[Qemu-devel] [PATCH v2 0/3] block: block-backup live backup command

[Stefan Hajnoczi]

This series adds a new QMP command, block-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:

  block-backup device=virtio0 format=qcow2 target=backup-20130401.qcow2

The original block-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.

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.

block-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 block-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 block-backup?
-----------------------------
The following enhancements are left for future patches:

1. QMP 'transaction' support.  It is handy to atomically snapshot multiple
   block devices.  We need qmp_transaction() support for this.  Wenchao Xia is
   currently making qmp_transaction() extensible so new action types, like
   block-backup, can be added.

2. Sync modes like drive-mirror (top, full, none).  This makes it possible to
   preserve the backing file chain.

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]

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

Stefan Hajnoczi (2):
  block: add block-backup QMP command
  qemu-iotests: add 054 block-backup test case

 block.c                    |  69 ++++++++++++-
 block/Makefile.objs        |   1 +
 block/backup.c             | 252 +++++++++++++++++++++++++++++++++++++++++++++
 blockdev.c                 |  92 +++++++++++++++++
 include/block/block.h      |   2 +
 include/block/block_int.h  |  16 +++
 include/block/blockjob.h   |  10 ++
 qapi-schema.json           |  31 ++++++
 qmp-commands.hx            |   6 ++
 tests/qemu-iotests/054     | 230 +++++++++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/054.out |   5 +
 tests/qemu-iotests/group   |   1 +
 12 files changed, 710 insertions(+), 5 deletions(-)
 create mode 100644 block/backup.c
 create mode 100755 tests/qemu-iotests/054
 create mode 100644 tests/qemu-iotests/054.out

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1.8.1.4