Mysql 采用多线程进行复制是从 Mysql 5.6 开始支持的内容，但是 5.6 版本下有缺陷，虽然支持多线程，但是每个数据库只能一个线程，也就是说如果我们只有一个数据库，则主从复制时也只有一个线程在工作。相当于还是以前的单线程。 从 Mysql 5.7 开始支持同一数据库下并行主从复制。不过默认情况下，还是单数据库单个线程，如果需要使用多线程，需要在从节点进行配置。
Mysql 5.7 对主从复制增加了一种类型，共有两种类型，如下：
DATABASE基于库的并行复制 ， 每个数据库对应一个复制线程LOGICAL_CLOCK基于组提交的并行复制方式，同一个数据库下可以有多个线程
下面的步骤，在从节点上进行配置。
查看当前配置
在开始配置之前，我们先看一下当前配置下的主从复制的进程数。
mysql> show processlist;
+----+-------------+-----------+------+---------+-------+--------------------------------------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-------------+-----------+------+---------+-------+--------------------------------------------------------+------------------+
| 1 | system user | | NULL | Connect | 91749 | Waiting for master to send event | NULL |
| 2 | system user | | NULL | Connect | 208 | Slave has read all relay log; waiting for more updates | NULL |
| 37 | root | localhost | NULL | Query | 0 | starting | show processlist |
+----+-------------+-----------+------+---------+-------+--------------------------------------------------------+------------------+
3 rows in set (0.00 sec)
从上面看出只有一个主进程在等待同步。
下面查看复制类型和并行数量配置
mysql> show variables like 'slave_parallel_type';
+---------------------+----------+
| Variable_name | Value |
+---------------------+----------+
| slave_parallel_type | DATABASE |
+---------------------+----------+
1 row in set (0.00 sec)
当前的复制类型是 DATABASE，也就是统一数据库下只有一个线程进行复制，不能并行复制。
mysql> show variables like 'slave_parallel_workers';
+------------------------+-------+
| Variable_name | Value |
+------------------------+-------+
| slave_parallel_workers | 0 |
+------------------------+-------+
1 row in set (0.01 sec)
当前并行工作的进程数是 0
配置多线程
1、停止从节点复制
mysql> stop slave;
Query OK, 0 rows affected (0.01 sec)
2、设置复制类型为 LOGICAL_CLOCK
mysql> set global slave_parallel_type='logical_clock';
Query OK, 0 rows affected (0.00 sec)
mysql> show variables like 'slave_parallel_type';
+---------------------+---------------+
| Variable_name | Value |
+---------------------+---------------+
| slave_parallel_type | LOGICAL_CLOCK |
+---------------------+---------------+
1 row in set (0.01 sec)
3、设置并行数量为 4
mysql> set global slave_parallel_workers=4;
Query OK, 0 rows affected (0.00 sec)
mysql> show variables like 'slave_parallel_workers';
+------------------------+-------+
| Variable_name | Value |
+------------------------+-------+
| slave_parallel_workers | 4 |
+------------------------+-------+
1 row in set (0.00 sec)
4、启动从节点复制
mysql> start slave;
Query OK, 0 rows affected (0.02 sec)
5、查看一下当前工作的线程数
mysql> show processlist;
+----+-------------+-----------+------+---------+------+--------------------------------------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-------------+-----------+------+---------+------+--------------------------------------------------------+------------------+
| 37 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 38 | system user | | NULL | Connect | 8 | Waiting for master to send event | NULL |
| 39 | system user | | NULL | Connect | 7 | Slave has read all relay log; waiting for more updates | NULL |
| 40 | system user | | NULL | Connect | 8 | Waiting for an event from Coordinator | NULL |
| 41 | system user | | NULL | Connect | 8 | Waiting for an event from Coordinator | NULL |
| 42 | system user | | NULL | Connect | 8 | Waiting for an event from Coordinator | NULL |
| 43 | system user | | NULL | Connect | 8 | Waiting for an event from Coordinator | NULL |
+----+-------------+-----------+------+---------+------+--------------------------------------------------------+------------------+
7 rows in set (0.00 sec)
最后说一下为什么需要多线程复制？因为主从之间的同步会有延时，多线程的目的是为了尽量减少这个延时时间。虽然如何优化主从是一个系统的功能，不同的场景需要不同的解决方案，但是多线程至少从基础上能减少延迟时间。另外根据数据库的实际情况，能否真正减少延时，以及配置多少线程，则需要反复的测试得出适合自己的数据。
实例配置:
cat backup-my.cnf # This MySQL options file was generated by innobackupex. # The MySQL server [mysqld] port = 3310 #innodb_checksum_algorithm=innodb #innodb_log_checksum_algorithm=innodb innodb_data_file_path=ibdata1:200M:autoextend innodb_log_files_in_group=2 innodb_log_file_size=1572864000 #innodb_fast_checksum=false #innodb_page_size=16384 #innodb_log_block_size=512 innodb_undo_directory=. innodb_undo_tablespaces=0 server-id = 1234 binlog-format = ROW gtid-mode = ON enforce-gtid-consistency = true relay_log_recovery = 1 sync-binlog = 0 skip_slave_start = 1 slave-parallel-type=LOGICAL_CLOCK slave-parallel-workers=32 master_info_repository=TABLE relay_log_info_repository=TABLE relay_log_recovery=ON #rds_encrypt_data=false #innodb_encrypt_algorithm=aes_128_ecb
mysql> show processlist; +----+-------------+-----------+------+---------+-------+----------------------------------------+------------------+ | Id | User | Host | db | Command | Time | State | Info | +----+-------------+-----------+------+---------+-------+----------------------------------------+------------------+ | 2 | root | localhost | NULL | Query | 0 | starting | show processlist | | 3 | system user | | NULL | Connect | 13 | Queueing master event to the relay log | NULL | | 4 | system user | | NULL | Connect | 0 | System lock | NULL | | 5 | system user | | NULL | Connect | 49775 | Waiting for an event from Coordinator | NULL | | 6 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 7 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 8 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 9 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 10 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 11 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 12 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 13 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 14 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 15 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 16 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 17 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 18 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 19 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 20 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 21 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 22 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 23 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 24 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 25 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 26 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 27 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 28 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 29 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 30 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 31 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 32 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 33 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 34 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 35 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | | 36 | system user | | NULL | Connect | 13 | Waiting for an event from Coordinator | NULL | +----+-------------+-----------+------+---------+-------+----------------------------------------+------------------+ 35 rows in set (0.00 sec) mysql> show global variables like 'slave_parallel_workers'; +------------------------+-------+ | Variable_name | Value | +------------------------+-------+ | slave_parallel_workers | 32 | +------------------------+-------+ 1 row in set (0.00 sec) mysql> show global variables like '%slave_parallel_type%'; +---------------------+---------------+ | Variable_name | Value | +---------------------+---------------+ | slave_parallel_type | LOGICAL_CLOCK | +---------------------+---------------+ 1 row in set (0.00 sec)
