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解决MySQL5.7中定位DDL被阻塞的问题

时间:2021-07-01 10:21:17 帮助过:24人阅读

在上篇文章《MySQL表结构变更,不可不知的Metadata Lock》中,我们介绍了MDL引入的背景,及基本概念,从“道”的层面知道了什么是MDL。下面就从“术”的层面看看如何定位MDL的相关问题。

在MySQL 5.7中,针对MDL,引入了一张新表performance_schema.metadata_locks,该表可对外展示MDL的相关信息,包括其作用对象,类型及持有等待情况。

开启MDL的instrument

但是相关instrument并没有开启(MySQL 8.0是默认开启的),其可通过如下两种方式开启,

临时生效

修改performance_schema.setup_instrume nts表,但实例重启后,又会恢复为默认值。

  1. UPDATE performance_schema.setup_instruments SET ENABLED = 'YES', TIMED = 'YES'
  2. WHERE NAME = 'wait/lock/metadata/sql/mdl';

永久生效

在配置文件中设置

  1. [mysqld]
  2. performance-schema-instrument='wait/lock/metadata/sql/mdl=ON'

测试场景

下面结合一个简单的Demo,来看看在MySQL 5.7中如何定位DDL操作的阻塞问题。

  1. session1> begin;
  2. Query OK, 0 rows affected (0.00 sec)
  3. session1> delete from slowtech.t1 where id=2;
  4. Query OK, 1 row affected (0.00 sec)
  5. session1> select * from slowtech.t1;
  6. +------+------+
  7. | id | name |
  8. +------+------+
  9. | 1 | a |
  10. +------+------+
  11. 1 row in set (0.00 sec)
  12. session1> update slowtech.t1 set name='c' where id=1;
  13. Query OK, 1 row affected (0.00 sec)
  14. Rows matched: 1 Changed: 1 Warnings: 0
  15. session2> alter table slowtech.t1 add c1 int; ##被阻塞
  16. session3> show processlist;
  17. +----+------+-----------+------+---------+------+---------------------------------+------------------------------------+
  18. | Id | User | Host | db | Command | Time | State | Info |
  19. +----+------+-----------+------+---------+------+---------------------------------+------------------------------------+
  20. | 2 | root | localhost | NULL | Sleep | 51 | | NULL |
  21. | 3 | root | localhost | NULL | Query | 0 | starting | show processlist |
  22. | 4 | root | localhost | NULL | Query | 9 | Waiting for table metadata lock | alter table slowtech.t1 add c1 int |
  23. +----+------+-----------+------+---------+------+---------------------------------+------------------------------------+
  24. 3 rows in set (0.00 sec)
  25. session3> select object_type,object_schema,object_name,lock_type,lock_duration,lock_status,owner_thread_id from performance_schema.metadata_locks;
  26. +-------------+--------------------+----------------+---------------------+---------------+-------------+-----------------+
  27. | object_type | object_schema | object_name | lock_type | lock_duration | lock_status | owner_thread_id |
  28. +-------------+--------------------+----------------+---------------------+---------------+-------------+-----------------+
  29. | TABLE | slowtech | t1 | SHARED_WRITE | TRANSACTION | GRANTED | 27 |
  30. | GLOBAL | NULL | NULL | INTENTION_EXCLUSIVE | STATEMENT | GRANTED | 29 |
  31. | SCHEMA | slowtech | NULL | INTENTION_EXCLUSIVE | TRANSACTION | GRANTED | 29 |
  32. | TABLE | slowtech | t1 | SHARED_UPGRADABLE | TRANSACTION | GRANTED | 29 |
  33. | TABLE | slowtech | t1 | EXCLUSIVE | TRANSACTION | PENDING | 29 |
  34. | TABLE | performance_schema | metadata_locks | SHARED_READ | TRANSACTION | GRANTED | 28 |
  35. +-------------+--------------------+----------------+---------------------+---------------+-------------+-----------------+
  36. 6 rows in set (0.00 sec)

这里,重点关注lock_status,"PENDING"代表线程在等待MDL,而"GRANTED"则代表线程持有MDL。

如何找出引起阻塞的会话

结合owner_thread_id,可以可到,是29号线程在等待27号线程的MDL,此时,可kill掉52号线程。

但需要注意的是,owner_thread_id给出的只是线程ID,并不是show processlist中的ID。如果要查找线程对应的processlist id,需查询performance_schema.threads表。

