mysql锁机制及原理（MySQL锁阻塞的深入分析）

mysql锁机制及原理

MySQL锁阻塞的深入分析

日常维护中，经常会碰到线程被阻塞，导致数据库响应非常慢，下面就看看如何获取是哪个线程导致了阻塞的。

1. 环境说明

RHEL 6.4 x86_64 + MySQL 5.6.19

事务隔离级别：RR

2. 测试过程

3. 查看锁阻塞线程信息

这里用几中方法进行分析：

3.1  使用show processlist查看

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MySQL [(none)]> show processlist; +----+------+-----------+------+---------+------+--------------+------------------------------------------+ | Id | User | Host  | db | Command | Time | State  | Info          | +----+------+-----------+------+---------+------+--------------+------------------------------------------+ | 2 | root | localhost | NULL | Query | 0 | init   | show processlist       | | 3 | root | localhost | test | Query | 70 | Sending data | select count(*) from t3 a,t3 b   | | 4 | root | localhost | test | Query | 65 | updating  | delete from emp where empno=7788   | | 7 | root | localhost | test | Query | 68 | updating  | update emp set sal=3500 where empno=7788 | +----+------+-----------+------+---------+------+--------------+------------------------------------------+ 4 rows in set (0.00 sec)

如果数据库存在较多线程的话，这种方法确实不太好确认的。

3.2  直接使用show engine innodb status查看

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------------ TRANSACTIONS ------------ Trx id counter 4131 Purge done for trx's n:o < 4119 undo n:o < 0 state: running but idle History list length 126 LIST OF TRANSACTIONS FOR EACH SESSION: ---TRANSACTION 0, not started MySQL thread id 2, OS thread handle 0x7f953ffff700, query id 115 localhost root init show engine innodb status ---TRANSACTION 4130, ACTIVE 41 sec starting index read mysql tables in use 1, locked 1 LOCK WAIT 2 lock struct(s), heap size 360, 1 row lock(s) MySQL thread id 4, OS thread handle 0x7f953ff9d700, query id 112 localhost root updating delete from emp where empno=7788 ------- TRX HAS BEEN WAITING 41 SEC FOR THIS LOCK TO BE GRANTED: ## 等待了41s RECORD LOCKS space id 16 page no 3 n bits 88 index `PRIMARY` of table `test`.`emp` trx id 4130 lock_mode X locks rec but not gap waiting Record lock, heap no 9 PHYSICAL RECORD: n_fields 10; compact format; info bits 0 ## 线程4在等待往test.emp中的主键上加X锁，page num=3  0: len 4; hex 80001e6c; asc l;;  1: len 6; hex 000000001018; asc  ;;  2: len 7; hex 91000001420084; asc  B ;;  3: len 5; hex 53434f5454; asc SCOTT;;  4: len 7; hex 414e414c595354; asc ANALYST;;  5: len 4; hex 80001d8e; asc  ;;  6: len 4; hex 208794f0; asc  ;;  7: len 4; hex 80000bb8; asc  ;;  8: SQL NULL;  9: len 4; hex 80000014; asc  ;;   ------------------ ---TRANSACTION 4129, ACTIVE 45 sec starting index read mysql tables in use 1, locked 1 LOCK WAIT 2 lock struct(s), heap size 360, 1 row lock(s) MySQL thread id 7, OS thread handle 0x7f953ff6c700, query id 111 localhost root updating update emp set sal=3500 where empno=7788 ------- TRX HAS BEEN WAITING 45 SEC FOR THIS LOCK TO BE GRANTED: ## 等待了45s RECORD LOCKS space id 16 page no 3 n bits 88 index `PRIMARY` of table `test`.`emp` trx id 4129 lock_mode X locks rec but not gap waiting Record lock, heap no 9 PHYSICAL RECORD: n_fields 10; compact format; info bits 0 ## 线程7在等待往test.emp中的主键上加X锁，page num=3  0: len 4; hex 80001e6c; asc l;;  1: len 6; hex 000000001018; asc  ;;  2: len 7; hex 91000001420084; asc  B ;;  3: len 5; hex 53434f5454; asc SCOTT;;  4: len 7; hex 414e414c595354; asc ANALYST;;  5: len 4; hex 80001d8e; asc  ;;  6: len 4; hex 208794f0; asc  ;;  7: len 4; hex 80000bb8; asc  ;;  8: SQL NULL;  9: len 4; hex 80000014; asc  ;;   ------------------ ---TRANSACTION 4128, ACTIVE 51 sec 2 lock struct(s), heap size 360, 1 row lock(s) MySQL thread id 3, OS thread handle 0x7f953ffce700, query id 110 localhost root cleaning up

