关于mysql:技术分享-MySQL-复制那点事-SecondsbehindMaster-参数调查笔记

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作者:戴骏贤
网易游戏 技术部资深数据库系统工程师。
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文章浏览工夫约 15 分钟,文中驰名参考文献在文末处有参考链接。

1. 问题背景

部门以后的数据库架构是 双主模式,既线上由 2 台互为主从的数据库搭建而成的集群。高可用通过 vip 和 headbeat 来做保障。通常状况下,vip 挂在主(本文称之为 da)上,当 da 产生了异样比方宕机等问题的时候,vip 主动漂移至从(本文称之为 dp)。架构如下图所示:

能够看出,所有的写入和读取操作都在 da 上进行。da 产生的 binlog 会被 dp 拉取重放。同时 dp 也会产生 binlog 并被 da 拉取重放(注:这里不会产生循环复制(既 da 产生的 binlog 事件在 dp 执行过了之后,再被 da 读取到再执行),因为 binlog 中每个 event 都有 serverid 标记,标记是哪个 server 产生的事物。当 da 读取到 binlog 的时候,发现 serverid 和本机的 serverid 统一,便会跳过以后的 binlog event)。失常状况下,da 的 Seconds_behind_Master 参数,应该放弃在 0 或者很小的数值才对。然而监控零碎发现,da 的 Seconds_behind_Master 参数常常产生跳变,上一秒为 0 下一秒就可能为一个很大值(比方 10000)。例如上面的数据,是通过脚本抓取到的线上 Seconds_behind_Master 状况:

2019-06-27 13:26:00
          Read_Master_Log_Pos: 472851480
               Relay_Log_File: da01-relay-bin.002323
                Relay_Log_Pos: 371
        Relay_Master_Log_File: dp01-bin.004267
          Exec_Master_Log_Pos: 472851480
        Seconds_Behind_Master: 0
2019-06-27 13:26:01
          Read_Master_Log_Pos: 473594815
               Relay_Log_File: da01-relay-bin.002323
                Relay_Log_Pos: 371
        Relay_Master_Log_File: dp01-bin.004267
          Exec_Master_Log_Pos: 473594649
        Seconds_Behind_Master: 622
2019-06-27 13:26:02
          Read_Master_Log_Pos: 474422859
               Relay_Log_File: da01-relay-bin.002323
                Relay_Log_Pos: 371
        Relay_Master_Log_File: dp01-bin.004267
          Exec_Master_Log_Pos: 474422859
        Seconds_Behind_Master: 0

2. 考察论断

前文讲到了本文考察的问题背景,在这里先给出下整个问题的考察论断。在 sql/rpl_slave.cc 中计算 Seconds_Behind_Master 逻辑如下【文献 2】

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)); 
}

当 Exec_Master_Log_Pos<Read_Master_Log_Pos 的时候进入 else 外面。在 da 的 IO 过程拉取 dp 的 binlog 的时候,会发现 dp 的 binlog 的 serverid 是本身产生的,所以不会去更新 last_master_timestamp。如果 dp 的 binlog 产生过轮转,会产生一个 ROTATE_EVENT 事件更新这个 last_master_timestamp。因而当 SQL 过程比 IO 过程慢的时候,计算出来的 time_diff 便是以后工夫与上次 ROTATE_EVENT 事件的差值。复现办法:

  1. dp 上 flush logs ;
  2. da 上 stop slave sql_thread ;
  3. da 上 DML 数据。此时察看 show slave status; 能够看到 Read_Master_Log_Pos 很超前
  4. da 上 start slave ; show slave status G 能够看到 Seconds_Behind_Master 变大

3. 考察与剖析过程

从前文的后果中能够看出,跟 Seconds_Behind_Master(下文简称 SBM)值无关的 MySQL 变量就是 last_master_timestamp。因而须要追踪下在收到的 binlog 的 serverid 和 MySQL 过程的 serverid 统一的状况下,last_master_timestamp 是如何更新的。依据前文的复现办法,须要去确认如下的 2 个点:

  1. dp 上 flush logs 的时候,da 获取到 dp 的 binglog 做的操作;
  2. 收到的 serverid 和本机统一的 binlog 的时候,da 的 MySQL 过程的操作。

为了便于察看 MySQL 的运行逻辑,本文参考【文献 9】的方法,编译了一个 debug 版本。在 MySQL 运行的过程中,tail -f mysqld.trace 文件察看 MySQL 命令执行的过程。

3.1 轮转 binlog 时的运行逻辑

3.1.1 写入 ROTATE_EVENT

在 dp 执行 flush logs 或者轮转 binlog 的时候,会产生一个 ROTATE_EVENT 事件(具体的解析能够参考【文献 11】)。在 mysqld.trace 文件中,能够察看到如下的信息。标记着 da 此时接管到了 dp 的 binlog 轮转事件的 ROTATE_EVENT。

handle_slave_io: info: IO thread received event of type Rotate

相应的代码在 sql/rpl_slave.cc 文件的 5819 行中:

 5817       THD_STAGE_INFO(thd, stage_queueing_master_event_to_the_relay_log);
 5818       event_buf= (const char*)mysql->net.read_pos + 1;
 5819       DBUG_PRINT("info", ("IO thread received event of type %s",
 5820                  Log_event::get_type_str(5821                             (Log_event_type)event_buf[EVENT_TYPE_OFFSET])));
          
          ... ...
              
