收到开发反应一库的sql频繁超时，系统几乎瘫痪，无法执行任何操作，我登上库先查看到当前的线程，发现有大量的线程状态是
Waiting for table flush
查看当前的事务
从昨天开始执行，到今天早晨还没执行完，具体原因还没深究，先将此线程释放，然后备份才可以flush table成功继而备份完成后后面一系列被阻塞的sql都得以正常运行
mysql> select * from information_schema.innodb_trx\G
*************************** 1. row ***************************
trx_id: 192611452
trx_state: RUNNING
trx_started: 2017-11-30 18:33:58
trx_requested_lock_id: NULL
trx_wait_started: NULL
trx_weight: 3688
trx_mysql_thread_id: 352932171
trx_query: DELETE FROM xx WHERE xx IN(SELECT xx
FROM xx WHERE Remarks LIKE xx)
trx_operation_state: unlock_row
trx_tables_in_use: 2
trx_tables_locked: 2
trx_lock_structs: 3688
trx_lock_memory_bytes: 368848
trx_rows_locked: 4
trx_rows_modified: 0
trx_concurrency_tickets: 0
trx_isolation_level: READ COMMITTED
trx_unique_checks: 1
trx_foreign_key_checks: 1
trx_last_foreign_key_error: NULL
trx_adaptive_hash_latched: 0
trx_adaptive_hash_timeout: 0
trx_is_read_only: 0
trx_autocommit_non_locking: 0
后来想了一下每天的凌晨两点有物理备份，于是查看备份日志，发现果然是上面的事务阻塞了物理备份;
物理备份的整个流程
先记录当前redo log的序列号
:: >> log scanned up to ()
xtrabackup: Generating a list of tablespaces
xtrabackup: using the full scan for incremental backup
xtrabackup: Starting threads for parallel data files transfer
然后备份innodb库表
:: [] Copying .
备份完之后flush table；因为被阻塞，所以知道释放完事务后才成功
:: Executing FLUSH NO_WRITE_TO_BINLOG TABLES...
接着开始备份非事务库表
:: Executing FLUSH TABLES WITH READ LOCK...
:: >> log scanned up to ()
:: Starting to backup non-InnoDB tables and files
:: [] Copying ....
xtrabackup: The latest check point (for incremental): ''
xtrabackup: Stopping log copying thread.
. :: >> log scanned up to ()
:: Executing FLUSH NO_WRITE_TO_BINLOG ENGINE LOGS...
备份完之后释放表锁
:: Executing UNLOCK TABLES
:: All tables unlocked
:: [] Copying ib_buffer_pool to xxx
:: [] ...done
:: Backup created in directory xxxx
MySQL binlog position: xxx
:: [] Writing backup-my.cnf
:: [] ...done
:: [] Writing xtrabackup_info
:: [] ...done
xtrabackup: Transaction log of lsn () to () was copied.
:: completed OK!
被阻塞的语句是FLUSH NO_WRITE_TO_BINLOG TABLES...
官方解释flush tables
Closes all open tables, forces all tables in use to be closed, and flushes the query cache and prepared statement cache.
没有涉及到锁相关的字眼；但是测试表明在执行查询或者变更还未完成时，如果另起一个会话执行flush tables 则会被阻塞，
如果此后如果有操作慢查询中的表的任何sql都会被阻塞；