From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55787) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z5VSB-0004jX-00 for qemu-devel@nongnu.org; Thu, 18 Jun 2015 04:45:45 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Z5VS6-0003lf-P1 for qemu-devel@nongnu.org; Thu, 18 Jun 2015 04:45:42 -0400 From: Wen Congyang Date: Thu, 18 Jun 2015 16:49:06 +0800 Message-ID: <1434617361-17778-2-git-send-email-wency@cn.fujitsu.com> In-Reply-To: <1434617361-17778-1-git-send-email-wency@cn.fujitsu.com> References: <1434617361-17778-1-git-send-email-wency@cn.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain Subject: [Qemu-devel] [PATCH COLO-Block v6 01/16] docs: block replication's description List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu devel , Fam Zheng , Max Reitz , Paolo Bonzini Cc: Kevin Wolf , qemu block , Lai Jiangshan , Jiang Yunhong , Dong Eddie , "Dr. David Alan Gilbert" , Gonglei , Stefan Hajnoczi , Yang Hongyang , zhanghailiang Signed-off-by: Wen Congyang Signed-off-by: Yang Hongyang Signed-off-by: zhanghailiang Signed-off-by: Gonglei --- docs/block-replication.txt | 179 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 179 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..a29f51a --- /dev/null +++ b/docs/block-replication.txt @@ -0,0 +1,179 @@ +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. + +Block replication is used for continuous checkpoints. It is designed +for COLO (COurse-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 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(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 + +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 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 be 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 be an empty disk, and +the driver supports bdrv_make_empty() and backing file. + +== Failure Handling == +There are 6 internal errors when block replication is running: +1. I/O error on primary disk +2. Forwarding primay write requests failed +3. Bacup failed or writing to secondary disk failed +4. I/O error on secondary disk +5. I/O error on active disk +6. Making active disk or hidden disk empty 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). +There is one internal error when doing failover: +1. Commiting the data in active disk/hidden disk to secondary disk failed +We just to report this error to FT/HA manager. + +== 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. The caller + must hold the I/O mutex lock if it is in migration/checkpoint + thread. +b. bdrv_do_checkpoint() + 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. bdrv_stop_replication() + 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. 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,no-connect=on,\ + children.0.file.filename=1.raw,\ + children.0.driver=raw,\ + children.1.file.driver=nbd,\ + children.1.file.host=xxx,\ + children.1.file.port=xxx,\ + children.1.file.export=xxx,\ + children.1.driver=replication,\ + children.1.mode=primary,\ + children.1.ignore-errors=on + 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. + 5. It is all a single argument to -drive, and you should ignore + the leading whitespace. + +Secondary: + -drive if=none,driver=raw,file=1.raw,id=nbd_target1 \ + -drive if=xxx,driver=replication,mode=secondary,export=xxx,\ + file.file.filename=active_disk.qcow2,\ + file.driver=qcow2,\ + file.backing_reference.drive_id=nbd_target1,\ + file.backing_reference.hidden-disk.file.filename=hidden_disk.qcow2,\ + file.backing_reference.hidden-disk.driver=qcow2,\ + file.backing_reference.hidden-disk.allow-write-backing-file=on + 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 + 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. -- 2.4.3