пробовал оптимизировать

mysql> optimize table molnia.messages;
+-----------------+----------+----------+-------------------------------------------------------------------+
| Table | Op | Msg_type | Msg_text |
+-----------------+----------+----------+-------------------------------------------------------------------+
| molnia.messages | optimize | note | Table does not support optimize, doing recreate + analyze instead |
| molnia.messages | optimize | status | OK |
+-----------------+----------+----------+-------------------------------------------------------------------+
2 rows in set (1 min 52.56 sec)

Из-за чего "Table does not support optimize, doing recreate + analyze instead"?
причём размер на диске увеличился...