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248 lines
11 KiB
Plaintext
Block replication
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----------------------------------------
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Copyright Fujitsu, Corp. 2016
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Copyright (c) 2016 Intel Corporation
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Copyright (c) 2016 HUAWEI TECHNOLOGIES CO., LTD.
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This work is licensed under the terms of the GNU GPL, version 2 or later.
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See the COPYING file in the top-level directory.
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Block replication is used for continuous checkpoints. It is designed
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for COLO (COarse-grain LOck-stepping) where the Secondary VM is running.
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It can also be applied for FT/HA (Fault-tolerance/High Assurance) scenario,
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where the Secondary VM is not running.
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This document gives an overview of block replication's design.
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== Background ==
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High availability solutions such as micro checkpoint and COLO will do
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consecutive checkpoints. The VM state of the Primary and Secondary VM is
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identical right after a VM checkpoint, but becomes different as the VM
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executes till the next checkpoint. To support disk contents checkpoint,
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the modified disk contents in the Secondary VM must be buffered, and are
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only dropped at next checkpoint time. To reduce the network transportation
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effort during a vmstate checkpoint, the disk modification operations of
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the Primary disk are asynchronously forwarded to the Secondary node.
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== Workflow ==
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The following is the image of block replication workflow:
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+----------------------+ +------------------------+
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|Primary Write Requests| |Secondary Write Requests|
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+----------------------+ +------------------------+
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| |
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| (4)
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| V
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| /-------------\
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| Copy and Forward | |
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|---------(1)----------+ | Disk Buffer |
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| | | |
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| (3) \-------------/
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| speculative ^
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| write through (2)
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| | |
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V V |
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+--------------+ +----------------+
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| Primary Disk | | Secondary Disk |
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+--------------+ +----------------+
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1) Primary write requests will be copied and forwarded to Secondary
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QEMU.
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2) Before Primary write requests are written to Secondary disk, the
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original sector content will be read from Secondary disk and
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buffered in the Disk buffer, but it will not overwrite the existing
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sector content (it could be from either "Secondary Write Requests" or
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previous COW of "Primary Write Requests") in the Disk buffer.
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3) Primary write requests will be written to Secondary disk.
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4) Secondary write requests will be buffered in the Disk buffer and it
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will overwrite the existing sector content in the buffer.
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== Architecture ==
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We are going to implement block replication from many basic
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blocks that are already in QEMU.
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virtio-blk ||
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^ || .----------
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| || | Secondary
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1 Quorum || '----------
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/ \ || virtio-blk
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/ \ || ^
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Primary 2 filter |
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disk ^ 7 Quorum
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| /
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3 NBD -------> 3 NBD /
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client || server 2 filter
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|| ^ ^
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--------. || | |
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Primary | || Secondary disk <--------- hidden-disk 5 <--------- active-disk 4
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--------' || | backing ^ backing
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|| | |
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|| | |
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|| '-------------------------'
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|| blockdev-backup sync=none 6
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1) The disk on the primary is represented by a block device with two
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children, providing replication between a primary disk and the host that
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runs the secondary VM. The read pattern (fifo) for quorum can be extended
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to make the primary always read from the local disk instead of going through
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NBD.
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2) The new block filter (the name is replication) will control the block
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replication.
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3) The secondary disk receives writes from the primary VM through QEMU's
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embedded NBD server (speculative write-through).
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4) The disk on the secondary is represented by a custom block device
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(called active-disk). It should start as an empty disk, and the format
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should support bdrv_make_empty() and backing file.
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5) The hidden-disk is created automatically. It buffers the original content
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that is modified by the primary VM. It should also start as an empty disk,
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and the driver supports bdrv_make_empty() and backing file.
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6) The blockdev-backup job (sync=none) is run to allow hidden-disk to buffer
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any state that would otherwise be lost by the speculative write-through
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of the NBD server into the secondary disk. So before block replication,
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the primary disk and secondary disk should contain the same data.
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7) The secondary also has a quorum node, so after secondary failover it
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can become the new primary and continue replication.
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== Failure Handling ==
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There are 7 internal errors when block replication is running:
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1. I/O error on primary disk
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2. Forwarding primary write requests failed
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3. Backup failed
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4. I/O error on secondary disk
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5. I/O error on active disk
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6. Making active disk or hidden disk empty failed
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7. Doing failover failed
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In case 1 and 5, we just report the error to the disk layer. In case 2, 3,
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4 and 6, we just report block replication's error to FT/HA manager (which
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decides when to do a new checkpoint, when to do failover).
