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

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(a)redhat.com&gt; > 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 virt-p2v converts physical machines to virtual machines. Boot with a live CD or over the network (PXE) and turn machines into KVM guests. http://libguestfs.org/virt-v2v