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# 使用show processlist语句查看会话状态信息，发现DDL语句的state列值为Waiting for table metadata lock，表示在等待MDL元数据锁。根据MySQL 5.7及其之后的版本中的online ddl特性，该语句应该立即执行完成（它只会修改元数，因为这里只是修改了字段长度，并没有修改字段的其他属性），因此，故障现象确认 admin@localhost : (none) 11:48:22> show processlist; +----+-------+-----------+--------+---------+------+---------------------------------+-----------------------------+ | Id | User | Host | db | Command | Time | State | Info | +----+-------+-----------+--------+---------+------+---------------------------------+-----------------------------+ | 27 | admin | localhost | sbtest | Sleep | 123 | | NULL | | 28 | admin | localhost | sbtest | Query | 102 | Waiting for table metadata lock | alter table sbtest1 modify column `pad` varchar(70) COLLATE utf8_bin NOT NULL DEFAULT '' | | 29 | admin | localhost | NULL | Query | 0 | starting | show processlist | | 30 | admin | localhost | NULL | Sleep | 93 | | NULL | +----+-------+-----------+--------+---------+------+---------------------------------+-------------------------------+ 4 rows in set (0.00 sec)
首先，我们查看主机负载信息，通过下图我们可以看到，主机基本处于空载状态，毫无压力
然后，我们查看数据库的活跃会话数量及其状态，我们可以看到数据库中并没有大量会话，也不存在正在执行的DML语句在操作表sbtest1，也不存在同时有其他会话同时使用DDL在操作相同的表，但这里无法确认是否存在未提交的事务
# 反复多执行几次show processlist语句 admin@localhost : (none) 11:49:10> show processlist; +----+-------+-----------+--------+---------+------+---------------------------------+-----------------------------------+ | Id | User | Host | db | Command | Time | State | Info | +----+-------+-----------+--------+---------+------+---------------------------------+------------------------------------+ | 27 | admin | localhost | sbtest | Sleep | 149 | | NULL | | 28 | admin | localhost | sbtest | Query | 128 | Waiting for table metadata lock | alter table sbtest1 modify column `pad` varchar(70) COLLATE utf8_bin NOT NULL DEFAULT '' | | 29 | admin | localhost | NULL | Query | 0 | starting | show processlist | | 30 | admin | localhost | NULL | Sleep | 119 | | NULL | +----+-------+-----------+--------+---------+------+---------------------------------+---------------------------------------+ 4 rows in set (0.00 sec)
先查看information_schema中记录的事务信息
# 发现并没有事务存在... admin@localhost : sbtest 05:49:17> select * from information_schema.innodb_trx\G Empty set (0.00 sec) # 也可以顺便使用sys.innodb_lock_waits视图确认是否存在一些事务锁等待 admin@localhost : performance_schema 06:27:35> select * from sys.innodb_lock_waits\G Empty set, 3 warnings (0.00 sec) # 查询结果为空 查看performance_schema下的MDL元数据锁记录信息 # WTF..居然为空 admin@localhost : sbtest 06:00:21> select * from performance_schema.metadata_locks; Empty set (0.00 sec) # 也可以顺便使用sys.schema_table_lock_waits视图查看表级别的锁等待 admin@localhost : performance_schema 06:28:12> select * from sys.schema_table_lock_waits\G Empty set (0.00 sec) # 查询结果为空
查看performance_schema下的handle持有信息
