merged into the parent disk on failover.
> +
> +**4.** On Primary VM's QEMU monitor, issue command::
> +
> +    {"execute":"qmp_capabilities"}
> +    {"execute": "human-monitor-command", "arguments": {"command-line": "drive_add -n buddy driver=replication,mode=primary,file.driver=nbd,file.host=127.0.0.2,file.port=9999,file.export=parent0,node-name=replication0"}}
> +    {"execute": "x-blockdev-change", "arguments":{"parent": "colo-disk0", "node": "replication0" } }
> +    {"execute": "migrate-set-capabilities", "arguments": {"capabilities": [ {"capability": "x-colo", "state": true } ] } }
> +    {"execute": "migrate", "arguments": {"uri": "tcp:127.0.0.2:9998" } }
> +
> +Note:
> +  a. There should be only one NBD Client for each primary disk.
> +  b. The qmp command line must be run after running qmp command line in
> +     secondary qemu.
> +
> +**5.** After the above steps, you will see, whenever you make changes to PVM, SVM will be synced.
> +You can issue command ``{ "execute": "migrate-set-parameters" , "arguments":{ "x-checkpoint-delay": 2000 } }``
> +to change the idle checkpoint period time
> +
> +Failover test
> +^^^^^^^^^^^^^
> +You can kill one of the VMs and Failover on the surviving VM:
> +
> +If you killed the Secondary, then follow "Primary Failover".
> +After that, if you want to resume the replication, follow "Primary resume replication"
> +
> +If you killed the Primary, then follow "Secondary Failover".
> +After that, if you want to resume the replication, follow "Secondary resume replication"
> +
> +Primary Failover
> +~~~~~~~~~~~~~~~~
> +The Secondary died, resume on the Primary::
> +
> +    {"execute": "x-blockdev-change", "arguments":{ "parent": "colo-disk0", "child": "children.1"} }
> +    {"execute": "human-monitor-command", "arguments":{ "command-line": "drive_del replication0" } }
> +    {"execute": "object-del", "arguments":{ "id": "comp0" } }
> +    {"execute": "object-del", "arguments":{ "id": "iothread1" } }
> +    {"execute": "object-del", "arguments":{ "id": "m0" } }
> +    {"execute": "object-del", "arguments":{ "id": "redire0" } }
> +    {"execute": "object-del", "arguments":{ "id": "redire1" } }
> +    {"execute": "x-colo-lost-heartbeat" }
> +
> +Secondary Failover
> +~~~~~~~~~~~~~~~~~~
> +The Primary died, resume on the Secondary and prepare to become the new Primary::
> +
> +    {"execute": "nbd-server-stop"}
> +    {"execute": "x-colo-lost-heartbeat"}
> +
> +    {"execute": "object-del", "arguments":{ "id": "f2" } }
> +    {"execute": "object-del", "arguments":{ "id": "f1" } }
> +    {"execute": "chardev-remove", "arguments":{ "id": "red1" } }
> +    {"execute": "chardev-remove", "arguments":{ "id": "red0" } }
> +
> +    {"execute": "chardev-add", "arguments":{ "id": "mirror0", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "0.0.0.0", "port": "9003" } }, "server": true } } } }
> +    {"execute": "chardev-add", "arguments":{ "id": "compare1", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "0.0.0.0", "port": "9004" } }, "server": true } } } }
> +    {"execute": "chardev-add", "arguments":{ "id": "compare0", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "127.0.0.1", "port": "9001" } }, "server": true } } } }
> +    {"execute": "chardev-add", "arguments":{ "id": "compare0-0", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "127.0.0.1", "port": "9001" } }, "server": false } } } }
> +    {"execute": "chardev-add", "arguments":{ "id": "compare_out", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "127.0.0.1", "port": "9005" } }, "server": true } } } }
> +    {"execute": "chardev-add", "arguments":{ "id": "compare_out0", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "127.0.0.1", "port": "9005" } }, "server": false } } } }
> +
> +Primary resume replication
> +~~~~~~~~~~~~~~~~~~~~~~~~~~
> +Resume replication after new Secondary is up.
