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Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command

From: Wenchao Xia
Subject: Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command
Date: Thu, 16 May 2013 14:16:20 +0800
User-agent: Mozilla/5.0 (Windows NT 5.1; rv:17.0) Gecko/20130328 Thunderbird/17.0.5

于 2013-5-15 22:34, Stefan Hajnoczi 写道:
> Note: These patches apply to my block-next tree.  You can also grab the code
> from git here:
> git://github.com/stefanha/qemu.git block-backup-core
> This series adds a new QMP command, drive-backup, which takes a point-in-time
> snapshot of a block device.  The snapshot is copied out to a target block
> device.  A simple example is:
>    drive-backup device=virtio0 format=qcow2 target=backup-20130401.qcow2
> The original drive-backup blockjob was written by Dietmar Maurer
> <address@hidden>.  He is currently busy but I feel the feature is worth
> pushing into QEMU since there has been interest.  This is my version of his
> patch, plus the QMP command and qemu-iotests test case.
> QMP 'transaction' support is included since v3.  It adds support for atomic
> snapshots of multiple block devices.  I also added an 'abort' transaction to
> allow testing of the .abort()/.cleanup() code path.  Thanks to Wenchao for
> making qmp_transaction() extensible.
> How is this different from block-stream and drive-mirror?
> ---------------------------------------------------------
> Both block-stream and drive-mirror do not provide immediate point-in-time
> snapshots.  Instead they copy data into a new file and then switch to it.  In
> other words, the point at which the "snapshot" is taken cannot be controlled
> directly.
> drive-backup intercepts guest writes and saves data into the target block
> device before it is overwritten.  The target block device can be a raw image
> file, backing files are not used to implement this feature.
> How can drive-backup be used?
> -----------------------------
> The simplest use-case is to copy a point-in-time snapshot to a local file.
> More advanced users may wish to make the target an NBD URL.  The NBD server
> listening on the other side can process the backup writes any way it wishes.  
> I
> previously posted an RFC series with a backup server that streamed Dietmar's
> VMA backup archive format.
> What's next for drive-backup?
> -----------------------------
> 1. Sync modes like drive-mirror (top, full, none).  This makes it possible to
>     preserve the backing file chain.
> v3:
>   * Rename to drive-backup for consistency with drive-mirror [kwolf]
>   * Add QMP transaction support [kwolf]
>   * Introduce bdrv_add_before_write_cb() to hook writes
>   * Mention 'query-block-jobs' lists job of type 'backup' [eblake]
>   * Rename rwlock to flush_rwlock [kwolf]
>   * Fix space in block/backup.c comment [kwolf]
> v2:
>   * s/block_backup/block-backup/ in commit message [eblake]
>   * Avoid funny spacing in QMP docs [eblake]
>   * Document query-block-jobs and block-job-cancel usage [eblake]

  After checking the code, I found it possible to add delta data backup
support also, If an additional dirty bitmap was added. Compared with
current solution, I think it is doing COW at qemu device level:

    qemu device
general block layer
virtual format layer
|                     |
qcow2             vmdk....

  This will make things complicated when more works comes, a better
place for block COW, is under general block layer. Maybe later we
can adjust block for it.

Best Regards

Wenchao Xia

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