【案例】主从替换之后的复制风暴-白红宇

一现象

一套MySQL主-备-备-备数据库，其中的备库升级到主库之后，系统监控报警 Seconds_Behind_Master 瞬间为0，瞬间为数十万秒。第一感觉是遇到了复制风暴--不同于主备server_id 的log event在主备库之间无限循环复制。升级的逻辑图如下：

二分析

在双主复制结构中，主库发生数据更新，会将更新记录为含有server_id_1 的log event事件发送到备库，然后备库更新数据,将含有server_id_1的log event 事件发送给主库，因此最初主库上的log event 更新事件又传了回来，这时候MySQL就要对复制事件的server_id进行判断，发现复制事件的server_id和自己的server_id相同时，放弃执行，如果不同则执行该log event 并记录到binlog 里面继续发送给备库。如果该event的server_id和主备的server_id都不相同,该log event 则在主备库中无限循环执行，也就是通常所说的复制风暴。

那为什么slave lag 为时大时小呢？

首先我们要了解MySQL 对于slave lag 的计算方式(sql/slave.cc )

bool show_master_info(THD* thd, Master_info* mi)

{

/*省略*/

Seconds_Behind_Master: if SQL thread is running and I/O thread is

connected, we can compute it otherwise show NULL (i.e. unknown).

if ((mi->slave_running == MYSQL_SLAVE_RUN_CONNECT) &&

mi->rli.slave_running)

{

long time_diff= ((long)(time(0) - mi->rli.last_master_timestamp)

- mi->clock_diff_with_master);

Apparently on some systems time_diff can be <0. Here are possible

reasons related to MySQL:

- the master is itself a slave of another master whose time is ahead.

- somebody used an explicit SET TIMESTAMP on the master.

Possible reason related to granularity-to-second of time functions

(nothing to do with MySQL), which can explain a value of -1:

assume the master's and slave's time are perfectly synchronized, and

that at slave's connection time, when the master's timestamp is read,

it is at the very end of second 1, and (a very short time later) when

the slave's timestamp is read it is at the very beginning of second

2. Then the recorded value for master is 1 and the recorded value for

slave is 2. At SHOW SLAVE STATUS time, assume that the difference

between timestamp of slave and rli->last_master_timestamp is 0

(i.e. they are in the same second), then we get 0-(2-1)=-1 as a result.

This confuses users, so we don't go below 0: hence the max().

last_master_timestamp == 0 (an "impossible" timestamp 1970) is a

special marker to say "consider we have caught up".

protocol->store((longlong)(mi->rli.last_master_timestamp ?

max(0, time_diff) : 0));

}

else

{

protocol->store_null();

}

/*省略*/

}

解释如下：

long_time_diff就是seconds_behind_master

seconds_behind_master=

slave系统时间 - master执行创建event的timestamp - ( slave系统时间 - master系统时间 )

(slave系统时间-master执行最新event的timestamp):得到最新event到slave执行还要多久。

(slave系统时间-master系统时间):可能存在主备系统时间差别，所以计算seconds_behind_master要减去，但实际情况，slave和master系统时间基本一致，得到结果应该接近0

所以seconds_behind_master的值是由于slave系统时间-master执行最新event的timestamp 决定的，当导致循环复制的log event创建时间越久远，slave lag 会越大，执行完这个event，会执行真正主库执行的log event ，此时slave lag 就会变成0 。

三解决

查看新主库的server_id

查看新备库的server_id

  主库上冲突的事务的server_id

备库上冲突事务的server_id

老的主库的server_id

解决方法
在新的备库更改server_id为冲突数据的server_id，等数据耗完毕，server_id改为原库的server_id。读者朋友可以思考一下为什么在备库上执行更改server_id的操作？

对于MySQL 本身，可以加上一层判断，在复制结构中检查 log envent的server_id是否属于复制结构中数据库的server_id，如果不是，则判断该事物属于复制风暴事物，予以抛弃。

四参考资料

[1]

[2]

[3]