 5831
 5832       /* XXX: 'synced' should be updated by queue_event to indicate
 5833          whether event has been synced to disk */
 5834       bool synced= 0;
 5835       if (queue_event(mi, event_buf, event_len))
 5836       {
 5837         mi->report(ERROR_LEVEL, ER_SLAVE_RELAY_LOG_WRITE_FAILURE,
 5838                    ER(ER_SLAVE_RELAY_LOG_WRITE_FAILURE),
 5839                    "could not queue event from master");
 5840         goto err;
 5841       }

能够看到在 SQL 过程接管到一个 binlog 之后,会调用 queue_event 进行写入 relaylog。queue_event 解决 ROTATE_EVENT 要害代码如下:

 8236  switch (event_type) {      
      ... ...
           
 8251  case binary_log::ROTATE_EVENT:
 8252  {
 8253     Rotate_log_event rev(buf, checksum_alg != binary_log::BINLOG_CHECKSUM_ALG_OFF ?
 8254                          event_len - BINLOG_CHECKSUM_LEN : event_len,
 8255                          mi->get_mi_description_event());
 8256 
 8257     if (unlikely(process_io_rotate(mi, &rev)))
 8258     {
 8259       mi->report(ERROR_LEVEL, ER_SLAVE_RELAY_LOG_WRITE_FAILURE,
 8260                  ER(ER_SLAVE_RELAY_LOG_WRITE_FAILURE),
 8261                  "could not queue event from master");
 8262       goto err;
 8263     }

在 switch 中判断 binlog 如果是 ROTATE_EVENT,则调用 process_io_rotate 进行解决。其中 buf 是 SQL 进行拉取到的 binlog 的缓存,mi 是代表了 masterinfo。process_io_rotate 次要的工作是更新 master_info 的相干信息,例如下一个 binglog 的地位,master 的 binlog 地位信息等。能够在 mysqld.trace 文件中察看到如下的信息:

process_io_rotate: info: new (master_log_name, master_log_pos): ('dp-bin.000029', 4)

process_io_rotate 函数正文信息如下:

 7767 /**
 7768   Used by the slave IO thread when it receives a rotate event from the
 7769   master.
 7770
 7771   Updates the master info with the place in the next binary log where
 7772   we should start reading.  Rotate the relay log to avoid mixed-format
 7773   relay logs.
 7774
 7775   @param mi master_info for the slave
 7776   @param rev The rotate log event read from the master
 7777
 7778   @note The caller must hold mi->data_lock before invoking this function.
 7779
 7780   @retval 0 ok
 7781   @retval 1 error
 <!--7782 */-->
 7783 static int process_io_rotate(Master_info *mi, Rotate_log_event *rev)
     
... ...
     
 7830 }

在实现相干的工作之后,来到 queue_event 函数的最初,写入 ROTATE_EVENT,在 mysqld.trace 中能够看到如下的信息:

queue_event: info: master_log_pos: 4
harvest_bytes_written: info: counter: 769  bytes_written: 44
queue_event: info: error: 0

相干代码如下:

 8656   {
 8657     bool is_error= false;
 8658     /* write the event to the relay log */
 8659     if (likely(rli->relay_log.append_buffer(buf, event_len, mi) == 0))
 8660     {8661       mi->set_master_log_pos(mi->get_master_log_pos() + inc_pos);
 8662       DBUG_PRINT("info", ("master_log_pos: %lu", (ulong) mi->get_master_log_pos()));
 8663       rli->relay_log.harvest_bytes_written(rli, true/*need_log_space_lock=true*/);
     
          ... ...
              
          }
         } 
  

到此,ROTATE_EVENT 事件被齐全写入到了 relaylog 中。借用一张经典 MySQL 复制原理图【文献 12】阐明这个过程,即实现了图中从 BinaryLog 读取到 RelayLog 的写入过程。

3.1.2 SQL 过程读取和执行 ROTATE_EVENT

在前文的形容中,能够察看到在 RelayLog 的写入过程中并没有去更新 last_master_timestamp 相干信息。因而更新 last_master_timestamp 的工作,只可能在 SQL 过程中去实现。接下来本节将摸索下 ROTATE_EVENT 在 SQL 过程中的执行过程。

MySQL 复制代码中,SQL 过程的入口函数是 handle_slave_sql:

 7158 /**
 7159   Slave SQL thread entry point.
 7160 
 7161   @param arg Pointer to Relay_log_info object that holds information
 7162   for the SQL thread.
 7163 
 7164   @return Always 0.
 7165 */
 7166 extern "C" void *handle_slave_sql(void *arg)

在 handle_slave_sql 中,要害的代码是 while (!sql_slave_killed(thd,rli)) 这个循环:

 7438   while (!sql_slave_killed(thd,rli))
 7439   {7440     THD_STAGE_INFO(thd, stage_reading_event_from_the_relay_log);
 7441     DBUG_ASSERT(rli->info_thd == thd);
 7442     THD_CHECK_SENTRY(thd);
 7443
 7444     if (saved_skip && rli->slave_skip_counter == 0)
 7445     {
     
             ... ...
                 