# 发现表sbtest1的handle被thread_id=70的线程持有 admin@localhost : (none) 11:49:36> select * from performance_schema.table_handles where OWNER_THREAD_ID!=0; +-------------+---------------+-------------+--------+-----------------+----------------+---------------+---------------+ | OBJECT_TYPE | OBJECT_SCHEMA | OBJECT_NAME | OBJECT_INSTANCE_BEGIN | OWNER_THREAD_ID | OWNER_EVENT_ID | INTERNAL_LOCK | EXTERNAL_LOCK | +-------------+---------------+-------------+---------+-----------------+----------------+---------------+---------------+ | TABLE | sbtest | sbtest1 | 140049018564288 | 70 | 6 | NULL | NULL | +-------------+---------------+-------------+----------+-----------------+----------------+---------------+---------------+ 1 row in set (0.00 sec) # 通过performance_schema.threads表查看是哪个线程（thread_id是数据库内部的线程ID，我们需要看到与之对应的processlist id） admin@localhost : (none) 11:50:03> select thread_id,processlist_id,type from performance_schema.threads where processlist_id is not null and type='FOREGROUND'; +-----------+----------------+------------+ | thread_id | processlist_id | type | +-----------+----------------+------------+ | 43 | 1 | FOREGROUND | | 69 | 27 | FOREGROUND | | 70 | 28 | FOREGROUND | # 发现processlist id为28 | 71 | 29 | FOREGROUND | | 72 | 30 | FOREGROUND | +-----------+----------------+------------+ 5 rows in set (0.00 sec) # 通过show processlist再次查看一下id号，额。。发现id列为28的居然就是执行DDL语句被hang住那个会话，好吧，白忙活了 admin@localhost : (none) 11:50:26> show processlist; +----+-------+-----------+--------+---------+------+---------------------------------+------------------------------------+ | Id | User | Host | db | Command | Time | State | Info | +----+-------+-----------+--------+---------+------+---------------------------------+--------------------------------------+ | 27 | admin | localhost | sbtest | Sleep | 228 | | NULL | | 28 | admin | localhost | sbtest | Query | 207 | Waiting for table metadata lock | alter table sbtest1 modify column `pad` varchar(70) COLLATE utf8_bin NOT NULL DEFAULT '' | | 29 | admin | localhost | NULL | Query | 0 | starting | show processlist | | 30 | admin | localhost | NULL | Sleep | 198 | | NULL | +----+-------+-----------+--------+---------+------+---------------------------------+-----------------------------------------+ 4 rows in set (0.00 sec)
通过show engine innodb status查看下锁与事务信息
# 仍然没有发现有效的锁信息 admin@localhost : performance_schema 06:14:13> show engine innodb status; ...... ------------ TRANSACTIONS ------------ Trx id counter 11559 Purge done for trx's n:o < 11557 undo n:o < 0 state: running but idle History list length 60 LIST OF TRANSACTIONS FOR EACH SESSION: ---TRANSACTION 421628104988048, not started 0 lock struct(s), heap size 1136, 0 row lock(s) ---TRANSACTION 421628104987136, not started 0 lock struct(s), heap size 1136, 0 row lock(s) ---TRANSACTION 421628104985312, not started 0 lock struct(s), heap size 1136, 0 row lock(s) ---TRANSACTION 421628104984400, not started 0 lock struct(s), heap size 1136, 0 row lock(s) ---TRANSACTION 421628104986224, not started 0 lock struct(s), heap size 1136, 0 row lock(s) ......
