mysqldump大致原理以及mysqldump备份过程中进行DDL操作的影响
- MySQL版本：5.7.18
隔离级别：REPEATABLE-READ
如果隔离级别不是RR，启动事务快照读的时候会报错：
root@localhost : (none) 10:38:48> show variables like '%isola%';+---------------+----------------+| Variable_name | Value |+---------------+----------------+| tx_isolation | READ-COMMITTED |+---------------+----------------+1 row in set (0.00 sec)root@localhost : (none) 10:38:50> START TRANSACTION /*!40100 WITH CONSISTENT SNAPSHOT */ ;Query OK, 0 rows affected, 1 warning (0.00 sec)Warning (Code 138): InnoDB: WITH CONSISTENT SNAPSHOT was ignored because this phrase can only be used with REPEATABLE READ isolation level.root@localhost : (none) 10:39:01> set global tx_isolation='REPEATABLE-READ';Query OK, 0 rows affected (0.00 sec)#退出客户端重进root@localhost : (none) 10:44:17> show variables like '%isola%';+---------------+-----------------+| Variable_name | Value |+---------------+-----------------+| tx_isolation | REPEATABLE-READ |+---------------+-----------------+1 row in set (0.00 sec)root@localhost : (none) 10:44:38> START TRANSACTION /*!40100 WITH CONSISTENT SNAPSHOT */ ;Query OK, 0 rows affected (0.00 sec)
mysqldump大致原理
- 1.FLUSH /!40101 LOCAL / TABLES
关闭所有在内存中打开的表，清理查询缓存。 - 2.FLUSH TABLES WITH READ LOCK
加一个全局读锁，只允许读，不允许更新操作。 - 3.SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ
设置当前会话的事务隔离等级为RR，RR可避免不可重复读和幻读，保证在备份期间，一个事务中所有相同的查询读取到同样的数据。 - 4.START TRANSACTION /!40100 WITH CONSISTENT SNAPSHOT /
获取当前数据库的快照，这个是由mysqldump中–single-transaction决定的,类似于开启事务并对所有表执行了一次SELECT操作，这样可保证备份时，在任意时间点执行select * from table得到的数据和执行START TRANSACTION WITH CONSISTENT SNAPSHOT时的数据一致。 - 5.SHOW MASTER STATUS
–master-data决定的，记录了开始备份时，binlog的状态信息，包括MASTER_LOG_FILE和MASTER_LOG_POS - 6.UNLOCK TABLES
释放锁，也就是说其实mysqldump这个语句，其实是有锁表，只不过锁了很短的时间 - 7.SHOW CREATE DATABASE IF NOT EXISTS
test
生成创库语句。 - 8.SAVEPOINT sp
设置SAVEPOINT，然后备份完每个表后再回滚到该SAVEPOINT。 - 9.show create table
test
生成创表语句 - 10.SELECT /!40001 SQL_NO_CACHE / * FROM
test
该语句会查询到表test1的所有数据，在备份文件中会生成相应的insert语句。
其中SQL_NO_CACHE的作用是查询的结果并不会缓存到查询缓存中。 - 11.SHOW TRIGGERS LIKE 'test'；
备份触发器。 - 12.ROLLBACK TO SAVEPOINT sp
如果不执行ROLLBACK TO SAVEPOINT sp，可能会阻塞同时进行的DDL操作，执行了该语句后，DDL操作可以继续执行。 - 13.SHOW FUNCTION STATUS WHERE Db = 'test'
SHOW CREATE FUNCTIONmycat_seq_currval
SHOW PROCEDURE STATUS WHERE Db = 'test'
用于备份存储过程和函数
第一种情况
session 1 session 2test 05:37:00> START TRANSACTION /*!40100 WITH CONSISTENT SNAPSHOT */ ;Query OK, 0 rows affected (0.00 sec)test 05:37:02> savepoint sp;Query OK, 0 rows affected (0.00 sec)test 05:37:08> show create table uniq_test;Query OK, 0 rows affected (0.00 sec)test 05:37:14> alter table uniq_test add column name7 varchar(10);Query OK, 0 rows affected (0.32 sec)test 05:37:24> select * from uniq_test;ERROR 1412 (HY000): Table definition has changed, please retry transactiontest 05:37:39> rollback to savepoint sp;Query OK, 0 rows affected (0.00 sec)
- START TRANSACTION /!40100 WITH CONSISTENT SNAPSHOT之后 /到select * from uniq_test之前，这中间，如果session 2执行ddl操作，则session 1中接着执行select * from uniq_test;就会报错：Table definition has changed, please retry transaction
体现在mysqldump（unlock tables语句到SELECT /!40001 SQL_NO_CACHE / * FROM test 语句之间）出来的文件就是无数据，无insert语句，表结构是在的。
第二种情况
session 1 session 2test 05:37:00> START TRANSACTION /*!40100 WITH CONSISTENT SNAPSHOT */ ;Query OK, 0 rows affected (0.00 sec)test 05:37:02> savepoint sp;Query OK, 0 rows affected (0.00 sec)test 05:37:08> show create table uniq_test;Query OK, 0 rows affected (0.00 sec)test 05:37:24> select * from uniq_test;+----+------+-------+-------+-------+-------+-------+| id | name | name1 | name2 | name3 | name4 | name5 |+----+------+-------+-------+-------+-------+-------+| 1 | NULL | NULL | NULL | NULL | NULL | NULL || 1 | NULL | NULL | NULL | NULL | NULL | NULL || 2 | NULL | NULL | NULL | NULL | NULL | NULL || 3 | NULL | NULL | NULL | NULL | NULL | NULL |+----+------+-------+-------+-------+-------+-------+4 rows in set (0.00 sec)test 05:37:44> alter table uniq_test add column name6 varchar(10);test 05:38:10> rollback to savepoint sp;test 05:38:46> alter table uniq_test add column name6 varchar(10);Query OK, 0 rows affected (21.73 sec)
- 在select * from uniq_test；之后到rollback to savepoint sp;之前，这中间，如果session 2执行ddl操作，则session 2的ddl操作会hang住，此时show processlist：
test 05:37:52> show processlist;+----+------+-----------+------+---------+------+---------------------------------+-----------------------------------------------------+| Id | User | Host | db | Command | Time | State | Info |+----+------+-----------+------+---------+------+---------------------------------+-----------------------------------------------------+| 21 | root | localhost | test | Query | 4 | Waiting for table metadata lock | alter table uniq_test add column name6 varchar(10) || 22 | root | localhost | test | Query | 0 | starting | show processlist |+----+------+-----------+------+---------+------+---------------------------------+-----------------------------------------------------+2 rows in set (0.00 sec)
可以看到DDL操作在等待元数据锁。当session 1中执行rollback to savepoint sp;后，session 2的ddl操作才会进行下去。所以这也是为什么要设置savepoint的原因。开启一个事务后，只有rollback或者commit才能把锁释放，但是这个大事务START TRANSACTION /!40100 WITH CONSISTENT SNAPSHOT /无法简单的对整个事务进行rollback或者commit，所以采用了：备份表之前设立一个savepoint每备份完成一个表就rollback这张表从而释放掉此表的元数据锁，从而对并发DDL操作的影响降到最低。
show create table uniq_test语句不会持有元数据锁。