我们知道，主要根因还是thread=3引起的，但从innodb status中却无法分析得到这个结果。

从上面来看，线程4和线程7都在等待往test.emp中的主键上加X锁，page num=3，但是线程7等待的时间为45s，而线程4等待的时间为41s，是较线程7之后申请的锁，所以可以判断是线程7阻塞了线程4。至于线程7为什么出现等待，这里分析不到根因。

3.3  使用mysqladmin debug查看

# mysqladmin -S /tmp/mysql3306.sock debug

然后在error日志中，会看到：

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Thread database.table_name   Locked/Waiting  Lock_type     3  test.t3      Locked - read   Low priority read lock 7  test.emp     Locked - write  High priority write lock

这种方法中，能找到线程ID=3和7是阻塞者，但还是不太准确，判断不出来线程7也是被线程ID=3阻塞的。

3.4  使用innodb_lock_monitor来获取阻塞锁线程

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MySQL [test]> CREATE TABLE innodb_lock_monitor (a INT) ENGINE=INNODB; ## 随便在一个数据库中创建这个表，就会打开lock monitor Query OK, 0 rows affected, 1 warning (0.07 sec)   MySQL [test]> show warnings\G *************************** 1. row ***************************  Level: Warning  Code: 131 Message: Using the table name innodb_lock_monitor to enable diagnostic output is deprecated and may be removed in future releases. Use INFORMATION_SCHEMA or PERFORMANCE_SCHEMA tables or SET GLOBAL innodb_status_output=ON. 1 row in set (0.00 sec)