 7456     }
 7457
 7458     if (exec_relay_log_event(thd,rli))
 7459     {
 7460        ... ...
 7525       }
 7526       goto err;
 7527     }
 7528   }

能够发现,每次循环的时候,都会去执行 exec_relay_log_event 这个函数:

 5098   Top-level function for executing the next event in the relay log.
 5099   This is called from the SQL thread.
 5100
 5101   This function reads the event from the relay log, executes it, and
 5102   advances the relay log position.  It also handles errors, etc.
 5103
 5104   This function may fail to apply the event for the following reasons:
 5105
 5106    - The position specfied by the UNTIL condition of the START SLAVE
 5107      command is reached.
 5108
 5109    - It was not possible to read the event from the log.
 5110
 5111    - The slave is killed.
 5112
 5113    - An error occurred when applying the event, and the event has been
 5114      tried slave_trans_retries times.  If the event has been retried
 5115      fewer times, 0 is returned.
 5116
 5117    - init_info or init_relay_log_pos failed. (These are called
 5118      if a failure occurs when applying the event.)
 5119
 5120    - An error occurred when updating the binlog position.
 5121
 5122   @retval 0 The event was applied.
 5123
 5124   @retval 1 The event was not applied.
 5125 */
 5126 static int exec_relay_log_event(THD* thd, Relay_log_info* rli)

在 exec_relay_log_event 中,执行对应的 relaylog 之后,会相应的更新 last_master_timestamp。在这里并行复制模式和非并行复制模式下,更新 last_master_timestamp 的形式是不同的。在这里先介绍下非并行复制模式下更新 last_master_timestamp 的步骤。

3.1.2.1 非并行复制模式下更新 last_master_timestamp

在 exec_relay_log_event 中判断是否是并行复制是通过 is_parallel_exec 函数实现的。如果是并行复制模式则返回 True,否则返回 False。在非并行复制的模式下,会进入如下的代码执行:

 5175     /*
 5176       Even if we don't execute this event, we keep the master timestamp,
 5177       so that seconds behind master shows correct delta (there are events
 5178       that are not replayed, so we keep falling behind).
 5179
 5180       If it is an artificial event, or a relay log event (IO thread generated
 5181       event) or ev->when is set to 0, or a FD from master, or a heartbeat
 5182       event with server_id '0' then  we don't update the last_master_timestamp.
 5183
 5184       In case of parallel execution last_master_timestamp is only updated when
 5185       a job is taken out of GAQ. Thus when last_master_timestamp is 0 (which
 5186       indicates that GAQ is empty, all slave workers are waiting for events from
 5187       the Coordinator), we need to initialize it with a timestamp from the first
 5188       event to be executed in parallel.
 5189     */
 5190     if ((!rli->is_parallel_exec() || rli->last_master_timestamp == 0) &&
 5191          !(ev->is_artificial_event() || ev->is_relay_log_event() ||
 5192           (ev->common_header->when.tv_sec == 0) ||
 5193           ev->get_type_code() == binary_log::FORMAT_DESCRIPTION_EVENT ||
 5194           ev->server_id == 0))
 5195     {
 5196       rli->last_master_timestamp= ev->common_header->when.tv_sec +
 5197                                   (time_t) ev->exec_time;
 5198       DBUG_ASSERT(rli->last_master_timestamp >= 0);
 5199     }

能够看到,last_master_timestamp 的值便是 ev->common_header->when.tv_sec 和 (time_t) ev->exec_time 的和。在代码中加上点调试信息,从新编译:

      DBUG_PRINT("info", ("before rli->last_master_timestamp = %lu", rli->last_master_timestamp));
      DBUG_PRINT("info", ("before rli->is_parallel_exec() = %d", int(rli->is_parallel_exec())));
    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_PRINT("info", ("after rli->last_master_timestamp = %lu", rli->last_master_timestamp));
      DBUG_PRINT("info", ("after rli->is_parallel_exec() = %d", int(rli->is_parallel_exec())));
      DBUG_ASSERT(rli->last_master_timestamp >= 0);
    }

