On 03/26/2015 02:31 PM, Fam Zheng wrote: > On Wed, 03/25 17:36, Wen Congyang wrote: >> Signed-off-by: Wen Congyang <we...@cn.fujitsu.com> >> Signed-off-by: Paolo Bonzini <pbonz...@redhat.com> >> Signed-off-by: Yang Hongyang <yan...@cn.fujitsu.com> >> Signed-off-by: zhanghailiang <zhang.zhanghaili...@huawei.com> >> Signed-off-by: Gonglei <arei.gong...@huawei.com> >> --- >> docs/block-replication.txt | 147 >> +++++++++++++++++++++++++++++++++++++++++++++ >> 1 file changed, 147 insertions(+) >> create mode 100644 docs/block-replication.txt >> >> diff --git a/docs/block-replication.txt b/docs/block-replication.txt >> new file mode 100644 >> index 0000000..874ed8e >> --- /dev/null >> +++ b/docs/block-replication.txt >> @@ -0,0 +1,147 @@ >> +Block replication >> +---------------------------------------- >> +Copyright Fujitsu, Corp. 2015 >> +Copyright (c) 2015 Intel Corporation >> +Copyright (c) 2015 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. >> + >> +The block replication is used for continuous checkpoints. It is designed >> +for COLO that Secondary VM is running. It can also be applied for FT/HA >> +scene that 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 checkpoint. The VM state of Primary VM 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 at the time of checkpoint, the disk modification operations of >> +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 in the Disk buffer. > > Could you elaborate a bit about the "existing sector content" here? IIUC, it > could be from either "Secondary Write Requests" or previous COW of "Primary > Write Requests". Is that right? > >> + 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 COLO block replication from many basic >> +blocks that are already in QEMU. >> + >> + virtio-blk || >> + ^ || .---------- >> + | || | Secondary >> + 1 Quorum || '---------- >> + / \ || >> + / \ || >> + Primary 2 NBD -------> 2 NBD >> + disk client || server >> virtio-blk >> + || ^ >> ^ >> +--------. || | >> | >> +Primary | || Secondary disk <--------- hidden-disk 4 >> <--------- active-disk 3 >> +--------' || | backing ^ >> backing >> + || | | >> + || | | >> + || '-------------------------' >> + || drive-backup sync=none >> + >> +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 for quorum can be extended to >> +make the primary always read from the local disk instead of going through >> +NBD. >> + >> +2) The secondary disk receives writes from the primary VM through QEMU's >> +embedded NBD server (speculative write-through). >> + >> +3) The disk on the secondary is represented by a custom block device >> +(called active-disk). It should be an empty disk, and the format should >> +be qcow2. >> + >> +4) The hidden-disk is created automatically. It buffers the original content >> +that is modified by the primary VM. It should also be an empty disk, and >> +the dirver supports bdrv_make_empty(). >> + >> +== New block driver interface == >> +We add three block driver interfaces to control block replication: >> +a. bdrv_start_replication() >> + Start block replication, called in migration/checkpoint thread. >> + We must call bdrv_start_replication() in secondary QEMU before >> + calling bdrv_start_replication() in primary QEMU. >> +b. bdrv_do_checkpoint() >> + This interface is called after all VM state is transfered 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. bdrv_stop_replication() >> + It is called when failover. We will flush the Disk buffer into >> + Secondary Disk and stop block replication. The vm should be stopped >> + before calling it. 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,\ >> + children.0.file.filename=1.raw,\ >> + children.0.driver=raw,\ >> + children.1.file.driver=nbd+colo,\ >> + children.1.file.host=xxx,\ >> + children.1.file.port=xxx,\ >> + children.1.file.export=xxx,\ >> + children.1.driver=raw,\ >> + children.1.ignore-errors=on > ^^^^^^^^^ > Won't the leading spaces cause trouble? :) > >> + Note: >> + 1. NBD Client should not be the first child of quorum. >> + 2. There should be only one NBD Client. >> + 3. host is the secondary physical machine's hostname or IP >> + 4. Each disk must have its own export name. >> + >> +Secondary: >> + -drive if=none,driver=raw,file=1.raw,id=nbd_target1 \ >> + -drive if=xxx,driver=qcow2+colo,file=active_disk.qcow2,export=xxx,\ >> + backing_reference.drive_id=nbd_target1,\ >> + backing_reference.hidden-disk.file.filename=hidden_disk.qcow2,\ >> + backing_reference.hidden-disk.driver=qcow2,\ >> + backing_reference.hidden-disk.allow-write-backing-file=on >> + Then run qmp command: >> + nbd_server_start host:port > > s/nbd_server_start/nbd-server-start
The command name in qmp-commands.hx is nbd-server-start, but (qemu) nbd-server-start 192.168.3.1:8889 unknown command: 'nbd-server-start' What's wrong? Thanks Wen Congyang > > For a few more too. > > >> + Note: >> + 1. The export name for the same disk must be the same in primary >> + and secondary QEMU command line >> + 2. The qmp command nbd_server_start must be run before running the >> + qmp command migrate on primary QEMU >> + 3. Don't use nbd_server_start's other options >> + 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. > > Fam > . >
