'Return size of the volume to be managed by manage_existing.
When calculating the size, round up to the next GB.
:param volume: Cinder volume to manage
:param existing_ref: Driver-specific information used to identify a
volume'

def manage_existing_get_size(self, volume, existing_ref):

    (_vm, disk) = self._get_existing(existing_ref)
    return int(math.ceil(((disk.capacityInKB * units.Ki) / float(units.Gi))))
