MySQL DeadLock故障排查过程

2019-01-02 14:13:26来源:cnblogs.com作者:携程DBA人点击

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【作者】

刘博:携程技术保障中心数据库高级经理,主要关注Sql server和Mysql的运维和故障处理。

【环境】

版本号:5.6.21
隔离级别:REPEATABLE READ

【问题描述】

接到监控报警,有一个线上的应用DeadLock报错,每15分钟会准时出现,报错统计如下图:

登录Mysql服务器查看日志:

mysql> show engine innodb status/G
*** (1) TRANSACTION:
TRANSACTION 102973, ACTIVE 11 sec starting index read
mysql tables in use 3, locked 3
LOCK WAIT 4 lock struct(s), heap size 1136, 3 row lock(s)
MySQL thread id 6, OS thread handle 140024996574976, query id 83 localhost us updating
UPDATE TestTable
SET column1 = 1,
Column2 = sysdate(),
Column3= '026’
Column4 = 0
AND column5 = 485
AND column6 = 'SEK'
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 417 page no 1493 n bits 1000 index idx_column6 of table test.TestTable trx id 102973 lock_mode X waiting
Record lock, heap no 859 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 3; hex 53454b; asc SEK;;
1: len 8; hex 80000000007e1452; asc ~ R;;

*** (2) TRANSACTION:
TRANSACTION 102972, ACTIVE 26 sec starting index read
mysql tables in use 3, locked 3
219 lock struct(s), heap size 24784, 2906 row lock(s), undo log entries 7
MySQL thread id 5, OS thread handle 140024996841216, query id 84 localhost us updating
UPDATE TestTable
SET Column1 = 1,
Column2 = sysdate(),
Column3 = '026'
Column4 = 0
AND Column5 = 485
AND Column6 = 'SEK'
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 417 page no 1493 n bits 1000 index idx_Column6 of table test.TestTable trx id 102972 lock_mode X
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
0: len 8; hex 73757072656d756d; asc supremum;;

Record lock, heap no 859 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 3; hex 53454b; asc SEK;;
1: len 8; hex 80000000007e1452; asc ~ R;;

*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 601 page no 89642 n bits 1000 index idx_column6 of table test.TestTable trx id 32231892482 lock_mode X locks rec but not gap waiting
Record lock, heap no 38 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 3; hex 53454b; asc SEK;;
1: len 8; hex 80000000007eea14; asc ~ ;;

大致一看,更新同一索引的同一行,应该是一个Block,报TimeOut的错才对,怎么会报DeadLock?

【初步分析】

先分析下(2) TRANSACTION,TRANSACTION 32231892482。
等待的锁信息为:

0: len 3; hex 53454b; asc SEK;;
1: len 8; hex 80000000007eea14; asc

持有的锁信息为:
0: len 3; hex 53454b; asc SEK;;
1: len 8; hex 80000000007eeac4; asc

再先分析下(1) TRANSACTION,TRANSACTION 32231892617。
等待的锁信息为:
0: len 3; hex 53454b; asc SEK;;
1: len 8; hex 80000000007eeac4; asc

于是可以画出的死锁表,两个资源相互依赖,造成死锁:

TRANSACTIONHoldWait
3223189261753454b/80000000007eea1453454b/80000000007eeac4
3223189248253454b/80000000007eeac453454b/80000000007eea14

让我们再看一下explain结果:

mysql>desc UPDATE TestTable SET Column1=1, Column2 = sysdate(),Column3 = '025' Column4 = 0 AND Column5 = 477 AND Column6 = 'SEK' /G;
*************************** 1. row ***************************
id: 1
select_type: UPDATE
table: TestTable
partitions: NULL
type: index_merge
possible_keys: column5_index,idx_column5_column6_Column1,idxColumn6
key: column5_index,idxColumn6
key_len: 8,9
ref: NULL
rows: 7
filtered: 100.00
Extra: Using intersect(column5_index,idxColumn6); Using where

可以看到 EXTRA 列:

Using intersect(column5_index,idxColumn6)

从5.1开始,引入了 index merge 优化技术,对同一个表可以使用多个索引分别进行条件扫描。
相关文档:http://dev.mysql.com/doc/refman/5.7/en/index-merge-optimization.html

The Index Merge method is used to retrieve rows with several range scans and to merge their results into one. The merge can produce unions, intersections, or unions-of-intersections of its underlying scans. This access method merges index scans from a single table; it does not merge scans across multiple tables.

