'Attach an existing volume to an existing instance.'

@check_instance_cell
def _attach_volume(self, context, instance, volume_id, device, disk_bus, device_type, tag=None):

    if tag:
        raise exception.VolumeTaggedAttachNotSupported()
    volume = self.volume_api.get(context, volume_id)
    self.volume_api.check_availability_zone(context, volume, instance=instance)
    return self._call_to_cells(context, instance, 'attach_volume', volume_id, device, disk_bus, device_type)
