延时复制 delayed replication

mysql5.6开始支持延时复制，默认master_delay为0秒，

CHANGE MASTER TO MASTER_DELAY = N;

表示延时N秒

原理：延时复制的本质是sql_thread需要等待延时时间之后才能执行。

延时复制适用场景：

（1）防止主库误操作，在复制同步之前，可以停止同步；

（2）用作测试,不用模拟负载就可以实现主从延迟；

（3）用来检查数据库以前的数据，如延迟设置为1周，这样不需要备份恢复就可以看到比对一周以前的数据

（4）reset slave会把SQL_delay的值清零，并且还会把Master_Log_File等值清空，但是不影响复制；

创建一个延时复制：slave:root@localhost [testdb]>stop slave;root@localhost [testdb]>change master to master_delay=60;root@localhost [testdb]>start slave;root@localhost [testdb]>show slave status\G......                    SQL_Delay: 60  --延时时间          SQL_Remaining_Delay: 56           --剩余时间      Slave_SQL_Running_State: Waiting until MASTER_DELAY seconds after master executed event   --等待延时......master：root@localhost [testdb]>delete from t1 where c1=4;slave：root@localhost [testdb]>select * from t1;+----+------+| c1 | c2   |+----+------+|  1 | aaa  ||  2 | bbb  ||  3 | ccc  ||  4 | ddd  |+----+------+root@localhost [testdb]>show processlist;+----+-------------+-----------+--------+---------+------+----------------------------------------------------------------+------------------+| Id | User        | Host      | db     | Command | Time | State                                                          | Info             |+----+-------------+-----------+--------+---------+------+----------------------------------------------------------------+------------------+| 26 | root        | localhost | testdb | Query   |    0 | starting                                                       | show processlist || 27 | system user |           | NULL   | Connect |  345 | Waiting for master to send event                               | NULL             || 28 | system user |           | NULL   | Connect |   10 | Waiting until MASTER_DELAY seconds after master executed event | NULL             |+----+-------------+-----------+--------+---------+------+----------------------------------------------------------------+------------------+#在没有达到60秒之前查看relay-log日志，发现已经写入relay-lo中，说明延时是阻塞SQL_thread线程[root@Darren1 data]# mysqlbinlog -vv --base64-output=decode-rows relay-bin.000003BEGIN/*!*/;# at 452#170409 22:12:27 server id 330622  end_log_pos 5624 CRC32 0x86f7edf4    Table_map: `testdb`.`t1` mapped to number 147# at 502#170409 22:12:27 server id 330622  end_log_pos 5668 CRC32 0x697c52ed    Delete_rows: table id 147 flags: STMT_END_F### DELETE FROM `testdb`.`t1`### WHERE###   @1=3 /* INT meta=0 nullable=0 is_null=0 */###   @2='ccc' /* VARSTRING(30) meta=30 nullable=1 is_null=0 */root@localhost [testdb]>select * from t1;+----+------+| c1 | c2   |+----+------+|  1 | aaa  ||  2 | bbb  ||  3 | ccc  |+----+------+