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

From: Stefan Hajnoczi
Subject: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command
Date: Wed, 15 May 2013 16:34:28 +0200

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

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.

 * 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]

 * 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]

Dietmar Maurer (1):
  block: add basic backup support to block driver

Stefan Hajnoczi (7):
  block: add bdrv_add_before_write_cb()
  block: add drive-backup QMP command
  qemu-iotests: add 055 drive-backup test case
  blockdev: rename BlkTransactionStates to singular
  blockdev: add DriveBackup transaction
  blockdev: add Abort transaction
  qemu-iotests: test 'drive-backup' transaction in 055

 block.c                    |  37 +++++
 block/Makefile.objs        |   1 +
 block/backup.c             | 282 ++++++++++++++++++++++++++++++++++++
 blockdev.c                 | 264 +++++++++++++++++++++++++++-------
 include/block/block_int.h  |  48 +++++++
 qapi-schema.json           |  65 ++++++++-
 qmp-commands.hx            |   6 +
 tests/qemu-iotests/055     | 348 +++++++++++++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/055.out |   5 +
 tests/qemu-iotests/group   |   1 +
 10 files changed, 1004 insertions(+), 53 deletions(-)
 create mode 100644 block/backup.c
 create mode 100755 tests/qemu-iotests/055
 create mode 100644 tests/qemu-iotests/055.out


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