[libvirt] Semantic change in v4.3 slows down image creation and snapshots (on ZFS)

Christian Ehrhardt christian.ehrhardt at canonical.com
Wed Oct 9 11:39:54 UTC 2019

when discussing bug [1] triggered by [2] Daniel and I have found that
the change [3] which is in since v4.3 was a semantic change to the
behavior of libvirt.

virt-manager used and still does issue a volume XML like:
    <format type="qcow2"/>

Up until the commit above that was using preallocation=metadata, but
now uses preallocation=falloc. In many cases (read: Fileystems) that
won't be very user-visible, but if you are running on ZFS (or anything
else that can't fallocate well) this is made worse by [4] slowing down
these operations a lot.

After the POV for virt-manager seems to have settled on the bug [1]
referenced the remaining question is if libvirt should revert/modify
the change [3] - and that we agreed required a wider discussion to be
had here, hence this mail.

[1]: https://bugzilla.redhat.com/show_bug.cgi?id=1759454
[2]: https://bugs.launchpad.net/virt-manager/+bug/1847105
[3]: https://libvirt.org/git/?p=libvirt.git;a=commit;h=c9ec7088c7a3f4cd26bb471f1f243931fff6f4f9
[4]: https://github.com/zfsonlinux/zfs/issues/326

Christian Ehrhardt
Software Engineer, Ubuntu Server
Canonical Ltd

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