COarse-grained LOck-stepping Virtual Machines for Non-stop Service
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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 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 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 ==
1. Startup qemu
Primary:
# qemu-system-x86_64 -accel kvm -m 2048 -smp 2 -qmp stdio -name primary \
  -device piix3-usb-uhci -vnc :7 \
  -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-system-x86_64 -accel kvm -m 2048 -smp 2 -qmp stdio -name secondary \
  -device piix3-usb-uhci -vnc :7 \
  -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': 'secondary-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.