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

On Tuesday 05 May 2015 04:22 PM, Ján Tomko wrote: > 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. Yes, and so I sent this patch that blocks a refresh for volumes being created. > 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. > I'm sorry, libvirt does not copy raw volumes as a 'fallback mechanism'. Libvirt chooses to copy raw volumes on its own, but calls on qemu-img to copy all other volume types. Is it not better to let qemu-img copy all volume types , including raw ones ? I wanted to check if there are reasons for this decision ? I'll be happy if the community could throw some light. Also cc'ing Cole, who had put in some fixes in this area.