'Return size of 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):

    if (existing_ref.get('source-name') or existing_ref.get('source-id')):
        with self._client.open_connection() as api:
            return api.get_unmanaged_volume_size(existing_ref)
    else:
        msg = _('Must    specify    source-name    or    source-id.')
        raise exception.ManageExistingInvalidReference(existing_ref=existing_ref, reason=msg)
