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

[Let's discuss this upstream] On Wed, Nov 04, 2015 at 12:18:48PM +0200, Yaniv Kaul wrote: 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