【模拟与验证】

根据以上初步分析,猜测应该就是intersect造成的,于是在测试环境模拟验证,开启2个session模拟死锁:

时间序列Session1Session2
1Begin;
2UPDATE TestTable SET Column2 = sysdate() Column4 = 0 AND Column5 = 47 AND Column6 = 'SEK
执行成功,影响7行
3Begin;
4UPDATE TestTable SET Column2 = sysdate(),Column4 = 0 AND Column5 = 485 AND Column6 = 'SEK';
被Blocking
5UPDATE TestTable SET Column2 = sysdate(),Column4 = 0 AND Column5 = 485 AND Column6 = 'SEK';
执行成功
ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction

依据以上信息可以发现Session2虽然被Block了,但也获取了一些Session1在时间序列5时所需资源的X锁,可以再开启一个查询select count(Column5) from TestTable where Column5 = 485,设置SET TRANSACTION ISOLATION LEVEL SERIALIZABLE,去查询Column5 = 485的行,观察锁等待的信息:

mysql> SELECT r.trx_id waiting_trx_id, r.trx_mysql_thread_id waiting_thread, r.trx_query waiting_query, b.trx_id blocking_trx_id, b.trx_mysql_thread_id blocking_thread, b.trx_query blocking_query FROM information_schema.innodb_lock_waits w INNER JOIN information_schema.innodb_trx b ON b.trx_id = w.blocking_trx_id INNER JOIN information_schema.innodb_trx r ON r.trx_id = w.requesting_trx_id /G;
*************************** 1. row ***************************
waiting_trx_id: 103006
waiting_thread: 36
waiting_query: UPDATE TestTable SET Column1 = 1, Column2 = sysdate(), Column3 = '026' Column4 = 0 AND Column5 = 485 AND Column6 = 'SEK'
blocking_trx_id: 103003
blocking_thread: 37
blocking_query: NULL
*************************** 2. row ***************************
waiting_trx_id: 421500433538672
waiting_thread: 39
waiting_query: select count(Column5) from TestTable where Column5 = 485
blocking_trx_id: 103006
blocking_thread: 36
blocking_query: UPDATE TestTable SET Column1 = 1, Column2 = sysdate(), Column3 = '026' Column4 = 0 AND Column5 = 485 AND Column6 = 'SEK'
2 rows in set, 1 warning (0.00 sec)

mysql> select * from information_schema.innodb_lock_waits /G;
*************************** 1. row ***************************
requesting_trx_id: 103006
requested_lock_id: 103006:417:1493:859
blocking_trx_id: 103003
blocking_lock_id: 103003:417:1493:859
*************************** 2. row ***************************
requesting_trx_id: 421500433538672
requested_lock_id: 421500433538672:417:749:2
blocking_trx_id: 103006
blocking_lock_id: 103006:417:749:2
2 rows in set, 1 warning (0.00 sec)

mysql> select * from INNODB_LOCKS /G;
*************************** 1. row ***************************
lock_id: 103006:417:1493:859
lock_trx_id: 103006
lock_mode: X
lock_type: RECORD
lock_table: test.TestTable
lock_index: idxColumn6
lock_space: 417
lock_page: 1493
lock_rec: 859
lock_data: 'SEK', 8262738
*************************** 2. row ***************************
lock_id: 103003:417:1493:859
lock_trx_id: 103003
lock_mode: X
lock_type: RECORD
lock_table:test.TestTable
lock_index: idxColumn6
lock_space: 417
lock_page: 1493
lock_rec: 859
lock_data: 'SEK', 8262738
*************************** 3. row ***************************
lock_id: 421500433538672:417:749:2
lock_trx_id: 421500433538672
lock_mode: S
lock_type: RECORD
lock_table: test.TestTable
lock_index: column5_index
lock_space: 417
lock_page: 749
lock_rec: 2
lock_data: 485, 8317620
*************************** 4. row ***************************
lock_id: 103006:417:749:2
lock_trx_id: 103006
lock_mode: X
lock_type: RECORD
lock_table: test.TestTable
lock_index: column5_index
lock_space: 417
lock_page: 749
lock_rec: 2
lock_data: 485, 8317620
4 rows in set, 1 warning (0.00 sec)

可以看到Session2,trx_id 103006阻塞了trx_id 421500433538672,而trx_id 421500433538672 requested_lock也正好是lock_data: 485, 8317620。由此可见Session2虽然别block了,但是还是获取到了Index column5_index相关的锁。被Block是因为intersect的原因,还需要idxColumn6的锁,至此思路已经清晰,对整个分配锁的信息简化一下,如下表格(请求到的锁用青色表示,需获取但未获取到的锁用红色表示):

时间点Session1Session2
1477 SEK
2485 SEK
3485 SEK死锁发生

可以看到485 SEK这两个资源形成了一个环状,最终发生死锁。

【解决方法】

  1. 最佳的方法是添加column5和Column6的联合索引。
  2. 我们环境当时的情况发现Column6的筛选度非常低,就删除了Column6的索引。
    10:55左右删除索引后,报错没有再发生:

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