在MySQL官方版本中，为了保证其的高可用性，一般情况我们会采用主从复制的方式来解决。当然，方法很多。而我们今天所要处理的是采用GTID方式并且开了多线程复制后，仍然延迟的情况，糟糕的是，延迟还在不断扩大！
| 序号 | 清单 | 说明 |
| 1 | 系统 | Redhat 6.x（4c，32g） |
| 2 | 数据库 | MySQL-5.7.25 |
| 3 | 同步方式 | 基于GTID主从同步 |
1）已经配置的重要参数：
#relay for slave
slave_parallel_type = LOGICAL_CLOCK
slave_parallel_workers = 6
master_info_repository = TABLE
relay_log_info_repository = TABLE
relay_log_recovery = on
sync_relay_log = 10000
注：此时没有设置slave_preserve_commit_order参数。
2）从库延迟状态查询
mysql> show slave statusG
*************************** 1. row ***************************
Slave_IO_State: Queueing master event to the relay log
Master_Host: xxx.xxx.xxx.xxx
Master_User: repl
Master_Port: 3306
Connect_Retry: 60
Master_Log_File: mysql-bin.008978
Read_Master_Log_Pos: 696914605
Relay_Log_File: DB41-relay-bin.001259
Relay_Log_Pos: 207377582
Relay_Master_Log_File: mysql-bin.008970
Slave_IO_Running: Yes
Slave_SQL_Running: Yes
Replicate_Do_DB: neteagle3
Replicate_Ignore_DB: mysql
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: 1068770059
Relay_Log_Space: 8425484286
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: 187358
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: 42
Master_UUID: eab7fcac-3cda-11e6-ada8-fa163e648db2
Master_Info_File: mysql.slave_master_info
SQL_Delay: 0
SQL_Remaining_Delay: NULL
Slave_SQL_Running_State: Waiting for dependent transaction to commit
Master_Retry_Count: 86400
Master_Bind:
Last_IO_Error_Timestamp:
Last_SQL_Error_Timestamp:
Master_SSL_Crl:
Master_SSL_Crlpath:
Retrieved_Gtid_Set: eab7fcac-3cda-11e6-ada8-fa163e648db2:58031191-59927276
Executed_Gtid_Set: eab7fcac-3cda-11e6-ada8-fa163e648db2:1-58080239:58080241
Auto_Position: 1
Replicate_Rewrite_DB:
Channel_Name:
Master_TLS_Version:
1 row in set (0.00 sec)
- Master_Log_File
- Read_Master_Log_Pos
- Seconds_Behind_Master
- Relay_Log_File
- Relay_Log_Pos
- Relay_Master_Log_File
- Exec_Master_Log_Pos
Master_Log_File, Read_Master_Log_Pos：
这两个参数是成对的，表示的是从库IO thread传输主库的binlog日志号及具体位置。
Relay_Log_File, Relay_Log_Pos：
这两个参数也是成对的，表示的是从库sql thread应用中继日志（relay log）号及具体位置。
Relay_Master_Log_File, Relay_Log_Pos：
这两个参数也是成对的，表示的是上一项中的中继日志对应的主库binlog日志及具体位置（有点绕）。
Seconds_Behind_Master：
此参数可简单理解为主从延迟时间，单位为秒。
从上面这段MySQL从状态信息中，我们可以看到，Seconds_Behind_Master: 187358这是从库sql应用延迟主库的时间为187358秒，转换成天，大概两天多。这说明，我们从库复制的数据是两天前的。
3)验证并行复制
mysql> show full processlist;
---- ------------- -------------- ----------- ------------------ -------- --------------------------------------------------------------- -----------------------
| Id | User | Host | db | Command | Time | State | Info |
---- ------------- -------------- ----------- ------------------ -------- --------------------------------------------------------------- -----------------------
| 1 | system user | | NULL | Connect | 18204 | Waiting for master to send event | NULL |
| 2 | system user | | NULL | Connect | 0 | Waiting for dependent transaction to commit | NULL |
| 3 | system user | | NULL | Connect | 154914 | System lock | NULL |
| 4 | system user | | NULL | Connect | 154914 | Waiting for an event from Coordinator | NULL |
| 5 | system user | | NULL | Connect | 154918 | Waiting for an event from Coordinator | NULL |
| 6 | system user | | NULL | Connect | 155525 | Waiting for an event from Coordinator | NULL |
| 7 | system user | | NULL | Connect | 180427 | Waiting for an event from Coordinator | NULL |
| 8 | system user | | NULL | Connect | 18204 | Waiting for an event from Coordinator | NULL |
| 10 | root | localhost | neteagle3 | Query | 0 | starting | show full processlist |
