Seconds_Behind_Master
今天同事遇到一个故障，xtrabackup备份中flush tables with read lock进程状态是flushing table，持续了大约60000s，首先分析是被长事务堵住了，show processlists，未看到长事务，因为slave上不提供应用服务，所以只有xtrabackup备份一个进程，show slave status\G发现Seconds_Behind_Master=0，但slave io thread执行的master binlog 要比 slave sql thread执行的master binlog多几十个，显然Seconds_Behind_Master不该为0（暂时未找到原因），抽样观察 slave sql thread执行的pos不变，怀疑是sql thread卡住引起flush tables with read lock 处于flushing table，mysqlbinlog 解析relaylog 发现当前slave正在执行delete操作，后检查binlog_format=row且对应表无主键，master上一条无主键表的delete执行记录在binlog中每row对应一个event，当master binlog传到slave上执行时，每个event对要在slave进行一次全表扫描，至此原因找到了，解决方案：
第一种
1.stop slave; #会hang 几个或十几个小时进行delete操作的回滚
2.对未主键表创建主键
第二种
1.master binlog_format=statment
2.master 备份
3.重做slave
Seconds_Behind_Master问题
show slave status\G 显示slave在复制中的信息，其中Seconds_Behind_Master常被用作衡量slave延迟秒数，虽然Seconds_Behind_Master值被许多认为不能够准确反映slave延迟情况，但由于show slave status\G简单易用，对于无须精确的场景足够了（精确方式如在master上表中定期插入timestamp,在slave上通过系统时间和timestamp进行对比得到精确slave延迟），简单看了下代码time_diff（Seconds_Behind_Master值）
sql/slave.cc
bool show_master_info(THD* thd, Master_info* mi)
long time_diff= ((long)(time(0) - mi->rli.last_master_timestamp)
- mi->clock_diff_with_master);
static int get_master_version_and_clock(MYSQL* mysql, Master_info* mi)
clock_diff_with_master= 0; /* The "most sensible" value */
void Relay_log_info::stmt_done(my_off_t event_master_log_pos,time_t event_creation_time)
last_master_timestamp= event_creation_time;
event_creation_time（代码中暂时未找到定义），从字面感觉在slave上对应的是Exec_Master_Log_Pos字段值，因此Seconds_Behind_Master值就是master当前系统时间戳(slave当前系统时间戳 - slave与master间的时间戳差) - slave sql thread当前已经执行完的master binlog pos.
manual中对Seconds_Behind_Master的解释如下，但与代码的计算公式不一致，Seconds_Behind_Master值是slave io thread当前已经获取的master binlog pos - slave sql thread当前已经执行完的master binlog pos
In essence, this field measures the time difference in seconds between the slave SQL thread and the slave I/O thread.
If the network connection between master and slave is fast, the slave I/O thread is very close to the master, so this field is a good approximation of how late the slave SQL thread is compared to the master. If the network is slow, this is not a good approximation; the slave SQL thread may quite often be caught up with the slow-reading slave I/O thread, soSeconds_Behind_Master often shows a value of 0, even if the I/O thread is late compared to the master. In other words, this column is useful only for fast networks.
个人倾向于代码中的解释，设计实验来验证，用取一天前的备份集新做了一个slave，change master且start slave后，Seconds_Behind_Master显示为60000多秒（20小时左右），显然是master当前系统时间戳 - slave sql thread当前已经执行完的master binlog pos的结果