[Let's discuss this upstream] On Wed, Nov 04, 2015 at 12:18:48PM +0200, Yaniv Kaul wrote: > I'm missing something here - what will happen to the tree structure? > Will we lose it? So essentially it performs a merge? In copying mode: virt-sparsify disk1 disk2 creates an overlay on top of disk1, writes zeroes to the overlay in the parts of disk1 which are not used (disk1 is not modified by this), and then copies the result to disk2. It doesn't even consider any snapshots or backing files of disk1. - - - This doesn't matter because you're proposing to use --in-place which works completely differently -- literally: it's a different code path. virt-sparsify --in-place disk opens `disk', mounts each filesystem it finds in turn, and runs fstrim on them. The question for this bug is does that do anything useful if `disk' has backing files? Here is a non-sparse Fedora 22 disk image: $ virt-builder fedora-22 $ mv fedora-22.img fedora-22.img.sparse $ cp --sparse=never fedora-22.img.sparse fedora-22.img $ du -sh fedora-22.img 6.1G fedora-22.img Let's add a snapshot on top: $ qemu-img create -f qcow2 -o compat=1.1 -b fedora-22.img overlay.qcow2 $ du -sh fedora-22.img overlay.qcow2 6.1G fedora-22.img 196K overlay.qcow2 and sparsify the overlay: $ virt-sparsify --in-place overlay.qcow2 $ du -sh fedora-22.img overlay.qcow2 6.1G fedora-22.img 3.2M overlay.qcow2 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). 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).

Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-builder quickly builds VMs from scratch http://libguestfs.org/virt-builder.1.html

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