MySQL DeadLock故障排查全过程记录

【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 statusG    *** (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：

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：

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

  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）：

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】

• 最佳的方法是添加column5和Column6的联合索引。
• 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：