问题描述：启动mysql数据库时报错unknown variable 'rpl_semi_sync_slave_enabled=1'.
数据库：mysql 5.7.21
系统：rhel 7.3
1、异常重现
--启动数据库
[mysql@mysql-leo-slave data]$ /usr/local/mysql/bin/mysqld_safe --defaults-file=/home/mysql/etc/my.cnf &

--告警信息
2023-08-26T05:18:40.824255Z 0 [Note] InnoDB: Loading buffer pool(s) from /mysql/data/ib_buffer_pool
2023-08-26T05:18:40.826048Z 0 [ERROR] unknown variable 'rpl_semi_sync_slave_enabled=1'
2023-08-26T05:18:40.826071Z 0 [ERROR] Aborting

说明：如上所示，数据库自动关闭.

2、异常分析
2.1、查配置文件
[mysql@mysql-leo-master etc]$ cat my.cnf
[mysql]
prompt="\u@mysql-leo-master[\\d]> "
#CLIENT#
port                                      = 3306
socket                                    = /mysql/data/mysql.sock

[mysqld]
basedir                                   = /usr/local/mysql
datadir                                   = /mysql/data/
server_id                                 = 1
port                                      = 3306
sql_mode                                  = ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
character_set_server                      = utf8mb4
socket                                    = /mysql/data/mysql.sock
secure-file-priv                          = /mysql/bak/file
log-bin                                   = /mysql/binlog/mysql-bin
log-bin-index                             = /mysql/data/mysql-bin.index
relay-log                                 = /mysql/data/relay-bin
relay-log-index                           = /mysql/data/relay-bin.index

#GTID
gtid_mode                                 = on
enforce_gtid_consistency                  = on
innodb_buffer_pool_size                   = 2G

#开启半同步复制
plugin-load=rpl_semi_sync_master=semisync_master.so
rpl_semi_sync_master_wait_no_slave        = 0
rpl_semi_sync_master_enabled              = 1
rpl_semi_sync_master_timeout              = 10000
rpl_semi_sync_slave_enabled               = 0
rpl_semi_sync_master_wait_for_slave_count = 1
rpl_semi_sync_master_wait_point           = AFTER_SYNC


[mysqld_safe]
pid_file                                  = /mysql/data/mysql.pid
log_error                                 = /mysql/data/mysql_error.log

说明：搭建mysql半同步在my.cnf文件中添加半同步参数时，global variable必须在相应的so文件安装后才有效.

3、解决方案
3.1、注释半同步参数
--将my.cnf相关半同步参数注释掉.
[mysql@mysql-leo-master etc]$ vi my.cnf
......
#plugin-load=rpl_semi_sync_master=semisync_master.so
#rpl_semi_sync_master_wait_no_slave        = 0
#rpl_semi_sync_master_enabled              = 1
#rpl_semi_sync_master_timeout              = 10000
#rpl_semi_sync_slave_enabled               = 0
#rpl_semi_sync_master_wait_for_slave_count = 1
#rpl_semi_sync_master_wait_point           = AFTER_SYNC
......

3.2、安装so文件
--查看so文件
[root@mysql-leo-master ~]# find / -name semisync_master.so
find: ‘/run/user/1000/gvfs’: Permission denied
/usr/local/Percona-Server-5.7.21-20-Linux.x86_64.ssl101/lib/mysql/plugin/semisync_master.so
[root@mysql-leo-master ~]# find / -name semisync_slave.so
find: ‘/run/user/1000/gvfs’: Permission denied
/usr/local/Percona-Server-5.7.21-20-Linux.x86_64.ssl101/lib/mysql/plugin/semisync_slave.so

--开启数据库
[mysql@mysql-leo-master etc]$ /usr/local/mysql/bin/mysqld_safe --defaults-file=/home/mysql/etc/my.cnf &

--安装插件
mysql> SELECT PLUGIN_NAME, PLUGIN_STATUS FROM INFORMATION_SCHEMA.PLUGINS WHERE PLUGIN_NAME LIKE '%semi%';
Empty set (0.01 sec)

mysql> INSTALL PLUGIN rpl_semi_sync_master SONAME 'semisync_master.so';
Query OK, 0 rows affected (0.00 sec)

mysql> install plugin rpl_semi_sync_slave soname 'semisync_slave.so';
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT PLUGIN_NAME, PLUGIN_STATUS FROM INFORMATION_SCHEMA.PLUGINS WHERE PLUGIN_NAME LIKE '%semi%';
+----------------------+---------------+
| PLUGIN_NAME          | PLUGIN_STATUS |
+----------------------+---------------+
| rpl_semi_sync_master | ACTIVE        |
| rpl_semi_sync_slave  | ACTIVE        |
+----------------------+---------------+
2 rows in set (0.00 sec)

3.3、重启数据库
[mysql@mysql-leo-master etc]$ mysql -uroot -p
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 12
Server version: 5.7.21-20-log Percona Server (GPL), Release 20, Revision ed217b06ca3

Copyright (c) 2009-2018 Percona LLC and/or its affiliates
Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> shutdown;
Query OK, 0 rows affected (0.00 sec)

mysql> exit
Bye
[mysql@mysql-leo-master etc]$ /usr/local/mysql/bin/mysqld_safe --defaults-file=/home/mysql/etc/my.cnf &

说明：如上所示，数据库成功开启.
参考文档：https://blog.csdn.net/weixin_36035750/article/details/113908654