| 11 | repl | DBSlave:9683 | NULL | Binlog Dump GTID | 18156 | Master has sent all binlog to slave; waiting for more updates | NULL |
| 13 | root | localhost | neteagle3 | Sleep | 4962 | | NULL |
---- ------------- -------------- ----------- ------------------ -------- --------------------------------------------------------------- -----------------------
mysql> select * from performance_schema.replication_applier_status_by_worker ;
-------------- ----------- ----------- --------------- ----------------------------------------------- ------------------- -------------------- ----------------------
| CHANNEL_NAME | WORKER_ID | THREAD_ID | SERVICE_STATE | LAST_SEEN_TRANSACTION | LAST_ERROR_NUMBER | LAST_ERROR_MESSAGE | LAST_ERROR_TIMESTAMP |
-------------- ----------- ----------- --------------- ----------------------------------------------- ------------------- -------------------- ----------------------
| | 1 | 51 | ON | eab7fcac-3cda-11e6-ada8-fa163e648db2:80240805 | 0 | | 0000-00-00 00:00:00 |
| | 2 | 52 | ON | eab7fcac-3cda-11e6-ada8-fa163e648db2:80240210 | 0 | | 0000-00-00 00:00:00 |
| | 3 | 53 | ON | eab7fcac-3cda-11e6-ada8-fa163e648db2:80235089 | 0 | | 0000-00-00 00:00:00 |
| | 4 | 54 | ON | eab7fcac-3cda-11e6-ada8-fa163e648db2:80191268 | 0 | | 0000-00-00 00:00:00 |
| | 5 | 55 | ON | eab7fcac-3cda-11e6-ada8-fa163e648db2:75296683 | 0 | | 0000-00-00 00:00:00 |
| | 6 | 56 | ON | | 0 | | 0000-00-00 00:00:00 |
-------------- ----------- ----------- --------------- ----------------------------------------------- ------------------- -------------------- ----------------------
6 rows in set (0.00 sec)
通过本条查询，可以看到开启了6个并行进行复制。
从库也同样查看了是否存在锁的情况，也没有发现。
因此反复的在查看slave状态，看能否发现一些细节，结果还真看到了一些异常现象。Relay_Log_Pos这个参数在频繁的刷slave状态时，发现时常会卡着不动（此时已确认没有看到锁）。或许问题真正的原因正在这里！
[mysql@xxx data]$ mysqlbinlog --no-defaults -v -v --base64-output=DECODE-ROWS relay-bin.001384 --start-position=420090430|more
/*!50530 SET @@SESSION.PSEUDO_SLAVE_MODE=1*/;
/*!50003 SET @OLD_COMPLETION_TYPE=@@COMPLETION_TYPE,COMPLETION_TYPE=0*/;
DELIMITER /*!*/;
# at 420090430
#190923 9:24:28 server id 42 end_log_pos 420090282 CRC32 0xd9097eaf GTID last_committed=57148 sequence_number=57149 rbr_only=yes
/*!50718 SET TRANSACTION ISOLATION LEVEL READ COMMITTED*//*!*/;
SET @@SESSION.GTID_NEXT= 'eab7fcac-3cda-11e6-ada8-fa163e648db2:69415610'/*!*/;
# at 420090495
#190923 9:24:28 server id 42 end_log_pos 420090364 CRC32 0x82b57dfd Query thread_id=95 exec_time=0 error_code=0
SET TIMESTAMP=1569201868/*!*/;
SET @@session.pseudo_thread_id=95/*!*/;
SET @@session.foreign_key_checks=1, @@session.sql_auto_is_null=0, @@session.unique_checks=1, @@session.autocommit=1/*!*/;
SET @@session.sql_mode=1075838976/*!*/;
SET @@session.auto_increment_increment=2, @@session.auto_increment_offset=1/*!*/;
/*!C gbk *//*!*/;
SET @@session.character_set_client=28,@@session.collation_connection=28,@@session.collation_server=8/*!*/;
SET @@session.lc_time_names=0/*!*/;
SET @@session.collation_database=DEFAULT/*!*/;
BEGIN
/*!*/;
# at 420090577
#190923 9:24:28 server id 42 end_log_pos 420090585 CRC32 0x752e27cf Table_map: `net`.`f_event` mapped to number 108
# at 420090798
#190923 9:24:28 server id 42 end_log_pos 420090812 CRC32 0x72b8e10d Table_map: `net`.`f_eventstorage` mapped to number 245
# at 420091025
#190923 9:24:28 server id 42 end_log_pos 420091039 CRC32 0x1797f9d8 Table_map: `net`.`f_eventstorage` mapped to number 245
# at 420091252
#190923 9:24:28 server id 42 end_log_pos 420091106 CRC32 0x8af14ad2 Table_map: `net`.`f_eventdetail` mapped to number 243
# at 420091319
#190923 9:24:28 server id 42 end_log_pos 420091177 CRC32 0xf1ce87c8 Table_map: `net`.`f_eventoperation` mapped to number 244
