def restore(self, res_id, backup_snap=None):
name = self._get_name()<EOL>out = self._cli.restore_snap(name, res_id, backup_snap)<EOL>ex.raise_if_err(out, '<STR_LIT>'.format(name),<EOL>default=ex.VNXSnapError)<EOL>
Restores a snapshot.
:param res_id: the LUN number of primary LUN or snapshot mount point to
    be restored.
:param backup_snap: the name of a backup snapshot to be created before
    restoring.