Re: [Qemu-devel] [PATCH v2 10/11] block: add option 'backing' to -drive options

[Ian Main]

On Mon, Jul 22, 2013 at 02:07:15PM +0800, Fam Zheng wrote:
> On Wed, 07/17 14:36, Paolo Bonzini wrote:
> > Il 17/07/2013 11:42, Fam Zheng ha scritto:
> > > This option allows overriding backing hd of drive. If the target drive
> > > exists, it's referenced as the backing file and refcount incremented.
> > > 
> > > Example:
> > >     qemu-system-x86_64 -drive \
> > >         file.filename=foo.qcow2,if=none,id=foo \
> > >         -drive file=bar.qcow2,backing=foo
> > 
> > I guess this is where we need the soft reference.
> > 
> > This has a _lot_ of potential for misuse, I think Kevin bashed me and
> > Federico very heavily when we tried to do something similar.
> > 
> > block/backup.c is the right place where we can override the backing hd
> > of the drive.  Perhaps we can add a way to open a file with
> > BDRV_O_NO_BACKING from the command line.
> 
> OK. If we get the override in block/backup.c, the only thing we need is
> naming the target so we can add it to nbd server. If refcounting,
> overriding backing with option and blockdev-backup are still good to
> have, I can split them and targeting to future releases.

In my latest patchset I have attempted to override the backing hd of the
target.  It is a little over my head however so any input there would be
appreciated.  It seems to me like applying that patch would get us a
working baseline and then you can add the more fancy stuff on top of
that?  That sound reasonable?

        Ian