Re: [Qemu-devel] [PATCH 0/2] block: commits of snapshots larger than backing files

[Jeff Cody <jcody@redhat.com>]

On Fri, Jan 17, 2014 at 03:17:10PM +0800, Stefan Hajnoczi wrote:
> On Mon, Jan 13, 2014 at 03:18:44PM -0500, Jeff Cody wrote:
> > If a snapshot is larger than a backing file, then the offline bdrv_commit and
> > the live active layer commit will fail with an i/o error (usually).  A live
> > commit of a non-active layer will complete successfully, as it runs
> > bdrv_truncate() on the backing image to resize it to the larger size.
> > 
> > For both bdrv_commit() and commit_active_start(), this series will resize
> > the underlying base image if needed.  If the resize fails, an error will
> > be returned.
> 
> This got me thinking about the opposite case: when the snapshot is
> smaller than the backing file.  We leave the backing file with its
> original size.  In practice this is "safe" because the partition and
> volume metadata should show the smaller size.  If the user really wants
> to shrink the device they can still truncate after completing the
> "commit" operation.
> 
> Can you update the QEMU documentation to explicitly cover both snapshot
>  > backing and snapshot < backing cases?
>

Yes, no problem.