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

[Richard W.M. Jones]

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
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