> +
> +Start the new Secondary (Steps 2 and 3 above), then on the Primary::
> +
> +    {"execute": "drive-mirror", "arguments":{ "device": "colo-disk0", "job-id": "resync", "target": "nbd://127.0.0.2:9999/parent0", "mode": "existing", "format": "raw", "sync": "full"} }
> +
> +Wait until disk is synced, then::
> +
> +    {"execute": "stop"}
> +    {"execute": "block-job-cancel", "arguments":{ "device": "resync"} }
> +
> +    {"execute": "human-monitor-command", "arguments":{ "command-line": "drive_add -n buddy driver=replication,mode=primary,file.driver=nbd,file.host=127.0.0.2,file.port=9999,file.export=parent0,node-name=replication0"}}
> +    {"execute": "x-blockdev-change", "arguments":{ "parent": "colo-disk0", "node": "replication0" } }
> +
> +    {"execute": "object-add", "arguments":{ "qom-type": "filter-mirror", "id": "m0", "netdev": "hn0", "queue": "tx", "outdev": "mirror0" } }
> +    {"execute": "object-add", "arguments":{ "qom-type": "filter-redirector", "id": "redire0", "netdev": "hn0", "queue": "rx", "indev": "compare_out" } }
> +    {"execute": "object-add", "arguments":{ "qom-type": "filter-redirector", "id": "redire1", "netdev": "hn0", "queue": "rx", "outdev": "compare0" } }
> +    {"execute": "object-add", "arguments":{ "qom-type": "iothread", "id": "iothread1" } }
> +    {"execute": "object-add", "arguments":{ "qom-type": "colo-compare", "id": "comp0", "primary_in": "compare0-0", "secondary_in": "compare1", "outdev": "compare_out0", "iothread": "iothread1" } }
> +
> +    {"execute": "migrate-set-capabilities", "arguments":{ "capabilities": [ {"capability": "x-colo", "state": true } ] } }
> +    {"execute": "migrate", "arguments":{ "uri": "tcp:127.0.0.2:9998" } }
> +
> +Note:
> +If this Primary previously was a Secondary, then we need to insert the
> +filters before the filter-rewriter by using the
> +""insert": "before", "position": "id=rew0"" Options. See below.
> +
> +Secondary resume replication
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +Become Primary and resume replication after new Secondary is up. Note
> +that now 127.0.0.1 is the Secondary and 127.0.0.2 is the Primary.
> +
> +Start the new Secondary (Steps 2 and 3 above, but with primary_ip=127.0.0.2),
> +then on the old Secondary::
> +
> +    {"execute": "drive-mirror", "arguments":{ "device": "colo-disk0", "job-id": "resync", "target": "nbd://127.0.0.1:9999/parent0", "mode": "existing", "format": "raw", "sync": "full"} }
> +
> +Wait until disk is synced, then::
> +
> +    {"execute": "stop"}
> +    {"execute": "block-job-cancel", "arguments":{ "device": "resync" } }
> +
> +    {"execute": "human-monitor-command", "arguments":{ "command-line": "drive_add -n buddy driver=replication,mode=primary,file.driver=nbd,file.host=127.0.0.1,file.port=9999,file.export=parent0,node-name=replication0"}}
> +    {"execute": "x-blockdev-change", "arguments":{ "parent": "colo-disk0", "node": "replication0" } }
> +
> +    {"execute": "object-add", "arguments":{ "qom-type": "filter-mirror", "id": "m0", "insert": "before", "position": "id=rew0", "netdev": "hn0", "queue": "tx", "outdev": "mirror0" } }
> +    {"execute": "object-add", "arguments":{ "qom-type": "filter-redirector", "id": "redire0", "insert": "before", "position": "id=rew0", "netdev": "hn0", "queue": "rx", "indev": "compare_out" } }
> +    {"execute": "object-add", "arguments":{ "qom-type": "filter-redirector", "id": "redire1", "insert": "before", "position": "id=rew0", "netdev": "hn0", "queue": "rx", "outdev": "compare0" } }
> +    {"execute": "object-add", "arguments":{ "qom-type": "iothread", "id": "iothread1" } }
> +    {"execute": "object-add", "arguments":{ "qom-type": "colo-compare", "id": "comp0", "primary_in": "compare0-0", "secondary_in": "compare1", "outdev": "compare_out0", "iothread": "iothread1" } }
> +
> +    {"execute": "migrate-set-capabilities", "arguments":{ "capabilities": [ {"capability": "x-colo", "state": true } ] } }
> +    {"execute": "migrate", "arguments":{ "uri": "tcp:127.0.0.1:9998" } }
> +
> +TODO
> +----
> +1. Support shared storage.
> +2. Develop the heartbeat part.
> +3. Reduce checkpoint VM’s downtime while doing checkpoint.

Reviewed-by: Fabiano Rosas <farosas@suse.de>