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

On Tue, May 05, 2015 at 03:24:31PM +0530, Prerna Saxena wrote: > 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. >> Oh, I was thinking of the soure volume for some reason. We correctly lock the pool before calling refreshVol, so changing the object should not be an issue. I think the bug is in storageVolCreateXMLFrom - it drops all the locks, but expects the allocation not to change.

In storageVolCreateXML we work around this by creating a shallow copy of the volume. >> 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 ? > This way, libvirt can create raw volumes even without qemu-img installed. I don't know if there's any other reason.