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

Hi, 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: <volume> <name>disk.qcow2</name> <capacity>16106127360</capacity> <allocation>16106127360</allocation> <target> <format type="qcow2"/> <features> <lazy_refcounts/> </features> </target> </volume> 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=c9ec7088c7a3f4cd26bb471... [4]: https://github.com/zfsonlinux/zfs/issues/326 -- Christian Ehrhardt Software Engineer, Ubuntu Server Canonical Ltd