MDL(Meta Data LocK)的作用
在MySQL5.1及之前的版本中，如果有未提交的事务trx，当执行DROP/RENAME/ALTER TABLE RENAME操作时，不会被其他事务阻塞住。这会导致如下问题(MySQL bug#989)
master： 未提交的事务，但SQL已经完成(binlog也准备好了)，表schema发生更改，在commit的时候不会被察觉到.
slave： 在binlog里是以事务提交顺序记录的，DDL隐式提交，因此在备库先执行DDL，后执行事务trx，由于trx作用的表已经发生了改变，因此trx会执行失败。 在DDL时的主库DML压力越大，这个问题触发的可能性就越高
在5.5引入了MDL（meta data lock）锁来解决在这个问题
MDL锁的类型
metadata lock也是一种锁。每个metadata lock都会定义锁住的对象，锁的持有时间和锁的类型
这些锁具有以下层级关系
MDL锁的简单示例
在实际工作中，最常见的MDL冲突就DDL的操作被没用提交的事务所阻塞。 我们下面通过一个具体的实例来演示DDL加MDL锁的过程。在这个实例中，利用gdb来跟踪DDL申请MDL锁的过程。
会话1:
mysql> create table ti(id int primary key, c1 int, key(c1)) engine=InnoDB stats_auto_recalc=default; Query OK, 0 rows affected (0.03 sec) mysql> insert into ti values (1,1), (2,2); Query OK, 2 rows affected (0.03 sec) Records: 2 Duplicates: 0 Warnings: 0 mysql> start transaction; Query OK, 0 rows affected (0.00 sec) mysql> select * from ti; +----+------+ | id | c1 | +----+------+ | 1 | 1 | | 2 | 2 | +----+------+ 2 rows in set (0.00 sec)AI 代码解读
再开启第二个会话,利用gdb来跟踪mysql加MDL的过程 会话2：
[root@localhost mysql]# ps -ef|grep mysql root 3336 2390 0 06:33 pts/2 00:00:01 /u02/mysql/bin/mysqld --basedir=/u02/mysql/ --datadir=/u02/mysql/data --plugin-dir=/u02/mysql//lib/plugin --user=root --log-error=/u02/mysql/tmp/error1.log --open-files-limit=10240 --pid-file=/u02/mysql/tmp/mysql.pid --socket=/u02/mysql/tmp/mysql.sock --port=3306 [root@localhost mysql]# gdb -p 3336 ----在GDB设置以下断点 (gdb) b MDL_context::acquire_lock Breakpoint 1 at 0x730cab: file /u02/mysql-server-5.6/sql/mdl.cc, line 2187. (gdb) b lock_rec_lock Breakpoint 2 at 0xb5ef50: file /u02/mysql-server-5.6/storage/innobase/lock/lock0lock.cc, line 2296. (gdb) c Continuing.....AI 代码解读
开启第三个会话
mysql> alter table ti stats_auto_recalc=1; 这个操作被hang住AI 代码解读
在会话2中执行下面的操作
(gdb) p mdl_request $1 = (MDL_request *) 0x7f697d1c3bd0 (gdb) p *mdl_request $2 = { type = MDL_INTENTION_EXCLUSIVE, duration = MDL_STATEMENT, next_in_list = 0x7f697002a560, prev_in_list = 0x7f697d1c3df8, ticket = 0x0, key = {m_length = 3, m_db_name_length = 0, m_ptr = '\000' <repeats 20 times>, "0|\002p\000\000\001\000\060<\034}i\177\000\000>\240\344\000\000\000\000\000\000\t\000pi\177\000\000\000\t\000pi\177\000\000`>\034}i\177\000\000V\312\344\000\000\000\000\000\240>\034}i\177\000\000\333\361\254\000b\001\000\000\a?\000\001", '\000' <repeats 20 times>, "0|\002p\000\000\001\000\220<\034}i\177\000\000>\240\344\000\000\000\000\000\340\236\002pi\177\000\000\333\361\254\000\000\000\000\000\a?\000\001", '\000' <repeats 12 times>"\340, >\034}i\177\000\000\060|\002p\000\000\001\000\350\062\220\003\000\000\000\000\333\361\254\000\000\000\000\000$\226\363", '\000' <repeats 14 times>, "?