From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40746) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YsrBC-0004oM-Im for qemu-devel@nongnu.org; Thu, 14 May 2015 07:19:56 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YsrBA-0001du-Pn for qemu-devel@nongnu.org; Thu, 14 May 2015 07:19:54 -0400 Date: Thu, 14 May 2015 12:19:39 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20150514111937.GD2576@work-vm> References: <1431076567-30371-1-git-send-email-wency@cn.fujitsu.com> <1431076567-30371-2-git-send-email-wency@cn.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1431076567-30371-2-git-send-email-wency@cn.fujitsu.com> Subject: Re: [Qemu-devel] [PATCH COLO v4 01/15] docs: block replication's description List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Wen Congyang Cc: Kevin Wolf , Fam Zheng , Lai Jiangshan , qemu block , Jiang Yunhong , Dong Eddie , qemu devel , Max Reitz , Gonglei , Stefan Hajnoczi , Paolo Bonzini , Yang Hongyang , zhanghailiang * Wen Congyang (wency@cn.fujitsu.com) wrote: > 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 Typo: Bacup -> Backup > +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. One thing I wanted to check I understand; how much RAM do the active and hidden disks use; lets say during the 1st checkpoint 10MB of disk is written, and during hte 2nd checkpoint a different 10MB of the disk is written (to different locations on the disk); how big are the active and hidden disks in RAM - are they 10MB or 20MB? Dave > + > +== 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. > -- > 2.1.0 > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK