Re: [Qemu-devel] [PATCH v4 0/7] Drop in_use from BlockDriverState and enable point-in-time snapshot exporting over NBD

[Kevin Wolf <kwolf@redhat.com>]

Am 22.11.2013 um 17:58 hat Stefan Hajnoczi geschrieben:
> On Fri, Nov 22, 2013 at 01:24:47PM +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 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

I haven't looked at the code yet, but the interface looks right.

> Interesting implementation, it looks pretty good.  I'll need to review
> it a second time to track all the operation block/unblocks.  It wasn't
> immediately clear to me whether these patches will restrict something
> that used to work.

We still have a lot of time for 1.8, so I think we can be radical here:
If something doesn't have a test case and we don't catch it during the
normal review, then let's just break it. Someone will find it and then
we'll get a test case for it, so that it doesn't happen again.

Kevin