求助！修改了tmpdir的问题

我修改了my.cnf，添加了tmpdir=/mnt/sdb1/tmp
然后打开数据库就提示Unknown table engine ‘InnoDb’  
然后用show engines;  Engine     | Support | Comment                                                   | Transactions | XA   | Savepoints |
+------------+---------+-----------------------------------------------------------+--------------+------+------------+
| MRG_MYISAM | YES     | Collection of identical MyISAM tables                     | NO           | NO   | NO         |
| CSV        | YES     | CSV storage engine                                        | NO           | NO   | NO         |
| MyISAM     | DEFAULT | Default engine as of MySQL 3.23 with great performance    | NO           | NO   | NO         |
| MEMORY     | YES     | Hash based, stored in memory, useful for temporary tables | NO           | NO   | NO         |
+------------+---------+-----------------------------------------------------------+--------------+------+------------+就这几项，把my.cnf的删掉的话数据库就又可以用了这是什么原因- - -