[Users] 'Drive' object has no attribute 'format'

Hi all! I'm getting this strange message on my vdsm.log Thread-187887::ERROR::2014-02-07 10:31:13,246::sampling::355::vm.Vm::(collect) vmId=`226fb21b-ce7e-45ec-8e6e-75b987374ead`::Stats function failed: <AdvancedStatsFunction _highWrite at 0x2d328f0> Traceback (most recent call last): File "/usr/share/vdsm/sampling.py", line 351, in collect statsFunction() File "/usr/share/vdsm/sampling.py", line 226, in __call__ retValue = self._function(*args, **kwargs) File "/usr/share/vdsm/vm.py", line 528, in _highWrite self._vm.extendDrivesIfNeeded() File "/usr/share/vdsm/vm.py", line 2285, in extendDrivesIfNeeded if not drive.blockDev or drive.format != 'cow': AttributeError: 'Drive' object has no attribute 'format' Anyone know what could be happening? Thanks.