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In case 7, if active commit failed, we use replication failover failed state
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in Secondary's write operation (what decides which target to write).
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== New block driver interface ==
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We add four block driver interfaces to control block replication:
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a. replication_start_all()
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Start block replication, called in migration/checkpoint thread.
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We must call block_replication_start_all() in secondary QEMU before
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calling block_replication_start_all() in primary QEMU. The caller
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must hold the I/O mutex lock if it is in migration/checkpoint
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thread.
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b. replication_do_checkpoint_all()
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This interface is called after all VM state is transferred to
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Secondary QEMU. The Disk buffer will be dropped in this interface.
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The caller must hold the I/O mutex lock if it is in migration/checkpoint
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thread.
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c. replication_get_error_all()
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This interface is called to check if error happened in replication.
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The caller must hold the I/O mutex lock if it is in migration/checkpoint
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thread.
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d. replication_stop_all()
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It is called on failover. We will flush the Disk buffer into
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Secondary Disk and stop block replication. The vm should be stopped
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before calling it if you use this API to shutdown the guest, or other
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things except failover. The caller must hold the I/O mutex lock if it is
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in migration/checkpoint thread.
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== Usage ==
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Primary:
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-drive if=xxx,driver=quorum,read-pattern=fifo,id=colo1,vote-threshold=1,\
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children.0.file.filename=1.raw,\
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children.0.driver=raw
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Run qmp command in primary qemu:
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{ "execute": "human-monitor-command",
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"arguments": {
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"command-line": "drive_add -n buddy driver=replication,mode=primary,file.driver=nbd,file.host=xxxx,file.port=xxxx,file.export=colo1,node-name=nbd_client1"
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}
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}
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{ "execute": "x-blockdev-change",
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"arguments": {
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"parent": "colo1",
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"node": "nbd_client1"
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}
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}
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Note:
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1. There should be only one NBD Client for each primary disk.
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2. host is the secondary physical machine's hostname or IP
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3. Each disk must have its own export name.
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4. It is all a single argument to -drive and you should ignore the
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leading whitespace.
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5. The qmp command line must be run after running qmp command line in
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secondary qemu.
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6. After primary failover we need remove children.1 (replication driver).
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Secondary:
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-drive if=none,driver=raw,file.filename=1.raw,id=colo1 \
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-drive if=none,id=childs1,driver=replication,mode=secondary,top-id=top-disk1
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file.file.filename=active_disk.qcow2,\
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file.driver=qcow2,\
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file.backing.file.filename=hidden_disk.qcow2,\
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file.backing.driver=qcow2,\
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file.backing.backing=colo1
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-drive if=xxx,driver=quorum,read-pattern=fifo,id=top-disk1,\
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vote-threshold=1,children.0=childs1
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Then run qmp command in secondary qemu:
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{ "execute": "nbd-server-start",
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"arguments": {
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"addr": {
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"type": "inet",
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"data": {
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"host": "xxx",
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"port": "xxx"
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}
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}
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}
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}
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{ "execute": "nbd-server-add",
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"arguments": {
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"device": "colo1",
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"writable": true
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}
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}
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Note:
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1. The export name in secondary QEMU command line is the secondary
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disk's id.
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2. The export name for the same disk must be the same
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3. The qmp command nbd-server-start and nbd-server-add must be run
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before running the qmp command migrate on primary QEMU
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4. Active disk, hidden disk and nbd target's length should be the
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same.
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5. It is better to put active disk and hidden disk in ramdisk.
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6. It is all a single argument to -drive, and you should ignore
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the leading whitespace.
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After Failover:
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Primary:
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The secondary host is down, so we should run the following qmp command
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to remove the nbd child from the quorum:
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{ "execute": "x-blockdev-change",
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"arguments": {
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"parent": "colo1",
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"child": "children.1"
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}
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}
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{ "execute": "human-monitor-command",
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"arguments": {
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"command-line": "drive_del xxxx"
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}
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}
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Note: there is no qmp command to remove the blockdev now
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Secondary:
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The primary host is down, so we should do the following thing:
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{ "execute": "nbd-server-stop" }
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Promote Secondary to Primary:
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see COLO-FT.txt
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TODO:
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1. Shared disk
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