能够在 mysqld.trace 中察看到如下的调试信息,印证了前文对于非并行复制的论断。同时也能够看出,非并行复制下 last_master_timestamp 的更新是在 binlogevent 执行之前。

exec_relay_log_event: info: before rli->last_master_timestamp = 0
exec_relay_log_event: info: before rli->is_parallel_exec() = 0
exec_relay_log_event: info: after rli->last_master_timestamp = 1562201989
exec_relay_log_event: info: after rli->is_parallel_exec() = 0

3.1.2.2 并行复制模式下更新 last_master_timestamp

前文介绍了非并行复制模式下的 last_master_timestamp 的更新逻辑。本节将介绍下在并行复制模式下,last_master_timestamp 是如何更新的。

在并行复制模式下 last_master_timestamp 的更新逻辑更为简单。同时对于 ROTATE_EVENT 事件和一般的 binlog 事件更新模式也有所不同。接下来首先介绍下一般的 binlog 事件下 last_master_timestamp 的更新逻辑:

3.1.2.2.1 一般的 binlog 事件下 last_master_timestamp 的更新逻辑

在【文献 1】和【文献 2】中对于并行复制逻辑的剖析如下:

​ 并行复制有一个散发队列 gaq,SQL 线程将 binlog 事务读取到 gaq,而后再分发给 worker 线程执行。并行复制时,binlog 事件是并发交叉执行的,gaq 中有一个 checkpoint 点称为 lwm,lwm 之前的 binlog 都曾经执行,而 lwm 之后的 binlog 有些执行有些没有执行。假如 worker 线程数为 2,gap 有 1,2,3,4,5,6,7,8 个事务。worker 1 已执行的事务为 1 4 6, woker 2 执行的事务为 2 3,那么 lwm 为 4。

​并行复制更新 gap checkpiont 时,会推动 lwm 点,同时更新 last_master_timestamp 为 lwm 所在事务完结的 event 的工夫。因而,并行复制是在事务执行实现后才更新 last_master_timestamp,更新是以事务为单位。同时更新 gap checkpiont 还受 slave_checkpoint_period 参数的影响。

​coordinator 会调用函数 mts_checkpoint_routune 定期做“checkpoint”操作,将 Relay_log_info.gaq 中曾经执行完结的事务移出队列,移除形式为从队列头开始查看,如果 done 为真则移出,否则进行扫描,并标记 Low Water Mark 为移出的事务中最大的 sequence_number(lock-interval 终止点)

能够看出,并行复制下 last_master_timestamp 的更新逻辑与 lwm 和 mts_checkpoint_routune 函数无关。mts_checkpoint_routune 函数头如下:

/**
   Processing rli->gaq to find out the low-water-mark (lwm) coordinates
   which is stored into the cental recovery table.

   @param rli            pointer to Relay-log-info of Coordinator
   @param period         period of processing GAQ, normally derived from
                         @c mts_checkpoint_period
   @param force          if TRUE then hang in a loop till some progress
   @param need_data_lock False if rli->data_lock mutex is aquired by
                         the caller.

   @return FALSE success, TRUE otherwise
*/
bool mts_checkpoint_routine(Relay_log_info *rli, ulonglong period,
                            bool force, bool need_data_lock)

在 coordinator 中会定期调用 mts_checkpoint_routine 将 Relay_log_info.gaq 中曾经执行完结的事务移出队列,并且更新 Low Water Mark 为移出的事务中最大的 sequence_number(参见 mts_checkpoint_routine 中如下):

 do
  {if (!is_mts_db_partitioned(rli))
      mysql_mutex_lock(&rli->mts_gaq_LOCK);

    cnt= rli->gaq->move_queue_head(&rli->workers);

    if (!is_mts_db_partitioned(rli))
      mysql_mutex_unlock(&rli->mts_gaq_LOCK);
#ifndef DBUG_OFF
    if (DBUG_EVALUATE_IF("check_slave_debug_group", 1, 0) &&
        cnt != opt_mts_checkpoint_period)
      sql_print_error("This an error cnt != mts_checkpoint_period");
#endif
  } while (!sql_slave_killed(rli->info_thd, rli) &&
           cnt == 0 && force &&
           !DBUG_EVALUATE_IF("check_slave_debug_group", 1, 0) &&
           (my_sleep(rli->mts_coordinator_basic_nap), 1));

在函数的最初,会执行 last_master_timestamp 的更新逻辑:

  /*
    Update the rli->last_master_timestamp for reporting correct Seconds_behind_master.