\034}i\177\000\000\060|\002p\000\000\001\000\000=\034}i\177\000\000>\240\344\000\000\000\000\000\000"..., static m_namespace_to_wait_state_name = { {m_key = 101, m_name = 0xf125a2 "Waiting for global read lock", m_flags = 0}, {m_key = 102, m_name = 0xf125c0 "Waiting for schema metadata lock", m_flags = 0}, {m_key = 103, m_name = 0xf125e8 "Waiting for table metadata lock", m_flags = 0}, {m_key = 104, m_name = 0xf12608 "Waiting for stored function metadata lock", m_flags = 0}, {m_key = 105, m_name = 0xf12638 "Waiting for stored procedure metadata lock", m_flags = 0}, {m_key = 106, m_name = 0xf12668 "Waiting for trigger metadata lock", m_flags = 0}, {m_key = 107, m_name = 0xf12690 "Waiting for event metadata lock", m_flags = 0}, {m_key = 108, m_name = 0xf126b0 "Waiting for commit lock", m_flags = 0}}}} (gdb)AI 代码解读
从上面的输出中，我只能看到申请了一个语句级别的MDL_INTENTION_EXCLUSIVE。并没有看到什么其他有意义的信息。我们继续gdb跟踪
(gdb) p *(mdl_request->next_in_list) $3 = {type = MDL_INTENTION_EXCLUSIVE, duration = MDL_TRANSACTION, next_in_list = 0x7f697002a388, prev_in_list = 0x7f697d1c3bd8, ticket = 0x0, key = {m_length = 7, m_db_name_length = 4, m_ptr = "\001test\000\000\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217\217", static m_namespace_to_wait_state_name = { {m_key = 101, m_name = 0xf125a2 "Waiting for global read lock", m_flags = 0}, {m_key = 102, m_name = 0xf125c0 "Waiting for schema metadata lock", m_flags = 0}, {m_key = 103, m_name = 0xf125e8 "Waiting for table metadata lock", m_flags = 0}, {m_key = 104, m_name = 0xf12608 "Waiting for stored function metadata lock", m_flags = 0}, {m_key = 105, m_name = 0xf12638 "Waiting for stored procedure metadata lock", m_flags = 0}, {m_key = 106, m_name = 0xf12668 "Waiting for trigger metadata lock", m_flags = 0}, {m_key = 107, m_name = 0xf12690 "Waiting for event metadata lock", m_flags = 0}, {m_key = 108, m_name = 0xf126b0 "Waiting for commit lock", m_flags = 0}}}}AI 代码解读
从上面的输出中，我们看到了需要在test（见输出中的 m_ptr = “\001test）数据库上加一把事务级的MDL_INTENTION_EXCLUSIVE锁。它并没有告诉我们最终的MDL会落在哪个对象上。我们继续跟踪
$4 = {type = MDL_SHARED_UPGRADABLE, duration = MDL_TRANSACTION, next_in_list = 0x0, prev_in_list = 0x7f697002a568, ticket = 0x0, key = {m_length = 9, m_db_name_length = 4, m_ptr = "\002test\000ti", '\000' <repeats 378 times>, static m_namespace_to_wait_state_name = { {m_key = 101, m_name = 0xf125a2 "Waiting for global read lock", m_flags = 0}, {m_key = 102, m_name = 0xf125c0 "Waiting for schema metadata lock", m_flags = 0}, {m_key = 103, m_name = 0xf125e8 "Waiting for table metadata lock", m_flags = 0}, {m_key = 104, m_name = 