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

[Stefan Hajnoczi <stefanha@redhat.com>]

On Tue, Jan 28, 2014 at 08:57:11AM -0800, Ian Main wrote:
> 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.

A Python NBD client is pretty easy to implement.  Here is a *server* I
wrote for testing a while back:

https://lists.gnu.org/archive/html/qemu-devel/2013-03/msg01535.html

Stefan