什么是FTWL FTWRL是FLUSH TABLES WITH READ LOCK的简称(FTWRL)，该命令主要用于保证备份一致性备份。为了达到这个目的，它需要关闭所有表对象，因此这个命令的杀伤性很大，执行命令时容易导致库hang住。
- 全局读锁(lock_global_read_lock) 会导致所有的更新操作被堵塞
- 全局COMMIT锁(make_global_read_lock_block_commit) 会导致所有的活跃事务无法提交
FLUSH TABLES WITH READ LOCK执行后整个系统会一直处于只读状态，直到显示执行UNLOCK TABLES。这点请切记。
[test]> flush tables with read lock;
Query OK, 0 rows affected (0.06 sec)
[test]> show full processlist\G
*************************** 1. row *************************** Id: 10 User: root Host: localhost db: test
Command: Query
Time: 0
State: init
Info: show full processlist
Progress: 0.000
*************************** 2. row *************************** Id: 11 User: root Host: localhost db: test
Command: Query
Time: 743
State: Waiting for global read lock
Info: delete from t0
Progress: 0.000
2 rows in set (0.00 sec)
[test]> select * from information_schema.processlist\G
*************************** 1. row *************************** ID: 11 USER: root HOST: localhost DB: test COMMAND: Query TIME: 954 STATE: Waiting for global read lock INFO: delete from t0 TIME_MS: 954627.587 STAGE: 0 MAX_STAGE: 0 PROGRESS: 0.000
MEMORY_USED: 67464
EXAMINED_ROWS: 0
QUERY_ID: 1457
INFO_BINARY: delete from t0
TID: 8838 *************************** 2. row *************************** ID: 10 USER: root HOST: localhost DB: test COMMAND: Query TIME: 0 STATE: Filling schema table INFO: select * from information_schema.processlist TIME_MS: 0.805 STAGE: 0 MAX_STAGE: 0 PROGRESS: 0.000
MEMORY_USED: 84576
EXAMINED_ROWS: 0
QUERY_ID: 1461
INFO_BINARY: select * from information_schema.processlist
TID: 8424
2 rows in set (0.02 sec)从上的输出中，我们只发现了会话11 在等候一个全局读锁。但这个锁被谁持有，从这个输出里面我们找不到任何线索。我现在再来看看INNODB STATUS输出：
...
------------
TRANSACTIONS
------------
Trx id counter 20439
Purge done for trx's n:o < 20422 undo n:o < 0 state: running but idle
History list length 176
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 0, not started
MySQL thread id 11, OS thread handle 0x7f7f5cdb8b00, query id 1457 localhost root Waiting for global read lock
delete from t0
---TRANSACTION 0, not started
MySQL thread id 10, OS thread handle 0x7f7f5ce02b00, query id 1462 localhost root init
show engine innodb status
--------
...- 如果你用的Mysql 5.7，那么你可以使用performance_schema.metadata_locks
- 如果你用的Mysql 5.6，那么你可以使用performance_schema.events_statements_history
- 如果你用的Mysql版本比较老，那么可以使用genearal log或者一些sql审计的日志来定位
会话1：
flush tables with read lock;
Query OK, 0 rows affected (0.00 sec)
会话2：
mysql> delete from t; --被hang住
会话3：
mysql> show processlist;
+----+------+-----------+------+---------+------+------------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------+------+---------+------+------------------------------+------------------+
| 7 | root | localhost | test | Query | 227 | Waiting for global read lock | delete from t |
| 8 | root | localhost | NULL | Sleep | 215 | | NULL |
| 9 | root | localhost | NULL | Query | 0 | init | show processlist |
+----+------+-----------+------+---------+------+------------------------------+------------------+由于会话1执行了FTWL,导致了会话2中的DML无法执行。接下来，我们演示如何通过gdb去定位执行了FTWL的会话。见下面的步骤
1.找出myql的进程id， ps -ef | grep mysql
root 7743 2366 0 05:07 ? 00:00:01 /u02/mysql/bin/mysqld (gdb) pset global_thread_list THD*
elem[0]: $1 = (THD *) 0x4a55de0
elem[1]: $2 = (THD *) 0x4a5cf10
elem[2]: $3 = (THD *) 0x4b24aa0
Set size = 3 上面的命令输出了三个会话的内存地址。接下来我们根据这些内存地址去查找每个会话各自对应的global_read_lock
4.依次在gdb中打印上面三个会话中的global_read_lock和thread_id的值
(gdb) p ((THD *) 0x4a55de0)->global_read_lock
$4 = {
static m_active_requests = 1,
m_state = Global_read_lock::GRL_NONE,
m_mdl_global_shared_lock = 0x0,
m_mdl_blocks_commits_lock = 0x0
} //这个会话的Global_read_lock为空，不是我们要找的
(gdb) p ((THD *) 0x4a5cf10)->global_read_lock
$5 = {
static m_active_requests = 1,
m_state = Global_read_lock::GRL_NONE,
m_mdl_global_shared_lock = 0x0,
m_mdl_blocks_commits_lock = 0x0
} //这个会话的Global_read_lock也为空，不是我们要找的
(gdb) p ((THD *) 0x4b24aa0)->global_read_lock
$6 = {
static m_active_requests = 1,
m_state = Global_read_lock::GRL_ACQUIRED_AND_BLOCKS_COMMIT,
m_mdl_global_shared_lock = 0x7f6034002bb0,
m_mdl_blocks_commits_lock = 0x7f6034002c20
}
//这个会话的Global_read_lock不为空，GRL_ACQUIRED_AND_BLOCKS_COMMIT表示全局读锁与commit锁，这个就是我们要好的。我接下来打印出它的thread_id
p ((THD *) 0x4b24aa0)->thread_id
$7 = 8 //8号会话执行了FTWL 5.我们可以通过执行kill 8结束这个会话来释放全局的锁。让被堵住的会话，继续运行下去。
在新开的mysql会话中，执行下面的语句
mysql> kill 8
以前被堵在的会话中，会看到下面的结果
mysql> delete from t;
Query OK, 0 rows affected (40 min 20.73 sec)