  1. session3> select * from performance_schema.threads where thread_id in (27,29)\G
  2. *************************** 1. row ***************************
  3. THREAD_ID: 27
  4. NAME: thread/sql/one_connection
  5. TYPE: FOREGROUND
  6. PROCESSLIST_ID: 2
  7. PROCESSLIST_USER: root
  8. PROCESSLIST_HOST: localhost
  9. PROCESSLIST_DB: NULL
  10. PROCESSLIST_COMMAND: Sleep
  11. PROCESSLIST_TIME: 214
  12. PROCESSLIST_STATE: NULL
  13. PROCESSLIST_INFO: NULL
  14. PARENT_THREAD_ID: 1
  15. ROLE: NULL
  16. INSTRUMENTED: YES
  17. HISTORY: YES
  18. CONNECTION_TYPE: Socket
  19. THREAD_OS_ID: 9800
  20. *************************** 2. row ***************************
  21. THREAD_ID: 29
  22. NAME: thread/sql/one_connection
  23. TYPE: FOREGROUND
  24. PROCESSLIST_ID: 4
  25. PROCESSLIST_USER: root
  26. PROCESSLIST_HOST: localhost
  27. PROCESSLIST_DB: NULL
  28. PROCESSLIST_COMMAND: Query
  29. PROCESSLIST_TIME: 172
  30. PROCESSLIST_STATE: Waiting for table metadata lock
  31. PROCESSLIST_INFO: alter table slowtech.t1 add c1 int
  32. PARENT_THREAD_ID: 1
  33. ROLE: NULL
  34. INSTRUMENTED: YES
  35. HISTORY: YES
  36. CONNECTION_TYPE: Socket
  37. THREAD_OS_ID: 9907
  38. 2 rows in set (0.00 sec)

将这两张表结合,借鉴sys.innodb_lock _waits的输出,实际上我们也可以直观地呈现MDL的等待关系。

  1. SELECT
  2. a.OBJECT_SCHEMA AS locked_schema,
  3. a.OBJECT_NAME AS locked_table,
  4. "Metadata Lock" AS locked_type,
  5. c.PROCESSLIST_ID AS waiting_processlist_id,
  6. c.PROCESSLIST_TIME AS waiting_age,
  7. c.PROCESSLIST_INFO AS waiting_query,
  8. c.PROCESSLIST_STATE AS waiting_state,
  9. d.PROCESSLIST_ID AS blocking_processlist_id,
  10. d.PROCESSLIST_TIME AS blocking_age,
  11. d.PROCESSLIST_INFO AS blocking_query,
  12. concat('KILL ', d.PROCESSLIST_ID) AS sql_kill_blocking_connection
  13. FROM
  14. performance_schema.metadata_locks a
  15. JOIN performance_schema.metadata_locks b ON a.OBJECT_SCHEMA = b.OBJECT_SCHEMA
  16. AND a.OBJECT_NAME = b.OBJECT_NAME
  17. AND a.lock_status = 'PENDING'
  18. AND b.lock_status = 'GRANTED'
  19. AND a.OWNER_THREAD_ID <> b.OWNER_THREAD_ID
  20. AND a.lock_type = 'EXCLUSIVE'
  21. JOIN performance_schema.threads c ON a.OWNER_THREAD_ID = c.THREAD_ID
  22. JOIN performance_schema.threads d ON b.OWNER_THREAD_ID = d.THREAD_ID\G
  23. *************************** 1. row ***************************
  24. locked_schema: slowtech
  25. locked_table: t1
  26. locked_type: Metadata Lock
  27. waiting_processlist_id: 4
  28. waiting_age: 259
  29. waiting_query: alter table slowtech.t1 add c1 int
  30. waiting_state: Waiting for table metadata lock
  31. blocking_processlist_id: 2
  32. blocking_age: 301
  33. blocking_query: NULL
  34. sql_kill_blocking_connection: KILL 2
  35. 1 row in set (0.00 sec)

输出一目了然,DDL操作如果要获得MDL,执行kill 2即可。

官方的sys.schematablelock_waits

实际上,MySQL 5.7在sys库中也集成了类似功能,同样的场景,其输出如下,

  1. mysql> select * from sys.schema_table_lock_waits\G
  2. *************************** 1. row ***************************
  3. object_schema: slowtech
  4. object_name: t1
  5. waiting_thread_id: 29
  6. waiting_pid: 4
  7. waiting_account: root@localhost
  8. waiting_lock_type: EXCLUSIVE
  9. waiting_lock_duration: TRANSACTION
  10. waiting_query: alter table slowtech.t1 add c1 int
  11. waiting_query_secs: 446
  12. waiting_query_rows_affected: 0
  13. waiting_query_rows_examined: 0
  14. blocking_thread_id: 27
  15. blocking_pid: 2
  16. blocking_account: root@localhost
  17. blocking_lock_type: SHARED_READ
  18. blocking_lock_duration: TRANSACTION
  19. sql_kill_blocking_query: KILL QUERY 2
  20. sql_kill_blocking_connection: KILL 2
  21. *************************** 2. row ***************************
  22. object_schema: slowtech
  23. object_name: t1
  24. waiting_thread_id: 29
  25. waiting_pid: 4
  26. waiting_account: root@localhost
  27. waiting_lock_type: EXCLUSIVE
  28. waiting_lock_duration: TRANSACTION
  29. waiting_query: alter table slowtech.t1 add c1 int
  30. waiting_query_secs: 446
  31. waiting_query_rows_affected: 0
  32. waiting_query_rows_examined: 0
  33. blocking_thread_id: 29
  34. blocking_pid: 4
  35. blocking_account: root@localhost
  36. blocking_lock_type: SHARED_UPGRADABLE
  37. blocking_lock_duration: TRANSACTION
  38. sql_kill_blocking_query: KILL QUERY 4
  39. sql_kill_blocking_connection: KILL 4
  40. 2 rows in set (0.00 sec)