    If GAQ is empty, set it to zero.
    Else, update it with the timestamp of the first job of the Slave_job_queue
    which was assigned in the Log_event::get_slave_worker() function.
  */
  ts= rli->gaq->empty()
    ? 0
    : reinterpret_cast<Slave_job_group*>(rli->gaq->head_queue())->ts;
  rli->reset_notified_checkpoint(cnt, ts, need_data_lock, true);

能够看出如果 GAP 为空(既 work 的队列为空)则更新 last_master_timestamp 为 0,否则更新为队列中第一个是元素的 timestamp。

3.1.2.2.2 ROTATE_EVENT 事件下 last_master_timestamp 的更新模式

前文提到在并行复制模式下,一般的 binlog 事件会被 coordinator 过程散发到 work 中去执行。并且在 mts_checkpoint_routine 中去推动 Low Water Mark 和更新 last_master_timestamp。然而如果执行的 binlog 是一个 ROTATE_EVENT 事件,则 coordinator 过程不会将事件散发到 work 中,而是在 coordinator 过程中本人执行并更新 last_master_timestamp。

对于 ROTATE_EVENT 事件的执行逻辑,入口仍然是 exec_relay_log_event 函数。在函数中调用了 apply_event_and_update_pos 函数执行 binlog 事件。

 5247     /* ptr_ev can change to NULL indicating MTS coorinator passed to a Worker */
 5248     exec_res= apply_event_and_update_pos(ptr_ev, thd, rli);
 5249     /*
 5250       Note: the above call to apply_event_and_update_pos executes
 5251       mysql_mutex_unlock(&rli->data_lock);
 5252     */
 5253
 5254     /* For deferred events, the ptr_ev is set to NULL
 5255         in Deferred_log_events::add() function.
 5256         Hence deferred events wont be deleted here.
 5257         They will be deleted in Deferred_log_events::rewind() funciton.
 5258     */

在 apply_event_and_update_pos 函数中,会首先调用 apply_event 去判断以后的 binlog 事件是否能够被散发到 work 过程执行。在这里收到的 binlog 事件是 ROTATE_EVENT,因而 apply_event 返回的 exec_res 为 0,ev->worker==rli(rli 代表了 coordinator 过程),因而不会进入上面的 if 逻辑中:

 4709   if (reason == Log_event::EVENT_SKIP_NOT)
 4710   {
 4711     // Sleeps if needed, and unlocks rli->data_lock.
 4712     if (sql_delay_event(ev, thd, rli))
 4713       DBUG_RETURN(SLAVE_APPLY_EVENT_AND_UPDATE_POS_OK);
 4714
 4715     exec_res= ev->apply_event(rli);
 4717     if (!exec_res && (ev->worker != rli))
 4718     {... ...}

在 mysqld.trace 中能够察看到如下的输入:

Log_event::shall_skip: info: skip reason=0=NOT
LOG_EVENT:apply_event: info: event_type=Rotate
apply_event_and_update_pos: info: apply_event error = 0

apply_event 函数定义和正文如下:

/**
   Scheduling event to execute in parallel or execute it directly.
   In MTS case the event gets associated with either Coordinator or a
   Worker.  A special case of the association is NULL when the Worker
   can't be decided yet.  In the single threaded sequential mode the
   event maps to SQL thread rli.

   @note in case of MTS failure Coordinator destroys all gathered
         deferred events.

   @return 0 as success, otherwise a failure.
*/
int Log_event::apply_event(Relay_log_info *rli)

因为无奈被散发到 work 执行,因而会进入如下的执行逻辑中:

 4832   DBUG_PRINT("info", ("apply_event error = %d", exec_res));
 4833   if (exec_res == 0)
 4834   {
 4835     /*
 4836       Positions are not updated here when an XID is processed. To make
 4837       a slave crash-safe, positions must be updated while processing a
 4838       XID event and as such do not need to be updated here again.
 4839
 4840       However, if the event needs to be skipped, this means that it
 4841       will not be processed and then positions need to be updated here.
 4842
 4843       See sql/rpl_rli.h for further details.
 4844     */
 4845     int error= 0;
 4846     if (*ptr_ev &&
 4847         (ev->get_type_code() != binary_log::XID_EVENT ||
 4848          skip_event || (rli->is_mts_recovery() && !is_gtid_event(ev) &&
 4849          (ev->ends_group() || !rli->mts_recovery_group_seen_begin) &&
 4850           bitmap_is_set(&rli->recovery_groups, rli->mts_recovery_index))))
 4851     {
 4852 #ifndef DBUG_OFF
 4853       /*
 4854         This only prints information to the debug trace.
 4855
 4856         TODO: Print an informational message to the error log?
 4857       */
            ... ...
   
 4873       error= ev->update_pos(rli);
                       
                       ... ...
           }
         }

能够发现,在此调用了 update_pos 进行更新。因为以后的 binlog 事件是 ROTATE_EVENT,因而会调用如下的代码:

/*
  Got a rotate log event from the master.

  This is mainly used so that we can later figure out the logname and
  position for the master.

  We can't rotate the slave's BINlog as this will cause infinitive rotations
  in a A -> B -> A setup.
  The NOTES below is a wrong comment which will disappear when 4.1 is merged.

  This must only be called from the Slave SQL thread, since it calls
  flush_relay_log_info().

