Re: [Qemu-devel] [PATCH v13 00/14] Drop in_use from BlockDriverState and enable point-in-time snapshot exporting over NBD

[Ian Main]

On Wed, Jan 29, 2014 at 01:07:27PM +0800, Fam Zheng wrote:
> This series adds for point-in-time snapshot NBD exporting based on
> blockdev-backup (variant of drive-backup with existing device as target).
> 
> We get a thin point-in-time snapshot by COW mechanism of drive-backup, and
> export it through built in NBD server. The steps are as below:
> 
>  1. (SHELL) qemu-img create -f qcow2 BACKUP.qcow2 <source size here>
> 
>     (Alternatively we can use -o backing_file=RUNNING-VM.img to omit 
> explicitly
>     providing the size by ourselves, but it's risky because RUNNING-VM.qcow2 
> is
>     used r/w by guest. Whether or not setting backing file in the image file
>     doesn't matter, as we are going to override the backing hd in the next
>     step)
> 
>  2. (QMP) blockdev-add backing=source-drive file.driver=file 
> file.filename=BACKUP.qcow2 id=target0 if=none driver=qcow2

It seems like we have a regression here.  I sent you a private email
earlier and after some more testing I am finding that the setup that I
had to test this before is no longer working.  Now when I try to use
blockdev-add I get:

rsp = srv.cmd(command, {'options': {
                        'backing': 'ide0-hd0',
                        'driver': 'qcow2',
                        'id': 'target0',
                        'file': {
                            'driver': 'file',
                            'filename': '/home/imain/tmp/BACKUP.qcow2'
                            }
                        } 
                   })

rsp is {u'error': {u'class': u'GenericError', u'desc': u"could not open
disk image target0: Block format 'qcow2' used by device 'target0'
doesn't support the option 'backing'"}}

It's possible I'm messing something up but I'm pretty sure something has
changed with the options handling in bdrv_open() to make this not work
anymore.

Thanks!

        Ian

 
>     (where source-drive is the running BlockDriverState name for
>     RUNNING-VM.img. This patch implements "backing=" option to override
>     backing_hd for added drive)
> 
>  3. (QMP) blockdev-backup device=source-drive sync=none target=target0
> 
>     (this is the QMP command introduced by this series, which use a named
>     device as target of drive-backup)
> 
>  4. (QMP) nbd-server-add device=target0
> 
> When image fleecing done:
> 
>  1. (QMP) block-job-cancel device=source-drive
> 
>  2. (HMP) drive_del target0
> 
>  3. (SHELL) rm BACKUP.qcow2
> 
> v12 -> v13: Address comments from Stefan.
> 
>     [05/15] block: Add bdrv_set_backing_hd()
>             Remove duplicated assignments.
>             
>     [10/15] qmp: Add command 'blockdev-backup'
>             Add check for source == target.
>             
>     [13/15] block: Add blockdev-backup to transaction
>             New.
>             
>     [14/15] qemu-iotests: Test blockdev-backup in 055
>             New.
>             
>     [15/15] qemu-iotests: Image fleecing test case 081
>             New. Hope this number has no conflict. I have a 080 on my
>             iothrottling test branch.
> 
> v11 -> v12: Rebase to qemu.git.
>      Call bdrv_refresh_limits() in bdrv_set_backing_hd().
> 
> 
> 
> Fam Zheng (14):
>   block: Add BlockOpType enum
>   block: Introduce op_blockers to BlockDriverState
>   block: Replace in_use with operation blocker
>   block: Move op_blocker check from block_job_create to its caller
>   block: Add bdrv_set_backing_hd()
>   block: Add backing_blocker in BlockDriverState
>   block: Parse "backing" option to reference existing BDS
>   block: Support dropping active in bdrv_drop_intermediate
>   stream: Use bdrv_drop_intermediate and drop close_unused_images
>   qmp: Add command 'blockdev-backup'
>   block: Allow backup on referenced named BlockDriverState
>   block: Add blockdev-backup to transaction
>   qemu-iotests: Test blockdev-backup in 055
>   qemu-iotests: Image fleecing test case 081
> 
>  block-migration.c               |   7 +-
>  block.c                         | 306 
> ++++++++++++++++++++++++++--------------
>  block/backup.c                  |  26 ++++
>  block/commit.c                  |   1 +
>  block/stream.c                  |  30 +---
>  blockdev.c                      | 118 ++++++++++++++--
>  blockjob.c                      |  14 +-
>  hw/block/dataplane/virtio-blk.c |  19 ++-
>  include/block/block.h           |  29 +++-
>  include/block/block_int.h       |   9 +-
>  include/block/blockjob.h        |   3 +
>  qapi-schema.json                |  50 +++++++
>  qmp-commands.hx                 |  44 ++++++
>  tests/qemu-iotests/055          | 275 ++++++++++++++++++++++++++++++------
>  tests/qemu-iotests/055.out      |   4 +-
>  tests/qemu-iotests/081          |  99 +++++++++++++
>  tests/qemu-iotests/081.out      |   5 +
>  tests/qemu-iotests/group        |   1 +
>  18 files changed, 836 insertions(+), 204 deletions(-)
>  create mode 100755 tests/qemu-iotests/081
>  create mode 100644 tests/qemu-iotests/081.out
> 
> -- 
> 1.8.5.3
>