On Wed, Oct 09, 2019 at 02:25:09PM +0200, Peter Krempa wrote: > On Wed, Oct 09, 2019 at 13:39:54 +0200, Christian Ehrhardt wrote: > > 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. > > Note that for raw files we would do preallocation for the full image but > only when fast allocation is supported. See 'createRawFile'. Hmm, yes, i see we report a fatal error if fallocate() does work, which is good. Annoyingly QEMU uses posix_fallocate() which automagically falls back to writing zeros, which is very bad. AFAIK it isn't possible to ask QEMU to allocate only if it is fast :-( > > 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. > > Semantically the change above would make sense if it weren't coupled to > the presence of VIR_STORAGE_VOL_CREATE_PREALLOC_METADATA. > > The flags semantics imply that metadata allocation is requested and thus > metadata should be allocated. > > On the other hand doing full allocation if the flag is not present but > the XML calls for it would semantically be okay IMO. Yeah, I agree with this. > I'm not sure if that is what you expect though. virt-manager passes VOL_CREATE_PREALLOC_METADATA, so honouring that flag correctly would fix the problem reported with ZFS. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
