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

[Prerna Saxena]

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 ?

-- 
Prerna Saxena

Linux Technology Centre,
IBM Systems and Technology Lab,
Bangalore, India