Re: [libvirt] [PATCH 1/2] Storage: Suppress metadata refresh for volumes being built.

On Tuesday 05 May 2015 03:20 PM, Prerna Saxena wrote: > On Tuesday 05 May 2015 01:52 PM, Ján Tomko wrote: >> On Tue, May 05, 2015 at 08:43:21AM +0530, Prerna Saxena wrote: >>> Libvirt periodically calls 'stat' on all volumes in a storage pool, >>> to update fields such as 'target.allocation'. >>> >>> The operation doesnt make sense for a volume which is curently being allocated. >> From the comments in the storage driver, the point of allowing refresh >> for a volume that is currently being allocated is to track the progress >> of the allocation. >> >>> Also, the 'target.allocation' sub-field is taken into account while copying a raw image. >>> To suppress any (potential) corruption, libvirt must not attempt to refresh a volume currently being built. >> What would be the corruption? >> >> We do not allow using a volume that is currently building as a >> source for cloning the volume - storageVolCreateXMLFrom checks for >> origvol->building: >> >> if (origvol->building) { >> virReportError(VIR_ERR_OPERATION_INVALID, >> _("volume '%s' is still being allocated."), >> origvol->name); >> goto cleanup; >> } >> > While running libvirt on PowerPC, I saw an interesting scenario. The 'target.allocation' field seemed to change for a volume getting allocated, and this would lead to incomplete copy. This would > happen at random intervals, not deterministically. While looking through the code, I found this to be the other place in code where the same field seemed to change without a lock. Hence the patch. >

> I have sent the second patch which fixes the erring code too : > > - remain = vol->target.allocation; > + remain = inputvol->target.capacity; > More fundamental question -- why do we offload the copying of non-raw images to qemu-img tool, but make libvirt responsible for copying raw volumes ? Would it not be better if libvirt called on 'qemu-img' to copy all types of volumes, including raw ones ?