Re: [Qemu-devel] [PATCH v7 for 2.1 4/4] block: add QAPI command to allow live backing file change

[Jeff Cody <jcody@redhat.com>]

On Wed, Jun 25, 2014 at 01:52:25PM -0600, Eric Blake wrote:
> On 06/25/2014 01:40 PM, Jeff Cody wrote:
> > This allows a user to make a live change to the backing file recorded in
> > an open image.
> > 
> > The image file to modify can be specified 2 ways:
> > 
> > 1) image filename
> > 2) image node-name
> > 
> > Note: this does not cause the backing file itself to be reopened; it
> > merely changes the backing filename in the image file structure, and
> > in internal BDS structures.
> > 
> > It is the responsibility of the user to pass a filename string that
> > can be resolved when the image chain is reopened, and the filename
> > string is not validated.
> > 
> > A good analogy for this command is that it is a live version of
> > 'qemu-img rebase -u', with respect to changing the backing file string.
> 
> For 2.1, did we want to limit this command to only affecting the active
> layer (and failing if it was attempted on a backing file), in order to
> simplify dealing with op-blocker work?  Having the command exist, even
> if it is limited in nature, is all the more libvirt wants right now; and
> it's easier to relax a constraint in a future release than it is to
> release something now and have to keep it working even after we rework
> op-blockers in the future.
> 

I left this as-is, because the blocker issue needs to be addressed
somehow for 2.1.  And once we know in what manner, I'll just update
this series to use that methodology (if necessary).  Unless Stefan or
Kevin would prefer that I change this to only operate on the active
layer, of course.

> > 
> > Reviewed-by: Eric Blake <eblake@redhat.com>
> > Signed-off-by: Jeff Cody <jcody@redhat.com>
> 
> I'm still okay with this patch as-is, if we are okay with the ability to
> rename the backing file of a non-active node.
> 
> -- 
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>