MySQL 5.7複製延遲之sync_relay_log

1、描述
MySQL 5.7版本主從複製，批量時候顯示延遲上萬秒。

2、現象

一、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

二、dm3是relay log 和binlog分區
$ 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

三、slave狀態
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

3、分析
經過以上現象發現備庫io使用率太高，超過90%。io太高的磁盤爲日誌盤，存放relay log和binlog。io thead一致在寫relay log，調用fdatasync寫磁盤。這裏涉及到一個參數sync_relay_log，默認值爲10000，查看當前系統參數值爲1.

4、解決方案
優化io thread線程和sql thread線程。sync_relay_log使用默認值，使用mts優化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;