Re: [Qemu-devel] [PATCH v12 00/11] Drop in_use from BlockDriverState and enable point-in-time snapshot exporting over NBD

[Ian Main]

On Tue, Jan 28, 2014 at 03:31:36PM +0100, Stefan Hajnoczi wrote:
> On Mon, Jan 27, 2014 at 11:48:06AM +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
> > 
> >     (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
> 
> Seems to work as advertized but do you have qemu-iotests for
> blockdev-backup and image fleecing in particular?

We have them for drive-backup but not blockdev-backup afaik.  Mostly the
same code but certainly a different API.

It shouldn't be too hard to duplicate the 055 drive-backup code and make
it a blockdev-backup test.  Testing nbd export would be difficult
however.

        Ian