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Re: [Qemu-block] [RFC PATCH COLO v2 01/13] docs: block replication's des
From: |
Fam Zheng |
Subject: |
Re: [Qemu-block] [RFC PATCH COLO v2 01/13] docs: block replication's description |
Date: |
Thu, 26 Mar 2015 14:31:45 +0800 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Wed, 03/25 17:36, Wen Congyang wrote:
> Signed-off-by: Wen Congyang <address@hidden>
> Signed-off-by: Paolo Bonzini <address@hidden>
> Signed-off-by: Yang Hongyang <address@hidden>
> Signed-off-by: zhanghailiang <address@hidden>
> Signed-off-by: Gonglei <address@hidden>
> ---
> 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
Re: [Qemu-block] [RFC PATCH COLO v2 01/13] docs: block replication's description,
Fam Zheng <=
[Qemu-block] [RFC PATCH COLO v2 02/13] quorum: allow ignoring child errors, Wen Congyang, 2015/03/25
[Qemu-block] [RFC PATCH COLO v2 06/13] NBD client: implement block driver interfaces for block replication, Wen Congyang, 2015/03/25