排查到这里，视乎已经没招了，也许我们还可以用mysqladmin debug命令试试看
# 执行debug命令，执行该命令之后，一些锁信息可能会被debug出来打印到错误日志中 [root@physical-machine ~]# mysqladmin debug # 很遗憾，在错误日志中仍然没有发现有效的锁信息 [root@physical-machine ~]# vim /data/mysqldata1/log/error.log
等等，我们好像忽略了点什么，既然DDL语句在等待MDL元数据锁，为啥在performance_schema.metadata_locks表中没有记录？查看一下MDL事件采集器试试看？
# 喔。。发现居然MDL锁信息的采集器开关并没有打开，难怪metadata_locks表中无法记录MDL元数据锁信息
admin@localhost : performance_schema 06:30:16> select * from performance_schema.setup_instruments where name like '%/mdl';
+----------------------------+---------+-------+
| NAME | ENABLED | TIMED |
+----------------------------+---------+-------+
| wait/lock/metadata/sql/mdl | NO | NO |
+----------------------------+---------+-------+
1 row in set (0.00 sec)
# 现在，我们启用mdl的采集器
admin@localhost : sbtest 07:18:52> call sys.ps_setup_enable_instrument('sql/mdl');
+-----------------------+
| summary |
+-----------------------+
| Enabled 6 instruments |
+-----------------------+
1 row in set (0.00 sec)
Query OK, 0 rows affected (0.00 sec)
然后，我们重新查询information_schema下的事务和锁信息
# 查看information_schema.innodb_trx表，仍然没有事务信息 admin@localhost : sbtest 07:17:03> select * from information_schema.innodb_trx\G Empty set (0.00 sec) # 查看事务锁等待信息，仍然为空 admin@localhost : sbtest 07:17:30> select * from sys.innodb_lock_waits\G Empty set, 3 warnings (0.01 sec)
查看performance_schema下的MDL元数据锁记录信息
# 查看metadata_locks表中的MDL锁信息，咦，有信息了！不过，有点乱啊（我们这里只查询sbtest库下的sbtest1表就可以了，因为我们的操作也只涉及到这张表） admin@localhost : (none) 11:52:46> select * from performance_schema.metadata_locks where OBJECT_SCHEMA='sbtest' and OBJECT_NAME='sbtest1'; +-------------+---------------+-------------+-----+--------+---------------+-------------+--------+-----------------+----------------+ | OBJECT_TYPE | OBJECT_SCHEMA | OBJECT_NAME | OBJECT_INSTANCE_BEGIN | LOCK_TYPE | LOCK_DURATION | LOCK_STATUS | SOURCE | OWNER_THREAD_ID | OWNER_EVENT_ID | +-------------+---------------+-------------+-----+--------+---------------+-------------+--------+-----------------+----------------+ # 从这行信息我们可以看到，表sbtest1上有一把SHARED_WRITE锁处于GRANTED状态，为thread_id=69的线程所持有。SHARED_WRITE类型的MDL锁是一把意向排他IX锁，通常在执行MDL或select ... for update时产生 | TABLE | sbtest | sbtest1 | 140048817276288 | SHARED_WRITE | TRANSACTION | GRANTED | | 69 | 11 | # 从这行信息我们可以看到，表sbtest1上有一把SHARED_UPGRADABLE的锁处于GRANTED状态，为thread_id=70的线程持有，从上文中的信息我们可以知道，thread_id=70的process id为28，也就是执行DDL语句被阻塞的那个会话id。SHARED_UPGRADABLE类型的MDL锁是一把共享升级锁，一般在执行online DDL语句时会产生。它的作用是在执行online ddl期间允许相同表的DML但防止DDL | TABLE | sbtest | sbtest1 | 140049018604784 | SHARED_UPGRADABLE | TRANSACTION | GRANTED | | 70 | 6 | # 从这行信息我们可以看到，表sbtest1上有一把EXCLUSIVE处于PENDING状态，为thread_id=70的线程在等待，从上文中的信息我们可以知道，thread_id=70的process id为28，也就是执行DDL语句被阻塞的那个会话id。EXCLUSIVE类型的MDL锁是一把排它X锁，用于阻止其他线程读写元数据信息，一般在执行DDL时产生 | TABLE | sbtest | sbtest1 | 140049018564112 | EXCLUSIVE | TRANSACTION | PENDING | | 70 | 6 | +-------------+---------------+-------------+-----+---------+---------------+-------------+--------+-----------------+----------------+ 3 rows in set (0.00 sec) # 通过上述信息我们可以得知，thread_id=70的线程需要获取的EXCLUSIVE锁与thread_id=69的线程所持有的SHARED_WRITE锁冲突了，但thread_id=69线程的process id是多少呢？