说明：记录一次启动数据库失败的异常
场景
systemctl start mysqld
使用systemctl start mysqld命令启动MySQL服务，报下面的错误：
Job for mysqld.service failed because the control process exited with error code. See "systemctl status mysqld.service" and "journalctl -xe" for details.
提示让我使用journalctl -xe命令，查看一下详细，内容如下：
4月 10 20:50:22 CentOS79 polkitd[848]: Unregistered Authentication Agent for unix-process:13016:187895386 (system bus name :1.1185, object path /org/freedesktop/PolicyKit1/AuthenticationAgent, locale zh_CN.UTF-8) (disconnected from bus)
4月 10 20:50:42 CentOS79 polkitd[848]: Registered Authentication Agent for unix-process:13026:187897598 (system bus name :1.1186 [/usr/bin/pkttyagent --notify-fd 5 --fallback], object path /org/freedesktop/PolicyKit1/AuthenticationAgent, locale zh_CN.UTF-8)
4月 10 20:50:42 CentOS79 systemd[1]: Starting mysqld.service...
-- Subject: Unit mysqld.service has begun start-up
-- Defined-By: systemd
-- Support: http:///mailman/listinfo/systemd-devel
--
-- Unit mysqld.service has begun starting up.
4月 10 20:50:45 CentOS79 systemd[1]: mysqld.service: control process exited, code=exited status=1
4月 10 20:50:45 CentOS79 systemd[1]: Failed to start mysqld.service.
-- Subject: Unit mysqld.service has failed
-- Defined-By: systemd
-- Support: http:///mailman/listinfo/systemd-devel
--
-- Unit mysqld.service has failed.
--
-- The result is failed.
4月 10 20:50:45 CentOS79 systemd[1]: Unit mysqld.service entered failed state.
4月 10 20:50:45 CentOS79 systemd[1]: mysqld.service failed.
4月 10 20:50:45 CentOS79 polkitd[848]: Unregistered Authentication Agent for unix-process:13026:187897598 (system bus name :1.1186, object path /org/freedesktop/PolicyKit1/AuthenticationAgent, locale zh_CN.UTF-8) (disconnected from bus)
毫无头绪，然后我查看了一下MySQL记录的日志信息，如下：
2024-04-10T12:50:42.738859Z 0 [Warning] Changed limits: max_open_files: 5000 (requested 7500)
2024-04-10T12:50:42.738955Z 0 [Warning] Changed limits: table_open_cache: 1745 (requested 2048)
2024-04-10T12:50:42.917286Z 0 [Note] --secure-file-priv is set to NULL. Operations related to importing and exporting data are disabled
2024-04-10T12:50:42.917339Z 0 [Note] /data/mysql/bin/mysqld (mysqld 5.7.36-log) starting as process 13032 ...
2024-04-10T12:50:42.922551Z 0 [Note] InnoDB: PUNCH HOLE support available
2024-04-10T12:50:42.922590Z 0 [Note] InnoDB: Mutexes and rw_locks use GCC atomic builtins
2024-04-10T12:50:42.922595Z 0 [Note] InnoDB: Uses event mutexes
2024-04-10T12:50:42.922598Z 0 [Note] InnoDB: GCC builtin __sync_synchronize() is used for memory barrier
2024-04-10T12:50:42.922602Z 0 [Note] InnoDB: Compressed tables use zlib 1.2.11
2024-04-10T12:50:42.922606Z 0 [Note] InnoDB: Using Linux native AIO
2024-04-10T12:50:42.923404Z 0 [Note] InnoDB: Number of pools: 1
2024-04-10T12:50:42.923536Z 0 [Note] InnoDB: Using CPU crc32 instructions
2024-04-10T12:50:42.925015Z 0 [Note] InnoDB: Initializing buffer pool, total size = 10G, instances = 8, chunk size = 128M
2024-04-10T12:50:43.486095Z 0 [Note] InnoDB: Completed initialization of buffer pool
2024-04-10T12:50:43.555443Z 0 [Note] InnoDB: If the mysqld execution user is authorized, page cleaner thread priority can be changed. See the man page of setpriority().
2024-04-10T12:50:43.567488Z 0 [Note] InnoDB: Highest supported file format is Barracuda.
2024-04-10T12:50:43.750980Z 0 [Note] InnoDB: Creating shared tablespace for temporary tables
2024-04-10T12:50:43.751049Z 0 [Note] InnoDB: Setting file './ibtmp1' size to 12 MB. Physically writing the file full; Please wait ...
2024-04-10T12:50:43.765705Z 0 [Note] InnoDB: File './ibtmp1' size is now 12 MB.
2024-04-10T12:50:43.766370Z 0 [Note] InnoDB: 96 redo rollback segment(s) found. 96 redo rollback segment(s) are active.
2024-04-10T12:50:43.766378Z 0 [Note] InnoDB: 32 non-redo rollback segment(s) are active.
2024-04-10T12:50:43.767368Z 0 [Note] InnoDB: Waiting for purge to start
2024-04-10T12:50:43.817571Z 0 [Note] InnoDB: 5.7.36 started; log sequence number 2750697
2024-04-10T12:50:43.817969Z 0 [Note] InnoDB: Loading buffer pool(s) from /data/mydata/ib_buffer_pool
2024-04-10T12:50:43.818543Z 0 [Note] Plugin 'FEDERATED' is disabled.
2024-04-10T12:50:43.819940Z 0 [Note] InnoDB: Buffer pool(s) load completed at 240410 20:50:43
2024-04-10T12:50:43.824078Z 0 [ERROR] Too many arguments (first extra is 'start').
2024-04-10T12:50:43.824097Z 0 [Note] Use --verbose --help to get a list of available options!
2024-04-10T12:50:43.824103Z 0 [ERROR] Aborting
2024-04-10T12:50:43.824114Z 0 [Note] Binlog end
…………
2024-04-10T12:50:45.650620Z 0 [Note] /data/mysql/bin/mysqld: Shutdown complete
有一条ERROR，[ERROR] Too many arguments (first extra is 'start').，于是我查了许多这方面的文章、博客，但都没有用；
解决
使用systemctl start mysql，即去掉最后一个d。我也不太清楚，前面的时候为什么要加上一个d，后面偶然漏敲了，反而可以正常启动。
systemctl start mysql