  @retval
    0   ok
*/
int Rotate_log_event::do_update_pos(Relay_log_info *rli)

在 do_update_pos 更新 last_master_timestamp 的要害逻辑如下:

if (rli->is_parallel_exec())
{bool real_event= server_id && !is_artificial_event();
      rli->reset_notified_checkpoint(0,
                           real_event ?
                           common_header->when.tv_sec +
                           (time_t) exec_time : 0,
                           true/*need_data_lock=true*/,
                           real_event? true : false);
}

能够在 mysqld.trace 中察看到如下的输入:

Rotate_log_event::do_update_pos: info: server_id=248; ::server_id=236
Rotate_log_event::do_update_pos: info: new_log_ident: dp-bin.000063
Rotate_log_event::do_update_pos: info: pos: 4
Rotate_log_event::do_update_pos: info: old group_master_log_name: 'dp-bin.000062'  old group_master_log_pos: 154
Relay_log_info::inc_group_relay_log_pos: info: log_pos: 4  group_master_log_pos: 154
Rotate_log_event::do_update_pos: info: new group_master_log_name: 'dp-bin.000063'  new group_master_log_pos: 4
Rotate_log_event::do_update_pos: info: ------> 1562684581(此处为本文退出的调试信息,在 reset_notified_checkpoint 更新 last_master_timestamp 的时候打印出。)

至此,对于并行复制和非并行复制下当收到 ROTATE_EVENT 事件时,更新 last_master_timestamp 的逻辑剖析全副实现。

3.2 da 执行与本实例 serverid 统一的 binlog 时的运行逻辑

前文剖析了当 MySQL 收到一个 ROTATE_EVENT 事件的时候所运行的逻辑。本节将剖析下在 MySQL 收到与本人的 serverid 统一的 binlog 事件的时候的运行逻辑。

3.2.1 io thread 解决与本实例 serverid 统一的 binlog

首先来看下 io thread 接管到与自身 serverid 统一的 binlog 的时候所做的操作。在 handle_slave_io 函数中,会在 while 循环中一直的调用 queue_even 函数。

 5740     while (!io_slave_killed(thd,mi))
 5741     {
 5742       ulong event_len;
           ... ...
 5750       event_len= read_event(mysql, mi, &suppress_warnings);
          
           ... ...
             
 5813       /* XXX: 'synced' should be updated by queue_event to indicate
 5814          whether event has been synced to disk */
 5815       bool synced= 0;
 5816       if (queue_event(mi, event_buf, event_len))
 5817       {
 5818         mi->report(ERROR_LEVEL, ER_SLAVE_RELAY_LOG_WRITE_FAILURE,
 5819                    ER(ER_SLAVE_RELAY_LOG_WRITE_FAILURE),
 5820                    "could not queue event from master");
 5821         goto err;
 5822       }   
           ... ...
          }

在 queue_even 函数中,对于收到与本人 serverid 统一的 binlog 的解决逻辑如下:

 8534   /*
 8535      If this event is originating from this server, don't queue it.
 8536      We don't check this for 3.23 events because it's simpler like this; 3.23
 8537      will be filtered anyway by the SQL slave thread which also tests the
 8538      server id (we must also keep this test in the SQL thread, in case somebody
 8539      upgrades a 4.0 slave which has a not-filtered relay log).
 8540
 8541      ANY event coming from ourselves can be ignored: it is obvious for queries;
 8542      for STOP_EVENT/ROTATE_EVENT/START_EVENT: these cannot come from ourselves
 8543      (--log-slave-updates would not log that) unless this slave is also its
 8544      direct master (an unsupported, useless setup!).
 8545   */
   
        ... ...
   
 8560   if ((s_id == ::server_id && !mi->rli->replicate_same_server_id) ||
 8561       /*
 8562         the following conjunction deals with IGNORE_SERVER_IDS, if set
 8563         If the master is on the ignore list, execution of
 8564         format description log events and rotate events is necessary.
 8565       */
 8566       (mi->ignore_server_ids->dynamic_ids.size() > 0 &&
 8567        mi->shall_ignore_server_id(s_id) &&
 8568        /* everything is filtered out from non-master */
 8569        (s_id != mi->master_id ||
 8570         /* for the master meta information is necessary */
 8571         (event_type != binary_log::FORMAT_DESCRIPTION_EVENT &&
 8572          event_type != binary_log::ROTATE_EVENT))))
 8573   {
 8574     /*
 8575       Do not write it to the relay log.
 8576       a) We still want to increment mi->get_master_log_pos(), so that we won't
 8577       re-read this event from the master if the slave IO thread is now
 8578       stopped/restarted (more efficient if the events we are ignoring are big
 8579       LOAD DATA INFILE).
 8580       b) We want to record that we are skipping events, for the information of
 8581       the slave SQL thread, otherwise that thread may let
 8582       rli->group_relay_log_pos stay too small if the last binlog's event is
 8583       ignored.
 8584       But events which were generated by this slave and which do not exist in
 8585       the master's binlog (i.e. Format_desc, Rotate & Stop) should not increment
 8586       mi->get_master_log_pos().
 8587       If the event is originated remotely and is being filtered out by
 8588       IGNORE_SERVER_IDS it increments mi->get_master_log_pos()
 8589       as well as rli->group_relay_log_pos.
 8590     */
 8591     if (!(s_id == ::server_id && !mi->rli->replicate_same_server_id) ||
 8592         (event_type != binary_log::FORMAT_DESCRIPTION_EVENT &&
 8593          event_type != binary_log::ROTATE_EVENT &&
 8594          event_type != binary_log::STOP_EVENT))
 8595     {8596       mi->set_master_log_pos(mi->get_master_log_pos() + inc_pos);
 8597       memcpy(rli->ign_master_log_name_end, mi->get_master_log_name(), FN_REFLEN);
 8598       DBUG_ASSERT(rli->ign_master_log_name_end[0]);
 8599       rli->ign_master_log_pos_end= mi->get_master_log_pos();
 8600     }
 8601     rli->relay_log.signal_update(); // the slave SQL thread needs to re-check
 8602     DBUG_PRINT("info", ("master_log_pos: %lu, event originating from %u server, ignored",
 8603                         (ulong) mi->get_master_log_pos(), uint4korr(buf + SERVER_ID_OFFSET)));
 8604   }

