and one more things, i often find the following info in error log,is this a bug?

120326 16:49:21 [Note] /usr/sbin/mysqld: ready for connections.
Version: '5.5.21-55-log' socket: '/var/lib/mysql/mysql.sock' port: 3306 Percona Server (GPL), Release rel25.0, Revision 227
120326 16:56:45 [ERROR] /usr/sbin/mysqld: Incorrect key file for table '/tmp/#sql_21b2_0.MYI'; try to repair it
120326 16:56:45 [ERROR] Got an error from unknown thread, /home/jenkins/workspace/percona-server-5.5-rpms/label_exp/ce ntos5-64/target/BUILD/Percona-Server-5.5.21-rel25.0/Percona
-Server-5.5.21-rel25.0/storage/myisam/mi_write.c:223
120326 16:56:45 [ERROR] /usr/sbin/mysqld: Sort aborted: Error writing file '/tmp/MYK74Kpi' (Errcode: 28)

Neither of these things is a bug. Item 1 is misconfiguration and item 2 is you running out of disk space.

This inspired me to write a quick blog post: http://www.mysqlperformanceblog.com/2012/03/26/how-to-diagno se-errors-in-the-mysql-error-log/

thanks, i changed mysql tmp dir and tmp_slave dir, so temporary table will not full the tmp dir,and one more questions is that
if i can see the max size of temporary table which ever created?