查看一下performance_schema.threads表 admin@localhost : (none) 11:53:47> select thread_id,processlist_id,type from performance_schema.threads where processlist_id is not null and type='FOREGROUND'; +-----------+----------------+------------+ | thread_id | processlist_id | type | +-----------+----------------+------------+ | 43 | 1 | FOREGROUND | | 69 | 27 | FOREGROUND | # 发现thread_id=69的线程process id为27 | 70 | 28 | FOREGROUND | | 71 | 29 | FOREGROUND | | 74 | 32 | FOREGROUND | +-----------+----------------+------------+ 5 rows in set (0.01 sec) # 到这里，我们知道了DDL语句就是被process id为27的线程阻塞的，但，还不是很直观，查看起来比较繁琐，我们还是直接使用sys.schema_table_lock_waits视图查看表级别的锁等待试试看吧，可以发现，该视图打印的信息看起来就很清晰了（可以清晰看到谁在等待，谁持有锁），不过，怎么有两行？以哪行为准呢？ admin@localhost : (none) 11:59:04> select * from sys.schema_table_lock_waits\G *************************** 1. row *************************** object_schema: sbtest object_name: sbtest1 waiting_thread_id: 70 waiting_pid: 28 waiting_account: admin@localhost waiting_lock_type: EXCLUSIVE waiting_lock_duration: TRANSACTION waiting_query: alter table sbtest1 modify col ... E utf8_bin NOT NULL DEFAULT '' waiting_query_secs: 744 waiting_query_rows_affected: 0 waiting_query_rows_examined: 0 blocking_thread_id: 69 blocking_pid: 27 blocking_account: admin@localhost blocking_lock_type: SHARED_WRITE blocking_lock_duration: TRANSACTION sql_kill_blocking_query: KILL QUERY 27 # 这一行表锁等待信息提示kill 掉process id为27的线程 sql_kill_blocking_connection: KILL 27 *************************** 2. row *************************** object_schema: sbtest object_name: sbtest1 waiting_thread_id: 70 waiting_pid: 28 waiting_account: admin@localhost waiting_lock_type: EXCLUSIVE waiting_lock_duration: TRANSACTION waiting_query: alter table sbtest1 modify col ... E utf8_bin NOT NULL DEFAULT '' waiting_query_secs: 744 waiting_query_rows_affected: 0 waiting_query_rows_examined: 0 blocking_thread_id: 70 blocking_pid: 28 blocking_account: admin@localhost blocking_lock_type: SHARED_UPGRADABLE blocking_lock_duration: TRANSACTION sql_kill_blocking_query: KILL QUERY 28 # 这一行表锁等待信息提示kill 掉process id为28的线程 sql_kill_blocking_connection: KILL 28 2 rows in set (0.01 sec)
DBA侧可通过在执行DDL语句的会话中，会话级别设置lock_wait_timeout系统变量为一个较小的值，在超过该时间值之后，仍然无法获得所需的锁时，自动放弃DDL操作（请自行评估需求）
root@localhost : sbtest 04:37:43> set lock_wait_timeout=10; Query OK, 0 rows affected (0.00 sec) root@localhost : sbtest 04:37:47> alter table sbtest1 modify column `pad` varchar(70) COLLATE utf8_bin NOT NULL DEFAULT ''; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
2）在会话1中，开启一个显式事务，并使用INSERT语句插入一行超过字段定义长度的数据行（模拟应用侧插入数据）
admin@localhost : sbtest:13: > begin; Query OK, 0 rows affected (0.00 sec) admin@localhost : sbtest:17: > insert into sbtest1 values(2,2,'40393031789-25132409365-58213491013-66541287984-65586459874-05762316127-59922091522-12151119251-49498591378-18011532520','test-29736863337-73672352543-26439979097-89323822066-87557735686'); ERROR 1406 (22001): Data too long for column 'pad' at row 1
3）在会话2中，将报错字段pad的长度加长到70个字符（模拟DBA侧使用DDL语句修改列长度定义）
# 查看表结构中的字段定义长度，可以发现，报错的pad列定义长度为varchar类型的60个字符长度
root@localhost : sbtest 04:12:03> show create table sbtest1;
+---------+------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+---------+-------------------------------------------------------------------------------------------------------+