联合正文,能够看出当 MySQL 收到与本实例 serverid 统一的 binlog 的时候,不会将以后的 binlog 事件写入 relaylog 中。同时会实现如下的 2 个事件:(ps:这是一个要害的中央,在下文中将提到它的作用)

  1. 将 mi->get_master_log_name() 拷贝到 rli->ign_master_log_name_end 中。
  2. rli->relay_log.signal_update()。此处会新 signal_cnt 这个变量的值。

在 mysqld.trace 中能够察看到如下的日志信息:

queue_event: info: master_log_pos: 219, event originating from 236 server, ignored
queue_event: info: error: 0

3.2.2 sql thread 解决与本实例 serverid 统一的 binlog

接下里看看 sql 过程对 binlog 的处理过程。入口的函数还是 exec_relay_log_event,在 exec_relay_log_event 函数中会调用 next_event 函数获取一个可执行的 binlog 事件。在这里 next_event 对于非并行复制会有一个非凡的解决:

 9175         if (!rli->is_parallel_exec())
 9176           rli->last_master_timestamp= 0;

如果是非并行复制,则当读取一个 binlog 的时候,都会把 last_master_timestamp 设置成 0。因而在非并行复制下,收到与本实例 serverid 统一的 binlog 的时候,mysqld.trace 中能够察看到 rli->last_master_timestamp 的值会始终为 0:

next_event: info: seeing an ignored end segment
handle_slave_io: info: IO thread received event of type Query
exec_relay_log_event: info: ================================before rli->last_master_timestamp = 0
exec_relay_log_event: info: ================================before rli->is_parallel_exec() = 0
apply_event_and_update_pos: info: thd->options: ; rli->last_event_start_time: 0
Log_event::shall_skip: info: ev->server_id=0, ::server_id=236, rli->replicate_same_server_id=0, rli->slave_skip_counter=0
Log_event::shall_skip: info: skip reason=0=NOT
LOG_EVENT:apply_event: info: event_type=Rotate

... ...
... ...

exec_relay_log_event: info: ================================before rli->last_master_timestamp = 0
exec_relay_log_event: info: ================================before rli->is_parallel_exec() = 0
apply_event_and_update_pos: info: thd->options: ; rli->last_event_start_time: 0
Log_event::shall_skip: info: ev->server_id=0, ::server_id=236, rli->replicate_same_server_id=0, rli->slave_skip_counter=0
Log_event::shall_skip: info: skip reason=0=NOT
LOG_EVENT:apply_event: info: event_type=Rotate
apply_event_and_update_pos: info: apply_event error = 0
apply_event_and_update_pos: info: OPTION_BEGIN: 0; IN_STMT: 0
Rotate_log_event::do_update_pos: info: server_id=0; ::server_id=236
Rotate_log_event::do_update_pos: info: new_log_ident: dp-bin.000069
Rotate_log_event::do_update_pos: info: pos: 282
MYSQL_BIN_LOG::signal_update: info: signal_cnt : 13
queue_event: info: master_log_pos: 326, event originating from 236 server, ignored
queue_event: info: error: 0

之后的逻辑中,如果发现 rli->ign_master_log_name_end[0] 不为空(对应了上一个大节中的第一项:将 mi->get_master_log_name() 拷贝到 rli->ign_master_log_name_end 中),则结构一个 serverid 为 0 的 Rotate event 并返回:

