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详解MySQL的Seconds_Behind_Master

【字号: 日期:2023-10-02 19:26:20浏览:68作者:猪猪
目录Seconds_Behind_Master原始实现last_master_timestampclock_diff_with_master其他exec_time时间函数总结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_routinets = 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 relatedrli->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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