说明：这个在5.6中有一个warning，但不影响使用。

然后再使用show engine innodb status查看：

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------------ TRANSACTIONS ------------ Trx id counter 4667 Purge done for trx's n:o < 4659 undo n:o < 0 state: running but idle History list length 138 LIST OF TRANSACTIONS FOR EACH SESSION: ---TRANSACTION 0, not started MySQL thread id 9, OS thread handle 0x7f813c5f7700, query id 152 localhost root init show engine innodb status ---TRANSACTION 4663, ACTIVE 78 sec starting index read mysql tables in use 1, locked 1 LOCK WAIT 2 lock struct(s), heap size 360, 1 row lock(s) MySQL thread id 4, OS thread handle 0x7f813c628700, query id 149 localhost root updating delete from emp where empno=7788 ------- TRX HAS BEEN WAITING 78 SEC FOR THIS LOCK TO BE GRANTED:  ## 等待了78s RECORD LOCKS space id 16 page no 3 n bits 88 index `PRIMARY` of table `test`.`emp` trx id 4663 lock_mode X locks rec but not gap waiting Record lock, heap no 9 PHYSICAL RECORD: n_fields 10; compact format; info bits 0 ## 线程4在等待往test.emp中的主键上加X锁，page num=3  0: len 4; hex 80001e6c; asc  l;;  1: len 6; hex 000000001018; asc    ;;  2: len 7; hex 91000001420084; asc   B ;;  3: len 5; hex 53434f5454; asc SCOTT;;  4: len 7; hex 414e414c595354; asc ANALYST;;  5: len 4; hex 80001d8e; asc   ;;  6: len 4; hex 208794f0; asc   ;;  7: len 4; hex 80000bb8; asc   ;;  8: SQL NULL;  9: len 4; hex 80000014; asc   ;;   ------------------ TABLE LOCK table `test`.`emp` trx id 4663 lock mode IX  ## 在给主键行上加X锁之前，先要在表上加意向锁IX RECORD LOCKS space id 16 page no 3 n bits 88 index `PRIMARY` of table `test`.`emp` trx id 4663 lock_mode X locks rec but not gap waiting Record lock, heap no 9 PHYSICAL RECORD: n_fields 10; compact format; info bits 0  0: len 4; hex 80001e6c; asc  l;;  1: len 6; hex 000000001018; asc    ;;  2: len 7; hex 91000001420084; asc   B ;;  3: len 5; hex 53434f5454; asc SCOTT;;  4: len 7; hex 414e414c595354; asc ANALYST;;  5: len 4; hex 80001d8e; asc   ;;  6: len 4; hex 208794f0; asc   ;;  7: len 4; hex 80000bb8; asc   ;;  8: SQL NULL;  9: len 4; hex 80000014; asc   ;;   ---TRANSACTION 4662, ACTIVE 81 sec starting index read mysql tables in use 1, locked 1 LOCK WAIT 2 lock struct(s), heap size 360, 1 row lock(s) MySQL thread id 7, OS thread handle 0x7f813c5c6700, query id 148 localhost root updating update emp set sal=3500 where empno=7788 ------- TRX HAS BEEN WAITING 81 SEC FOR THIS LOCK TO BE GRANTED: ## 等待了81s RECORD LOCKS space id 16 page no 3 n bits 88 index `PRIMARY` of table `test`.`emp` trx id 4662 lock_mode X locks rec but not gap waiting Record lock, heap no 9 PHYSICAL RECORD: n_fields 10; compact format; info bits 0 ## 线程7在等待往test.emp中的主键上加X锁，page num=3  0: len 4; hex 80001e6c; asc  l;;  1: len 6; hex 000000001018; asc    ;;  2: len 7; hex 91000001420084; asc   B ;;  3: len 5; hex 53434f5454; asc SCOTT;;  4: len 7; hex 414e414c595354; asc ANALYST;;  5: len 4; hex 80001d8e; asc   ;;  6: len 4; hex 208794f0; asc   ;;  7: len 4; hex 80000bb8; asc   ;;  8: SQL NULL;  9: len 4; hex 80000014; asc   ;;   ------------------ TABLE LOCK table `test`.`emp` trx id 4662 lock mode IX  ## 在给主键行上加X锁之前，先要在表上加意向锁IX RECORD LOCKS space id 16 page no 3 n bits 88 index `PRIMARY` of table `test`.`emp` trx id 4662 lock_mode X locks rec but not gap waiting Record lock, heap no 9 PHYSICAL RECORD: n_fields 10; compact format; info bits 0  0: len 4; hex 80001e6c; asc  l;;  1: len 6; hex 000000001018; asc    ;;  2: len 7; hex 91000001420084; asc   B ;;  3: len 5; hex 53434f5454; asc SCOTT;;  4: len 7; hex 414e414c595354; asc ANALYST;;  5: len 4; hex 80001d8e; asc   ;;  6: len 4; hex 208794f0; asc   ;;  7: len 4; hex 80000bb8; asc   ;;  8: SQL NULL;  9: len 4; hex 80000014; asc   ;;   ---TRANSACTION 4615, ACTIVE 1579 sec, thread declared inside InnoDB 1222 mysql tables in use 2, locked 0 2 lock struct(s), heap size 360, 1 row lock(s) MySQL thread id 3, OS thread handle 0x7f813c659700, query id 147 localhost root Sending data select count(*) from t3 a,t3 b  ## 这是线程3当前正在执行的SQL Trx read view will not see trx with id >= 4662, sees < 4659 TABLE LOCK table `test`.`emp` trx id 4615 lock mode IX ## 线程3中正在拥有表上的意向IX锁，并且有test.emp表上主键的行级X锁，page num=3 RECORD LOCKS space id 16 page no 3 n bits 88 index `PRIMARY` of table `test`.`emp` trx id 4615 lock_mode X locks rec but not gap Record lock, heap no 9 PHYSICAL RECORD: n_fields 10; compact format; info bits 0  0: len 4; hex 80001e6c; asc  l;;  1: len 6; hex 000000001018; asc    ;;  2: len 7; hex 91000001420084; asc   B ;;  3: len 5; hex 53434f5454; asc SCOTT;;  4: len 7; hex 414e414c595354; asc ANALYST;;  5: len 4; hex 80001d8e; asc   ;;  6: len 4; hex 208794f0; asc   ;;  7: len 4; hex 80000bb8; asc   ;;  8: SQL NULL;  9: len 4; hex 80000014; asc   ;;

为什么线程3当前执行的是一个select t3表操作，但却锁住了test.emp表上page num=3？

有可能是线程3之前对test.emp表的操作事务没有及时提交导致。

所以得出：线程3阻塞了线程7，而线程7又阻塞了线程4，所以根因就是线程3，让线程3尽快提交或是kill掉即可。

4. 结论

在分析innodb中锁阻塞时，几种方法的对比情况：

（1）使用show processlist查看不靠谱；

（2）直接使用show engine innodb status查看，无法判断到问题的根因；

（3）使用mysqladmin debug查看，能看到所有产生锁的线程，但无法判断哪个才是根因；

（4）开启innodb_lock_monitor后，再使用show engine innodb status查看，能够找到锁阻塞的根因。

原文链接：https://blog.csdn.net/hw_libo/article/details/39080809

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