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 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