 9181         if (rli->ign_master_log_name_end[0])
 9182         {
 9183           /* We generate and return a Rotate, to make our positions advance */
 9184           DBUG_PRINT("info",("seeing an ignored end segment"));
 9185           ev= new Rotate_log_event(rli->ign_master_log_name_end,
 9186                                    0, rli->ign_master_log_pos_end,
 9187                                    Rotate_log_event::DUP_NAME);
 9188           rli->ign_master_log_name_end[0]= 0;
 9189           mysql_mutex_unlock(log_lock);
 9190           if (unlikely(!ev))
 9191           {
 9192             errmsg= "Slave SQL thread failed to create a Rotate event"
 9193               "(out of memory?), SHOW SLAVE STATUS may be inaccurate";
 9194             goto err;
 9195           }
 9196           ev->server_id= 0; // don't be ignored by slave SQL thread
 9197           DBUG_RETURN(ev);
 9198         }

紧接着调用 apply_event_and_update_pos 函数。在 apply_event_and_update_pos 函数中,如前所述如果是 ROTATE EVENT 则返回 0,不会被 work 过程并行执行,并进入 update_pos 逻辑中。在 update_pos 逻辑中要害的代码如下(sql/log_event.cc 文件):

 6649     if (rli->is_parallel_exec())
 6650     {6651       bool real_event= server_id && !is_artificial_event();
 6652       rli->reset_notified_checkpoint(0,
 6653                            real_event ?
 6654                            common_header->when.tv_sec +
 6655                            (time_t) exec_time : 0,
 6656                            true/*need_data_lock=true*/,
 6657                            real_event? true : false);
 6658     }

能够见得 real_event 在 server_id 是 0 的时候为 false。因而当进入 reset_notified_checkpoint 函数后,因为 update_timestamp 条件 (传入的值便是 real_event 的值) 为 false 便不会更新 last_master_timestamp。

 262 /**
 263    This method is called in mts_checkpoint_routine() to mark that each
 264    worker is required to adapt to a new checkpoint data whose coordinates
 265    are passed to it through GAQ index.
 266
 267    Worker notices the new checkpoint value at the group commit to reset
 268    the current bitmap and starts using the clean bitmap indexed from zero
 269    of being reset checkpoint_seqno.
 270
 271     New seconds_behind_master timestamp is installed.
 272
 273    @param shift            number of bits to shift by Worker due to the
 274                            current checkpoint change.
 275    @param new_ts           new seconds_behind_master timestamp value
 276                            unless zero. Zero could be due to FD event
 277                            or fake rotate event.
 278    @param need_data_lock   False if caller has locked @c data_lock
 279    @param update_timestamp if true, this function will update the
 280                            rli->last_master_timestamp.
 281 */
 282 void Relay_log_info::reset_notified_checkpoint(ulong shift, time_t new_ts,
 283                                                bool need_data_lock,
 284                                                bool update_timestamp)
 285 {
 
       ... ...
       
 330   if (update_timestamp)
 331   {332     if (need_data_lock)
 333       mysql_mutex_lock(&data_lock);
 334     else
 335       mysql_mutex_assert_owner(&data_lock);
 336     last_master_timestamp= new_ts;
 337     if (need_data_lock)
 338       mysql_mutex_unlock(&data_lock);
 339   }
 340 }

3.3 小结

从上文的剖析能够得出如下的 2 个论断:

  1. 并行复制下会发现 last_master_timestamp 的值,始终是 ROTATE EVENT 产生的工夫。当 Exec_Master_Log_Pos < Read_Master_Log_Pos 的时候,便会发现 Seconds_Behind_Master 的值渐变(渐变的值是以后工夫和 ROTATE EVENT 产生的时间差的秒数)
  2. 在非并行复制下,因为 rli->last_master_timestamp 会在 next_event 被置为 0。同时之后的代码也不会去更新这个参数的值,因而 rli->last_master_timestamp 始终为 0。show slave status 的时候,当 Exec_Master_Log_Pos < Read_Master_Log_Pos 的时候,Seconds_Behind_Master 的值不会渐变。

4. 总结

本文剖析了并行复制和非并行复制下,Seconds_Behind_Master 参数值更新的相干逻辑。限于本文的作者程度无限,文中的谬误在劫难逃,恳请大家批评指正。

5. 参考文献

序号 文献
1 MySQL 5.7 MTS 源码剖析
2 MySQL · 答疑解惑 · 备库 Seconds_Behind_Master 计算
3 MySQL · 答疑释惑 · server_id 为 0 的 Rotate)
4 【案例】主从替换之后的复制风暴
5 MySQL 复制源码解析
6 MySQL 主库 02 设置宕机导致的主库数据失落解决办法和起因
7 Mysql rpl_slave.cc:handle_slave_io 源码的一些集体剖析)
8 mysql slave 备库提早是怎么失去的
9 MySQL 编译装置并且开启 DEBUG 模式
10 Bug #72376:Seconds_behind_master distorted because of previous_gtid event
11 MySQL Binlog 解析(1)
12 了解 MySQL——复制(Replication)

ps: 如上的参考资料并非齐全在本文中被援用,其余没有被援用的材料在本文的造成的工作中,提供了思路上和其余方面的参考,因而一并列入参考文献。感激如上的文献作者提供的参考。

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