'Returns size of volume to be managed by `manage_existing`.
The driver does some check here:
1. The LUN `existing_ref` should be managed by the `volume.host`.'

def manage_existing_get_size(self, volume, existing_ref):

    lun = self._get_referenced_lun(existing_ref)
    target_pool_name = utils.get_pool_name(volume)
    lun_pool_name = lun.pool.name
    if (target_pool_name and (lun_pool_name != target_pool_name)):
        reason = (_('The    imported    LUN    is    in    pool    %(pool_name)s    which    is    not    managed    by    the    host    %(host)s.') % {'pool_name': lun_pool_name, 'host': volume.host})
        raise exception.ManageExistingInvalidReference(existing_ref=existing_ref, reason=reason)
    return utils.byte_to_gib(lun.size_total)
