On Wed, Oct 9, 2019 at 8:33 AM Daniel P. Berrangé <berrange@xxxxxxxxxx> wrote: > 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: > > > Up until the commit above that was using preallocation=metadata, but > > > now uses preallocation=falloc. > > > 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. I know that this is an old message, but I'm just getting around to updating some of my libvirt-python scripts and I'm encountering a similar problem to virt-manager when using libvirt >=4.3, without the ZFS part. Prior to libvirt v4.3, my script created qcow2-format StorageVolumes with a <capacity/> element, and no <allocation/> element. It used the flag VIR_STORAGE_VOL_CREATE_PREALLOC_METADATA to request metadata-only prealloc, which the documentation of virStorageVolCreateXml describes as creating a sparse file: > Since 1.0.1 VIR_STORAGE_VOL_CREATE_PREALLOC_METADATA in flags > can be used to get higher performance with qcow2 image files which don't support > full preallocation, by creating a sparse image file with metadata. The patch identified in this thread does not create a sparse file even in the presence of this flag when the requested capacity is equal to the requested allocation. You could just update the documentation to match current behavior, but I think that this is a bug and that qemu-img's prealloc=metadata option should always be used when VIR_STORAGE_VOL_CREATE_PREALLOC_METADATA is specified. It also seems that the documentation is out-of-date in regards to qcow2 not supporting full preallocation. As mentioned previously, using the StorageVolume's <allocation /> element as an input does not work for qcow2. The requested allocation size is never honored by qemu-img. There is no concept of partial preallocation. There is only full prealloc and sparse metadata prealloc. Thanks, Mike
