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[Qemu-devel] [RFC PATCH 01/14] docs: block replication's description
From: |
Wen Congyang |
Subject: |
[Qemu-devel] [RFC PATCH 01/14] docs: block replication's description |
Date: |
Thu, 12 Feb 2015 11:07:05 +0800 |
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 | 129 +++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 129 insertions(+)
create mode 100644 docs/block-replication.txt
diff --git a/docs/block-replication.txt b/docs/block-replication.txt
new file mode 100644
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@@ -0,0 +1,129 @@
+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.
+ 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 <--------- COLO buffer 3
+--------' || backing
+
+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
+("COLO buffer"). The disk buffer's backing image is the secondary disk,
+and the disk buffer uses bdrv_add_before_write_notifier to implement
+copy-on-write, similar to block/backup.c.
+
+== 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.
+c. bdrv_stop_replication()
+ It is called when failover. We will flush the Disk buffer into
+ Secondary Disk and stop block replication.
+
+== Usage ==
+Primary:
+ -drive if=xxx,driver=quorum,read-pattern=first,\
+ 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
+ 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=xxx,driver=blkcolo,export=xxx,\
+ backing.file.filename=1.raw,\
+ backing.driver=raw
+ Then run qmp command:
+ nbd_server_start host:port
+ 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
--
2.1.0
- [Qemu-devel] [RFC PATCH 00/14] Block replication for continuous checkpoints, Wen Congyang, 2015/02/11
- [Qemu-devel] [RFC PATCH 02/14] quorom: add a new read pattern, Wen Congyang, 2015/02/11
- [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description,
Wen Congyang <=
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Fam Zheng, 2015/02/12
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Wen Congyang, 2015/02/12
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Fam Zheng, 2015/02/12
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Wen Congyang, 2015/02/12
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Fam Zheng, 2015/02/12
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Wen Congyang, 2015/02/12
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, famz, 2015/02/12
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Wen Congyang, 2015/02/13
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, Fam Zheng, 2015/02/13
- Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description, John Snow, 2015/02/13