Flush Tables With Read Lock

【Flush Tables With Read Lock】

　　FLUSH TABLES

　　Closes all open tables, forces all tables in use to be closed, and flushes the query cache. FLUSH TABLES also removes all query results from the query cache, like the RESET QUERY CACHE statement.

　　

　　FLUSH TABLES WITH READ LOCK

　　Closes all open tables and locks all tables for all databases with a global read lock. This is a very convenient way to get backups if you have a file system such as Veritas or ZFS that can take snapshots in time. Use UNLOCK TABLES to release the lock.

参考：http://dev.mysql.com/doc/refman/5.6/en/flush.html