具体分析下官方的输出,

只有一个alter table操作,却产生了两条记录,而且两条记录的kill对象竟然还不一样,对表结构不熟悉及不仔细看记录内容的话,难免会kill错对象。

不仅如此,如果有N个查询被DDL操作堵塞,则会产生N*2条记录。在阻塞操作较多的情况下,这N*2条记录完全是个噪音。

而之前的SQL,无论有多少操作被阻塞,一个alter table操作,就只会输出一条记录。

如何查看阻塞会话已经执行过的操作

但上面这个SQL也有遗憾,其blocking_query为NULL,而在会话1中,其明明已经执行了三个SQL。

这个与performance_schema.threads(类似于show processlist)有关,其只会输出当前正在运行的SQL,对于已经执行过的,实际上是没办法看到。

但在线上,kill是一个需要谨慎的操作,毕竟你很难知道kill的是不是业务关键操作?又或者,是个批量update操作?那么,有没有办法抓到该事务之前的操作呢?

答案,有。

即Performance Schema中记录Statement Event(操作事件)的表,具体包括

events_statements_current,events_statements_history,events_statements_history_long,prepared_statements_instances。

常用的是前面三个。

三者的表结构完全一致,其中,events_statements_history又包含了events_statements_current的操作,所以我们这里会使用events_statements_history。

终极SQL如下,

  1. SELECT
  2. locked_schema,
  3. locked_table,
  4. locked_type,
  5. waiting_processlist_id,
  6. waiting_age,
  7. waiting_query,
  8. waiting_state,
  9. blocking_processlist_id,
  10. blocking_age,
  11. substring_index(sql_text,"transaction_begin;" ,-1) AS blocking_query,
  12. sql_kill_blocking_connection
  13. FROM
  14. (
  15. SELECT
  16. b.OWNER_THREAD_ID AS granted_thread_id,
  17. a.OBJECT_SCHEMA AS locked_schema,
  18. a.OBJECT_NAME AS locked_table,
  19. "Metadata Lock" AS locked_type,
  20. c.PROCESSLIST_ID AS waiting_processlist_id,
  21. c.PROCESSLIST_TIME AS waiting_age,
  22. c.PROCESSLIST_INFO AS waiting_query,
  23. c.PROCESSLIST_STATE AS waiting_state,
  24. d.PROCESSLIST_ID AS blocking_processlist_id,
  25. d.PROCESSLIST_TIME AS blocking_age,
  26. d.PROCESSLIST_INFO AS blocking_query,
  27. concat('KILL ', d.PROCESSLIST_ID) AS sql_kill_blocking_connection
  28. FROM
  29. performance_schema.metadata_locks a
  30. JOIN performance_schema.metadata_locks b ON a.OBJECT_SCHEMA = b.OBJECT_SCHEMA
  31. AND a.OBJECT_NAME = b.OBJECT_NAME
  32. AND a.lock_status = 'PENDING'
  33. AND b.lock_status = 'GRANTED'
  34. AND a.OWNER_THREAD_ID <> b.OWNER_THREAD_ID
  35. AND a.lock_type = 'EXCLUSIVE'
  36. JOIN performance_schema.threads c ON a.OWNER_THREAD_ID = c.THREAD_ID
  37. JOIN performance_schema.threads d ON b.OWNER_THREAD_ID = d.THREAD_ID
  38. ) t1,
  39. (
  40. SELECT
  41. thread_id,
  42. group_concat( CASE WHEN EVENT_NAME = 'statement/sql/begin' THEN "transaction_begin" ELSE sql_text END ORDER BY event_id SEPARATOR ";" ) AS sql_text
  43. FROM
  44. performance_schema.events_statements_history
  45. GROUP BY thread_id
  46. ) t2
  47. WHERE
  48. t1.granted_thread_id = t2.thread_id \G
  49. *************************** 1. row ***************************
  50. locked_schema: slowtech
  51. locked_table: t1
  52. locked_type: Metadata Lock
  53. waiting_processlist_id: 4
  54. waiting_age: 294
  55. waiting_query: alter table slowtech.t1 add c1 int
  56. waiting_state: Waiting for table metadata lock
  57. blocking_processlist_id: 2
  58. blocking_age: 336
  59. blocking_query: delete from slowtech.t1 where id=2;select * from slowtech.t1;update slowtech.t1 set name='c' where id=1
  60. sql_kill_blocking_connection: KILL 2
  61. 1 row in set, 1 warning (0.00 sec)

从上面的输出可以看到,blocking_query中包含了会话1中当前事务的所有操作,按执行的先后顺序输出。

需要注意的是,默认情况下,events_statements_history只会保留每个线程最近的10个操作,如果事务中进行的操作较多,实际上也是没办法抓全的。

总结

以上所述是小编给大家介绍的解决MySQL 5.7中如何定位DDL被阻塞的问题,希望对大家有所帮助,如果大家有任何疑问请给我留言,小编会及时回复大家的。在此也非常感谢大家对脚本之家网站的支持!

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