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 <[email protected]> > > 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
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