# at 420091390
#190923 9:24:28 server id 42 end_log_pos 420091244 CRC32 0x586c0b9d Table_map: `net`.`f_eventaudit` mapped to number 242
# at 420091457
#190923 9:24:28 server id 42 end_log_pos 420093382 CRC32 0x505e5408 Update_rows: table id 108
# at 420093595
#190923 9:24:28 server id 42 end_log_pos 420098858 CRC32 0x0f404509 Update_rows: table id 245
# at 420099071
#190923 9:24:28 server id 42 end_log_pos 420098910 CRC32 0xb8d9ed15 Write_rows: table id 243
# at 420099123
#190923 9:24:28 server id 42 end_log_pos 420098966 CRC32 0x3c489a7f Write_rows: table id 244 flags: STMT_END_F
我们查看了中继日志relay-bin.001384卡住的位置号420090430，为设置GTID_NEXT，信息没什么用。
继续看在卡住时刻，数据库open的是什么表？
mysql> show open tables where In_use=1;
----------- --------------------- -------- -------------
| Database | Table | In_use | Name_locked |
----------- --------------------- -------- -------------
| net | f_currentxxx | 1 | 0 |
----------- --------------------- -------- -------------
这张表有什么特殊的么？查看其表结构
mysql> show create table net.f_currentxxxG
*************************** 1. row ***************************
Table: f_currentxxx
Create Table: CREATE TABLE `f_currentxxx` (
`serial` int(20) NOT NULL COMMENT 'xxx',
`audittime` bigint(20) NOT NULL COMMENT 'xxx',
`type` int(11) DEFAULT NULL COMMENT 'xxx',
`severity` int(11) DEFAULT NULL COMMENT 'xxx,
KEY `audittime` (`audittime`)
) ENGINE=MEMORY DEFAULT CHARSET=gbk COMMENT='xxx'
1 row in set (0.00 sec)
有没有看到什么与众不同？
没错，就是表的存储引擎ENGINE=MEMORY。MEMORY的表进行主从复制，首先来说如果从库不做查询，一点意义没有，另外对Memory表做复制，性能是非常堪忧的。如果必须进行同步，考虑将表的存储引擎改为InnoDB
mysql> select table_name from information_schema.tables where TABLE_SCHEMA='net' and ENGINE='memory';
----------------------
| table_name |
----------------------
| f_currentxxx |
----------------------
1 row in set, 6 warnings (0.01 sec)
出于严谨，我们将要复制的数据库进行彻底排查，确实只有这一张表是Memory存储引擎。
mysql> show slave statusG
*************************** 1. row ***************************
Slave_IO_State: Waiting for master to send event
Master_Host: xxx.xxx.xxx.xxx
Master_User: repl
Master_Port: 3306
Connect_Retry: 60
Master_Log_File: mysql-bin.009194
Read_Master_Log_Pos: 939698255
Relay_Log_File: relay-bin.001964
Relay_Log_Pos: 444060572
Relay_Master_Log_File: mysql-bin.009027
Slave_IO_Running: Yes
Slave_SQL_Running: Yes
Replicate_Do_DB: net
Replicate_Ignore_DB: mysql
Replicate_Do_Table:
Replicate_Ignore_Table: net.f_currentxxx
Replicate_Wild_Do_Table:
Replicate_Wild_Ignore_Table:
Last_Errno: 0
Last_Error:
Skip_Counter: 0
Exec_Master_Log_Pos: 444060359
Relay_Log_Space: 180287882098
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: 179221
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: 42
Master_UUID: eab7fcac-3cda-11e6-ada8-fa163e648db2
Master_Info_File: mysql.slave_master_info
SQL_Delay: 0
SQL_Remaining_Delay: NULL
Slave_SQL_Running_State: Waiting for dependent transaction to commit
Master_Retry_Count: 86400
Master_Bind:
Last_IO_Error_Timestamp:
Last_SQL_Error_Timestamp:
Master_SSL_Crl:
Master_SSL_Crlpath:
Retrieved_Gtid_Set: eab7fcac-3cda-11e6-ada8-fa163e648db2:69497322-107886661
Executed_Gtid_Set: 1264a536-da12-11e9-81ea-005056856ba5:1,
eab7fcac-3cda-11e6-ada8-fa163e648db2:1-71980857
Auto_Position: 1
Replicate_Rewrite_DB:
Channel_Name:
Master_TLS_Version:
1 row in set (0.00 sec)
我们可以看到net.f_currentxxx表已经被忽略复制。持续观察一段时间后，Seconds_Behind_Master在逐渐缩小。
中继日志应用速度大约5分钟一个（每个中继日志为1GB大小），而主库binlog日志大约为10分钟一个（每个binlog日志为1GB大小）。
在梳理了整个处理过程后，其实难度不高，主要是要细心，细心去排查每一个想到的点。 在非轻量级的数据库中问题发生的概率也会随着量级的增加而增多。而这恰恰是能够磨练个人的成长。
同时，知识的储备也要充足，这是进阶高手的必要前提！