0xf12608 "Waiting for stored function metadata lock", m_flags = 0}, {m_key = 105, m_name = 0xf12638 "Waiting for stored procedure metadata lock", m_flags = 0}, {m_key = 106, m_name = 0xf12668 "Waiting for trigger metadata lock", m_flags = 0}, {m_key = 107, m_name = 0xf12690 "Waiting for event metadata lock", m_flags = 0}, {m_key = 108, m_name = 0xf126b0 "Waiting for commit lock", m_flags = 0}}}}AI 代码解读
从上面的输出中，我们可以看出最终是要在test数据库的ti对象上加一把MDL_SHARED_UPGRADABLE锁。在做DDL时会先加MDL_SHARED_UPGRADABLE锁，然后升级到MDL_EXCLUSIVE锁
我来执行下面的过程 会话1
mysql> commit; Query OK, 0 rows affected (5.51 sec)AI 代码解读
会话2
(gdb) p *mdl_request $5 = {type = MDL_EXCLUSIVE, duration = MDL_TRANSACTION, next_in_list = 0x20302000000, prev_in_list = 0x200000001, ticket = 0x0, key = {m_length = 9, m_db_name_length = 4, m_ptr = "\002test\000ti\000\000\000\000@\031\220\003\000\000\000\000\333\361\254\000\000\000\000\000\260<\034}i\177\000\000\302\362\254\000\000\000\000\000\300<\034}i\177\000\000\060|\002pi\177\000\000\320<\034}i\177\000\000\360\236\344\000\000\000\000\000\000\t\000pi\177\000\000(}\002pi\177\000\000\360<\034}i\177\000\000\234\312\344\000\000\000\000\000H\245\002pi\177\000\000\333\361\254\000\000\000\000\000\023\360\000\001", '\000' <repeats 12 times>, "`S\005pi\177\000\000\060|\002p\000\000\001\000\060=\034}i\177\000\000>\240\344\000\000\000\000\000\000\t\000pi\177\000\000\000\t\000pi\177\000\000\200=\034}i\177\000\000\231\310\344\000\000\000\000\000\240=\034}i\177\000\000l-d0t\b\000\000H\344\000\001\000\000\000\000\023\360\000\001\000\000\000\000\226"..., static m_namespace_to_wait_state_name = { {m_key = 101, m_name = 0xf125a2 "Waiting for global read lock", m_flags = 0}, {m_key = 102, m_name = 0xf125c0 "Waiting for schema metadata lock", m_flags = 0}, {m_key = 103, m_name = 0xf125e8 "Waiting for table metadata lock", m_flags = 0}, {m_key = 104, m_name = 0xf12608 "Waiting for stored function metadata lock", m_flags = 0}, {m_key = 105, m_name = 0xf12638 "Waiting for stored procedure metadata lock", m_flags = 0}, {m_key = 106, m_name = 0xf12668 "Waiting for trigger metadata lock", m_flags = 0}, {m_key = 107, m_name = 0xf12690 "Waiting for event metadata lock", m_flags = 0}, {m_key = 108, m_name = 0xf126b0 "Waiting for commit lock", m_flags = 0}}}}AI 代码解读
从上面的输出中，我们看到了最终是在test.ti上申请了事务级别的MDL_EXCLUSIVE锁。
会话3
mysql> alter table ti stats_auto_recalc=1; Query OK, 0 rows affected (22 min 58.99 sec) Records: 0 Duplicates: 0 Warnings: 0AI 代码解读
小结
本例只是简单的演示了，在同一个事务的不同时期加的不同的MDL的锁。MYSQL中DDL的操作不属于事务操作的范围。这就给mysql主备基于语句级别同步带来了困难。mysql主备在同步的过程中，为了保证主备结构一致性，而引入了MDL机制。为了尽可能的降低MDL带来的影响。请在业务低谷的时候，执行DDL操作。
