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diff --git a/docs/block-replication.txt b/docs/block-replication.txt new file mode 100644 index 0000000000..6bde6737fb --- /dev/null +++ b/docs/block-replication.txt @@ -0,0 +1,239 @@ +Block replication +---------------------------------------- +Copyright Fujitsu, Corp. 2016 +Copyright (c) 2016 Intel Corporation +Copyright (c) 2016 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 (COarse-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 the Primary 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 during a vmstate checkpoint, the disk modification operations of +the 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 6 + +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 (fifo) 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 start as 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 start as an empty disk, +and the driver supports bdrv_make_empty() and backing file. + +6) The drive-backup job (sync=none) is run to allow hidden-disk to buffer +any state that would otherwise be lost by the speculative write-through +of the NBD server into the secondary disk. So before block replication, +the primary disk and secondary disk should contain the same data. + +== Failure Handling == +There are 7 internal errors when block replication is running: +1. I/O error on primary disk +2. Forwarding primary write requests failed +3. Backup failed +4. I/O error on secondary disk +5. I/O error on active disk +6. Making active disk or hidden disk empty failed +7. Doing failover 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). +In case 7, if active commit failed, we use replication failover failed state +in Secondary's write operation (what decides which target to write). + +== New block driver interface == +We add four block driver interfaces to control block replication: +a. replication_start_all() + Start block replication, called in migration/checkpoint thread. + We must call block_replication_start_all() in secondary QEMU before + calling block_replication_start_all() in primary QEMU. The caller + must hold the I/O mutex lock if it is in migration/checkpoint + thread. +b. replication_do_checkpoint_all() + 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. replication_get_error_all() + This interface is called to check if error happened in replication. + The caller must hold the I/O mutex lock if it is in migration/checkpoint + thread. +d. replication_stop_all() + 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 if you use this API to shutdown the guest, or other + things except failover. 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,id=colo1,vote-threshold=1,\ + children.0.file.filename=1.raw,\ + children.0.driver=raw + + Run qmp command in primary qemu: + { 'execute': 'human-monitor-command', + 'arguments': { + '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' + } + } + { 'execute': 'x-blockdev-change', + 'arguments': { + 'parent': 'colo1', + 'node': 'nbd_client1' + } + } + Note: + 1. There should be only one NBD Client for each primary disk. + 2. host is the secondary physical machine's hostname or IP + 3. Each disk must have its own export name. + 4. It is all a single argument to -drive and you should ignore the + leading whitespace. + 5. The qmp command line must be run after running qmp command line in + secondary qemu. + 6. After failover we need remove children.1 (replication driver). + +Secondary: + -drive if=none,driver=raw,file.filename=1.raw,id=colo1 \ + -drive if=xxx,id=topxxx,driver=replication,mode=secondary,top-id=topxxx\ + file.file.filename=active_disk.qcow2,\ + file.driver=qcow2,\ + file.backing.file.filename=hidden_disk.qcow2,\ + file.backing.driver=qcow2,\ + file.backing.backing=colo1 + + Then run qmp command in secondary qemu: + { 'execute': 'nbd-server-start', + 'arguments': { + 'addr': { + 'type': 'inet', + 'data': { + 'host': 'xxx', + 'port': 'xxx' + } + } + } + } + { 'execute': 'nbd-server-add', + 'arguments': { + 'device': 'colo1', + 'writable': true + } + } + + Note: + 1. The export name in secondary QEMU command line is the secondary + disk's id. + 2. The export name for the same disk must be the same + 3. The qmp command nbd-server-start and nbd-server-add must be run + before running the qmp command migrate on primary QEMU + 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. + +After Failover: +Primary: + The secondary host is down, so we should run the following qmp command + to remove the nbd child from the quorum: + { 'execute': 'x-blockdev-change', + 'arguments': { + 'parent': 'colo1', + 'child': 'children.1' + } + } + { 'execute': 'human-monitor-command', + 'arguments': { + 'command-line': 'drive_del xxxx' + } + } + 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. Continuous block replication +2. Shared disk |