[Libguestfs] Fwd: [Bug 1277705] virt-sparsify --in-place should not sparsify a snapshot

[Shahar Havivi]

Adding Francesco and Michal.

On 04.11.15 11:45, Richard W.M. Jones wrote:
> On Wed, Nov 04, 2015 at 01:10:04PM +0200, Yaniv Kaul wrote:
> > On Wed, Nov 4, 2015 at 12:49 PM, Richard W.M. Jones <rjones at redhat.com>
> > wrote:
> > > All that happened was that the overlay got bigger (because it's now
> > > storing a bunch of qcow2 zero clusters marking the places in the
> > > backing file which are zero).
> 
>                            ^^^
> Here I should have more accurately written "unused".
> 
> > Perhaps I should run 'zerofree' when the VM is up, so the blocks become
> > zero'ed on the right snapshot? Not sure how that would help a lot, though.
> > It might on newer storage, which will recognize zero blocks as free on the
> > underlying storage level.
> 
> This shouldn't make any difference, since fstrim finds all unused
> space in filesystems, whether zeroes or deleted files.
> 
> > > In *theory* it should be possible to commit the changes to the backing
> > > file, making the backing file sparse.  But in reality that doesn't
> > > work:
> > >
> > > $ qemu-img commit overlay.qcow2
> > > Image committed.
> > > $ du -sh fedora-22.img overlay.qcow2
> > > 6.1G fedora-22.img
> > > 260K overlay.qcow2
> > >
> > > So really there's no use for virt-sparsify on a snapshot (although you
> > > could also argue this is a bug or missing feature in qemu-img).
> > >
> > 
> > Indeed, as in real life, I expect that in any level of the snapshot tree
> > there are opportunities to sparsify blocks.
> 
> The trouble is you can't run virt-sparsify on the backing files -
> you'll just end up with a corrupted disk.  This is actually because
> virt-sparsify mounts the filesystems within the backing file, and the
> mount could replay the journal.
> 
> I think this is a qemu bug or missing feature.  It should be possible
> to "push" the trimmed clusters to the backing file.  I'll ask Paolo.
> 
> Rich.
> 
> -- 
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
> Read my programming and virtualization blog: http://rwmj.wordpress.com
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