| sbtest1 | CREATE TABLE `sbtest1` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`k` int(11) NOT NULL DEFAULT '0',
`c` char(120) COLLATE utf8_bin NOT NULL DEFAULT '',
`pad` varchar(60) COLLATE utf8_bin NOT NULL DEFAULT '', # pad字段的定义长度为60
PRIMARY KEY (`id`),
KEY `k_1` (`k`)
) ENGINE=InnoDB AUTO_INCREMENT=20000001 DEFAULT CHARSET=utf8 COLLATE=utf8_bin |
+---------+-------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)
# 查看INSERT语句中，pad列给出数据字符串的字符长度与字节长度
## INSERT语句中给出的pad列值字符长度为64
root@localhost : sbtest 11:01:33> select char_length('test-29736863337-73672352543-26439979097-89323822066-87557735686');
+---------------------------------------------------------------------------------+
| char_length('test-29736863337-73672352543-26439979097-89323822066-87557735686') |
+---------------------------------------------------------------------------------+
| 64 |
+---------------------------------------------------------------------------------+
1 row in set (0.00 sec)
## INSERT语句中给出的pad列值字节长度为64
root@localhost : sbtest 11:02:19> select length('test-29736863337-73672352543-26439979097-89323822066-87557735686');
+----------------------------------------------------------------------------+
| length('test-29736863337-73672352543-26439979097-89323822066-87557735686') |
+----------------------------------------------------------------------------+
| 64 |
+----------------------------------------------------------------------------+
1 row in set (0.00 sec)
# 使用alter语句修改pad列定义长度为70
root@localhost : sbtest 04:12:47> alter table sbtest1 modify column `pad` varchar(70) COLLATE utf8_bin NOT NULL DEFAULT '';
## 发生阻塞
4）在会话3中，查看数据库中的会话状态信息（模拟DBA侧排查故障问题）
# 可以看到info列的alter语句的state列值为Waiting for table metadata lock，表示在等待MDL锁 admin@localhost : (none) 11:50:55> show processlist; +----+-------+-----------+--------+---------+------+---------------------------------+--------------------------------+ | Id | User | Host | db | Command | Time | State | Info | +----+-------+-----------+--------+---------+------+---------------------------------+--------------------------------+ | 27 | admin | localhost | sbtest | Sleep | 919 | | NULL | | 28 | admin | localhost | sbtest | Query | 898 | Waiting for table metadata lock | alter table sbtest1 modify column `pad` varchar(70) COLLATE utf8_bin NOT NULL DEFAULT '' | | 29 | admin | localhost | NULL | Query | 0 | starting | show processlist | | 32 | admin | localhost | NULL | Sleep | 154 | | NULL | +----+-------+-----------+--------+---------+------+---------------------------------+------------------------------------+ 4 rows in set (0.00 sec)
PS：MDL元数据锁的类型有很多种，根据官方手册中对performance_schema.metadata_locks表的LOCK_TYPE字段的描述可得知，一共有9种（INTENTION_EXCLUSIVE、SHARED、SHARED_HIGH_PRIO、SHARED_READ、SHARED_WRITE、SHARED_UPGRADABLE、SHARED_NO_WRITE、SHARED_NO_READ_WRITE、EXCLUSIVE），但，官方手册中并未找到每一种MDL锁的具体含义和发生的场景，关于MDL锁更详细的信息可参考如下这两个链接
https://blog.csdn.net/finalkof1983/article/details/88063328
| 作者简介
罗小波·沃趣科技高级数据库技术专家
IT从业多年，主要负责MySQL 产品的数据库支撑与售后二线支撑。曾参与版本发布系统、轻量级监控系统、运维管理平台、数据库管理平台的设计与编写，熟悉MySQL体系结构，Innodb存储引擎，喜好专研开源技术，多次在公开场合做过线下线上数据库专题分享，发表过多篇数据库相关的研究文章。
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