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 :|