MySQL复制中seconds_behind_master的计算
1.seconds_behind_master定义为从库相对于主库的延迟时间。
具体的计算公式为：
clock_of_slave - last_timestamp_executed_by_SQL_thread-clock_diff_with_master.
备注：位于rpl_mi.h中定义了clock_diff_with_master，该值为从库相对于主库的时间差。
/*
The difference in seconds between the clock of the master and the clock of
the slave (second - first). It must be signed as it may be <0 or >0.
clock_diff_with_master is computed when the I/O thread starts; for this the
I/O thread does a SELECT UNIX_TIMESTAMP() on the master.
"how late the slave is compared to the master" is computed like this:
clock_of_slave - last_timestamp_executed_by_SQL_thread - clock_diff_with_master
*/
long clock_diff_with_master;
// clock_diff_with_master
// mi->clock_diff_with_master=
(long) (time((time_t*) 0) - strtoul(master_row[0], 0, 10));
master_row[0]就是在主库上执行SELECT UNIX_TIMESTAMP()的操作。
从如下到(rpl_slave.cc)可以看出：
start_slave_thread-> //启动start slave
handle_slave_io-> //启动start io thread
get_master_version_and_clock //获取当前slave和主机之间的时间差(clock_diff_with_master)
/*
Used to defer stopping the SQL thread to give it a chance
to finish up the current group of events.
The timestamp is set and reset in @c sql_slave_killed().
*/
time_t last_event_start_time; //slave端最后执行的reply log的最后一个事件的时间
