docs: Add documentation for COLO feature

Introduce the design of COLO, and how to test it.
Signed-off-by: Nzhanghailiang <zhang.zhanghailiang@huawei.com>
Reviewed-by: NAmit Shah <amit.shah@redhat.com>
Signed-off-by: NAmit Shah <amit@amitshah.net>

docs/COLO-FT.txt

+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-----------+  |    +-------------+
+ |  VM Monitor |    | |  COLO Proxy      |       |    COLO Proxy    |  |    | VM Monitor  |
+ |             |    | |(compare packet)  |       | (adjust sequence)|  |    |             |
+ +-------------+    | +----------|----^--+       +------------------+  |    +-------------+
+                    |            |    |                                |
+ +------------------|------------|----|--+       +---------------------|------------------+
+ |   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 manger 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 Seconday, 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 informations.
+
+Note:
+HeartBeat has not been implemented yet, so you need to trigger failover process
+by using 'x-colo-lost-heartbeat' command.
+
+== Test procedure ==
+1. Startup qemu
+Primary:
+# qemu-kvm -enable-kvm -m 2048 -smp 2 -qmp stdio -vnc :7 -name primary \
+  -device piix3-usb-uhci \
+  -device usb-tablet -netdev tap,id=hn0,vhost=off \
+  -device virtio-net-pci,id=net-pci0,netdev=hn0 \
+  -drive if=virtio,id=primary-disk0,driver=quorum,read-pattern=fifo,vote-threshold=1,\
+         children.0.file.filename=1.raw,\
+         children.0.driver=raw -S
+Secondary:
+# qemu-kvm -enable-kvm -m 2048 -smp 2 -qmp stdio -vnc :7 -name secondary \
+  -device piix3-usb-uhci \
+  -device usb-tablet -netdev tap,id=hn0,vhost=off \
+  -device virtio-net-pci,id=net-pci0,netdev=hn0 \
+  -drive if=none,id=secondary-disk0,file.filename=1.raw,driver=raw,node-name=node0 \
+  -drive if=virtio,id=active-disk0,driver=replication,mode=secondary,\
+         file.driver=qcow2,top-id=active-disk0,\
+         file.file.filename=/mnt/ramfs/active_disk.img,\
+         file.backing.driver=qcow2,\
+         file.backing.file.filename=/mnt/ramfs/hidden_disk.img,\
+         file.backing.backing=secondary-disk0 \
+  -incoming tcp:0:8888
+
+2. On Secondary VM's QEMU monitor, issue command
+{'execute':'qmp_capabilities'}
+{ 'execute': 'nbd-server-start',
+  'arguments': {'addr': {'type': 'inet', 'data': {'host': 'xx.xx.xx.xx', 'port': '8889'} } }
+}
+{'execute': 'nbd-server-add', 'arguments': {'device': 'secondeary-disk0', '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.
+
+3. 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=xx.xx.xx.xx,file.port=8889,file.export=secondary-disk0,node-name=nbd_client0'}}
+{ 'execute':'x-blockdev-change', 'arguments':{'parent': 'primary-disk0', 'node': 'nbd_client0' } }
+{ 'execute': 'migrate-set-capabilities',
+      'arguments': {'capabilities': [ {'capability': 'x-colo', 'state': true } ] } }
+{ 'execute': 'migrate', 'arguments': {'uri': 'tcp:xx.xx.xx.xx:8888' } }
+
+  Note:
+  a. There should be only one NBD Client for each primary disk.
+  b. xx.xx.xx.xx is the secondary physical machine's hostname or IP
+  c. The qmp command line must be run after running qmp command line in
+     secondary qemu.
+
+4. 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 checkpoint period time
+
+5. Failover test
+You can kill Primary VM and run 'x_colo_lost_heartbeat' in Secondary VM's
+monitor at the same time, then SVM will failover and client will not detect this
+change.
+
+Before issuing '{ "execute": "x-colo-lost-heartbeat" }' command, we have to
+issue block related command to stop block replication.
+Primary:
+  Remove the nbd child from the quorum:
+  { 'execute': 'x-blockdev-change', 'arguments': {'parent': 'colo-disk0', 'child': 'children.1'}}
+  { 'execute': 'human-monitor-command','arguments': {'command-line': 'drive_del blk-buddy0'}}
+  Note: there is no qmp command to remove the blockdev now
+
+Secondary:
+  The primary host is down, so we should do the following thing:
+  { 'execute': 'nbd-server-stop' }
+
+== TODO ==
+1. Support continuous VM replication.
+2. Support shared storage.
+3. Develop the heartbeat part.
+4. Reduce checkpoint VM’s downtime while doing checkpoint.