Re: [Qemu-devel] [PATCH v6 00/10] Drop in_use from BlockDriverState and enable point-in-time snapshot exporting over NBD

[Ian Main]

On Wed, Dec 11, 2013 at 10:46:49AM +0800, Fam Zheng wrote:
> On 2013年11月28日 16:39, 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
> >
> >     (where ide0-hd0 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-complete device=ide0-hd0
> >
> >  2. (HMP) drive_del target0
> >
> >  3. (SHELL) rm BACKUP.qcow2
> >
> >v6: Address Paolo's comments, (except for bitmask):
> >     - Add blocker for all backing_hd references, a relatively big change, 
> > some
> >       patches are reordered.
> >     - Introduce a few other necessary patches.
> >     - Move two snapshot checks into bdrv_snapshot_*.
> >
> >     The interface is unchanged.
> >
> 
> Hi,
> 
> Based on the size of change, this series needs some more review
> before merging. And I'd like to know if there is any concern or
> objection with op_blocker introduced here. I would like to base my
> next series (incremental backup with dirty bitmap) on it.
> 
> Any more reviews/comments?

I really like it.  Just did some more testing and this seems to have
solved the issues from before.. I suspect this will resolve some issues
with other commands and will be a useful framework for the future.

I love this:

(qemu) drive_del target0
block device is in use by block job: backup
(qemu) drive_del target1
block device is in use by block job: backup
(qemu) drive_del ide0-hd0
device is used as backing hd of 'target0'

$ python qmp --path=/home/imain/qmp-sock eject --device=target0
<snip>
Exception: block device is in use by block job: backup

I've started working on a backup/snapshot API for libvirt already.

        Ian