MySQL 5.7复制延迟之sync_relay_log

一、描述
MySQL 5.7版本主从复制，批量时候显示延迟上万秒。

二、现象

1、io使用率高
#iostat -dxm 1 1000
Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await  svctm  %util
scd0              0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00   0.00   0.00
vda               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00   0.00   0.00
vdb               0.00    96.00    0.00 2596.00     0.00     8.54     6.74     1.33    0.51   0.37  95.30
vdc               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00   0.00   0.00
vdd               0.00     0.00    0.00   11.00     0.00     0.06    11.64     0.00    0.09   0.09   0.10
vde               0.00     0.00    0.00    7.00     0.00     0.00     0.00     0.00    0.00   0.00   0.00
vdf               0.00     0.00    0.00  511.00     0.00     0.00     0.00     0.05    0.09   0.09   4.60
vdg               0.00     0.00    0.00  511.00     0.00     0.00     0.00     0.05    0.09   0.09   4.80
dm-0              0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00   0.00   0.00
dm-1              0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00   0.00   0.00
dm-2              0.00     0.00    0.00   34.00     0.00     0.23    13.65     0.02    0.59   0.38   1.30
dm-3              0.00     0.00    0.00 2144.00     0.00     8.38     8.00     1.40    0.65   0.45  97.20
dm-4              0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00   0.00   0.00
dm-5              0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00   0.00   0.00

2、dm3是relay log 和binlog分区
$ ls -l /dev/mapper
total 0
lrwxrwxrwx 1 root root      7 Jul 23 23:20 backup-backup -> ../dm-0
crw-rw---- 1 root root 10, 58 Jul 23 23:20 control
lrwxrwxrwx 1 root root      7 Jul 23 23:20 VG00-lv_root -> ../dm-4
lrwxrwxrwx 1 root root      7 Jul 23 23:20 zxmysql-zxdba -> ../dm-1
lrwxrwxrwx 1 root root      7 Jul 23 23:20 zxmysql-zxlog -> ../dm-3

3、slave状态
mysql> show slave status \G;
*************************** 1. row ***************************
               Slave_IO_State: Queueing master event to the relay log
                略.........................................
                Connect_Retry: 60
              Master_Log_File: mysql-bin.011494
          Read_Master_Log_Pos: 21037034
               Relay_Log_File: relay-log.001904
                Relay_Log_Pos: 3154097
        Relay_Master_Log_File: mysql-bin.011494
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB: 
          Replicate_Ignore_DB: 
           Replicate_Do_Table: 
       Replicate_Ignore_Table: 
      Replicate_Wild_Do_Table: 
  Replicate_Wild_Ignore_Table: 
                   Last_Errno: 0
                   Last_Error: 
                 Skip_Counter: 0
          Exec_Master_Log_Pos: 3153884
              Relay_Log_Space: 21037535
              Until_Condition: None
               Until_Log_File: 
                Until_Log_Pos: 0
           Master_SSL_Allowed: No
           Master_SSL_CA_File: 
           Master_SSL_CA_Path: 
              Master_SSL_Cert: 
            Master_SSL_Cipher: 
               Master_SSL_Key: 
        Seconds_Behind_Master: 471
Master_SSL_Verify_Server_Cert: No
                Last_IO_Errno: 0
                Last_IO_Error: 
               Last_SQL_Errno: 0
               Last_SQL_Error: 
  Replicate_Ignore_Server_Ids: 
             Master_Server_Id: 400011
                  Master_UUID: 0f8507ea-6da1-11e8-8646-005056873c4a
             Master_Info_File: mysql.slave_master_info
                    SQL_Delay: 0
          SQL_Remaining_Delay: NULL
      Slave_SQL_Running_State: Reading event from the relay log
           Master_Retry_Count: 86400
                  Master_Bind: 
      Last_IO_Error_Timestamp: 
     Last_SQL_Error_Timestamp: 
               Master_SSL_Crl: 
           Master_SSL_Crlpath: 
           Retrieved_Gtid_Set: 0f8507ea-6da1-11e8-8646-005056873c4a:14137114-19288497
            Executed_Gtid_Set: 0f8507ea-6da1-11e8-8646-005056873c4a:1-19288446
                Auto_Position: 1
         Replicate_Rewrite_DB: 
                 Channel_Name: 
           Master_TLS_Version: 
1 row in set (0.01 sec)

ERROR: 
No query specified

三、分析
通过以上现象发现备库io使用率过高，超过90%。io过高的磁盘为日志盘，存放relay log和binlog。io thead一致在写relay log，调用fdatasync写磁盘。这里涉及到一个参数sync_relay_log，默认值为10000，查看当前系统参数值为1.

四、解决方案
优化io thread线程和sql thread线程。sync_relay_log使用默认值，使用mts优化sql thread。

stop slave;
set global slave_parallel_type=logical_clock;
set global slave_parallel_workers=8;
set global sync_master_info=10000;
set global sync_relay_log=10000;
set global sync_relay_log_info=10000;
start slave;