详解MySQL的Seconds_Behind_Master

seconds_behind_master

对于mysql主备实例，seconds_behind_master是衡量master与slave之间延时的一个重要参数。通过在slave上执行"show slave status;"可以获取seconds_behind_master的值。

原始实现

definition：the number of seconds that the slave sql thread is behind processing the master binary log.

type：time_t（long）

计算方式如下：

rpl_slave.cc::show_slave_status_send_data()
if ((mi->get_master_log_pos() == mi->rli->get_group_master_log_pos()) &&
       (!strcmp(mi->get_master_log_name(),
                mi->rli->get_group_master_log_name()))) {
     if (mi->slave_running == mysql_slave_run_connect)
       protocol->store(0ll);
     else
       protocol->store_null();
   } else {
     long time_diff = ((long)(time(0) - mi->rli->last_master_timestamp) -
                       mi->clock_diff_with_master);
     protocol->store(
         (longlong)(mi->rli->last_master_timestamp ? max(0l, time_diff) : 0));
   }

主要分为以下两种情况：

• sql线程等待io线程获取主机binlog，此时seconds_behind_master为0，表示备机与主机之间无延时；
• sql线程处理relay log，此时seconds_behind_master通过(long)(time(0) – mi->rli->last_master_timestamp) – mi->clock_diff_with_master计算得到；

last_master_timestamp

定义：

主库binlog中事件的时间。

type: time_t (long)

计算方式：

last_master_timestamp根据备机是否并行复制有不同的计算方式。

非并行复制：

rpl_slave.cc:exec_relay_log_event()
if ((!rli->is_parallel_exec() || rli->last_master_timestamp == 0) &&
    !(ev->is_artificial_event() || ev->is_relay_log_event() ||
     (ev->common_header->when.tv_sec == 0) ||
     ev->get_type_code() == binary_log::format_description_event ||
     ev->server_id == 0))
{
 rli->last_master_timestamp= ev->common_header->when.tv_sec +
                             (time_t) ev->exec_time;
 dbug_assert(rli->last_master_timestamp >= 0);
}

在该模式下，last_master_timestamp表示为每一个event的结束时间，其中when.tv_sec表示event的开始时间，exec_time表示事务的执行时间。该值的计算在apply_event之前，所以event还未执行时，last_master_timestamp已经被更新。由于exec_time仅在query_log_event中存在，所以last_master_timestamp在应用一个事务的不同event阶段变化。以一个包含两条insert语句的事务为例，在该代码段的调用时，打印出event的类型、时间戳和执行时间

create table t1(a int primary key auto_increment ,b longblob) engine=innodb;
begin;
insert into t1(b) select repeat('a',104857600);
insert into t1(b) select repeat('a',104857600);
commit;

10t06:41:32.628554z 11 [note] [my-000000] [repl] event_type: 33 gtid_log_event

2020-02-10t06:41:32.628601z 11 [note] [my-000000] [repl] event_time: 1581316890

2020-02-10t06:41:32.628614z 11 [note] [my-000000] [repl] event_exec_time: 0

2020-02-10t06:41:32.628692z 11 [note] [my-000000] [repl] event_type: 2   query_event

2020-02-10t06:41:32.628704z 11 [note] [my-000000] [repl] event_time: 1581316823

2020-02-10t06:41:32.628713z 11 [note] [my-000000] [repl] event_exec_time: 35

2020-02-10t06:41:32.629037z 11 [note] [my-000000] [repl] event_type: 19   table_map_event

2020-02-10t06:41:32.629057z 11 [note] [my-000000] [repl] event_time: 1581316823

2020-02-10t06:41:32.629063z 11 [note] [my-000000] [repl] event_exec_time: 0

2020-02-10t06:41:33.644111z 11 [note] [my-000000] [repl] event_type: 30    write_rows_event

2020-02-10t06:41:33.644149z 11 [note] [my-000000] [repl] event_time: 1581316823

2020-02-10t06:41:33.644156z 11 [note] [my-000000] [repl] event_exec_time: 0

2020-02-10t06:41:43.520272z 0 [note] [my-011953] [innodb] page cleaner took 9185ms to flush 3 and evict 0 pages

