慢查询阻塞了xtrabackup进而阻塞以后的sql导致的系统瘫痪问题

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；