有一回碰上在 mysql 5.5 中,一些表没法执行 DDL ,会一直被阻塞
有一回碰上在 mysql 5.5 中,一些表没法执行 DDL ,会一直被阻塞住。show processlist 会看到
mysql> show full processlist; +----+------+-----------------+------+---------+------+---------------------------------+-----------------------+ | Id | User | Host | db | Command | Time | State | Info | +----+------+-----------------+------+---------+------+---------------------------------+-----------------------+ ... | 6 | root | localhost:56841 | test | Query | 0 | init | show full processlist | | 7 | root | localhost:56843 | test | Query | 13 | Waiting for table metadata lock | rename table u to uu | ... +----+------+-----------------+------+---------+------+---------------------------------+-----------------------+
可以看出,是被一个 metadata lock 阻塞住了。但是从 process list 里是没法看出是哪个线程持有了这个锁。用别的方式,也只能得到通过 lock table 方式获取的锁或 innodb 事务的信息。对于一些比较隐蔽的方式,比如
mysql> begin; mysql> select * from test.u;
这样也同样会获取 metadata lock 。在 autocommit 为 0 的时候,没有显式 begin 也是如此。这类情况没法从任何途径获取究竟是谁上了锁。
于是我就试着做了一个 mysql 的 information schema 插件。 https://github.com/xiezhenye/mysql-plugin-mdl-lock 安装后,只要 select * from information_schema.MDL_LOCKS 就可以获取到所有 metadata lock ,也就是 mdl lock 的信息。
mysql> select * from information_schema.MDL_LOCKS; +-----------+-------------+---------------------+-----------+----------+------+ | THREAD_ID | DURATION | TYPE | NAMESPACE | DATABASE | NAME | +-----------+-------------+---------------------+-----------+----------+------+ | 7 | STATEMENT | INTENTION_EXCLUSIVE | GLOBAL | | | | 7 | TRANSACTION | SHARED_WRITE | TABLE | test | t | +-----------+-------------+---------------------+-----------+----------+------+ 2 rows in set (0.00 sec)
在写这个插件的时候,发现 mysql 把 mdl lock 的信息封装的相当严密,即使是写插件也没法用常规途径获取。只好用了一些很恶心的技巧。如果各位有碰到类似的问题,也可以先在测试环境试试。
原文地址:显示 metadata lock 信息的 mysql 插件, 感谢原作者分享。

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