2020-02-10t06:42:05.982458z 11 [note] [my-000000] [repl] event_type: 19   table_map_event

2020-02-10t06:42:05.982488z 11 [note] [my-000000] [repl] event_time: 1581316858

2020-02-10t06:42:05.982495z 11 [note] [my-000000] [repl] event_exec_time: 0

2020-02-10t06:42:06.569345z 11 [note] [my-000000] [repl] event_type: 30    write_rows_event

2020-02-10t06:42:06.569376z 11 [note] [my-000000] [repl] event_time: 1581316858

2020-02-10t06:42:06.569384z 11 [note] [my-000000] [repl] event_exec_time: 0

2020-02-10t06:42:16.506176z 0 [note] [my-011953] [innodb] page cleaner took 9352ms to flush 8 and evict 0 pages

2020-02-10t06:42:37.202507z 11 [note] [my-000000] [repl] event_type: 16    xid_event

2020-02-10t06:42:37.202539z 11 [note] [my-000000] [repl] event_time: 1581316890

2020-02-10t06:42:37.202546z 11 [note] [my-000000] [repl] event_exec_time: 0

并行复制：

rpl_slave.cc   mts_checkpoint_routine
ts = rli->gaq->empty()
          ? 0
          : reinterpret_cast<slave_job_group *>(rli->gaq->head_queue())->ts;
 rli->reset_notified_checkpoint(cnt, ts, true);
 /* end-of "coordinator::"commit_positions" */

在该模式下备机上存在一个分发队列gaq，如果gaq为空，则设置last_commit_timestamp为0；如果gaq不为空，则此时维护一个checkpoint点lwm，lwm之前的事务全部在备机上执行完成，此时last_commit_timestamp被更新为lwm所在事务执行完成后的时间。该时间类型为time_t类型。

ptr_group->ts = common_header->when.tv_sec +
                   (time_t)exec_time;  // seconds_behind_master related
rli->rli_checkpoint_seqno++;

if (update_timestamp) {
 mysql_mutex_lock(&data_lock);
 last_master_timestamp = new_ts;
 mysql_mutex_unlock(&data_lock);
}

在并行复制下，event执行完成之后才会更新last_master_timestamp，所以非并行复制和并行复制下的seconds_behind_master会存在差异。

clock_diff_with_master

定义：

• 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.
• type: long

rpl_slave.cc::get_master_version_and_clock()
if (!mysql_real_query(mysql, string_with_len("select unix_timestamp()")) &&
     (master_res= mysql_store_result(mysql)) &&
     (master_row= mysql_fetch_row(master_res)))
 {
   mysql_mutex_lock(&mi->data_lock);
   mi->clock_diff_with_master=
     (long) (time((time_t*) 0) - strtoul(master_row[0], 0, 10));
   dbug_execute_if("dbug.mts.force_clock_diff_eq_0",
     mi->clock_diff_with_master= 0;);
   mysql_mutex_unlock(&mi->data_lock);
 }

该差值仅被计算一次，在master与slave建立联系时处理。

其他

exec_time

定义：

• the difference from the statement's original start timestamp and the time at which it completed executing.
• type: unsigned long

struct timeval end_time;
ulonglong micro_end_time = my_micro_time();
my_micro_time_to_timeval(micro_end_time, &end_time);
exec_time = end_time.tv_sec - thd_arg->query_start_in_secs();

时间函数

（1）time_t time(time_t timer) time_t为long类型，返回的数值仅精确到秒；

（2）int gettimeofday (struct timeval *tv, struct timezone *tz) 可以获得微秒级的当前时间；

（3）timeval结构

#include <time.h>
stuct timeval {
   time_t tv_sec; /*seconds*/
   suseconds_t tv_usec; /*microseconds*/
}

总结

使用seconds_behind_master衡量主备延时只能精确到秒级别，且在某些场景下，seconds_behind_master并不能准确反映主备之间的延时。主备异常时，可以结合seconds_behind_master源码进行具体分析。

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