CentOS 7下MySQL服务启动失败的解决思路

今天，启动MySQL服务器失败，如下所示：

[root@spark01 ~]# /etc/init.d/mysqld start
Starting mysqld (via systemctl):  Job for mysqld.service failed because the control process exited with error code. See "systemctl status mysqld.service" and "journalctl -xe" for details.
                                                           [FAILED]

根据提示，分别使用systemctl status mysqld.service和journalctl -xe查看服务启动失败的原因

[root@spark01 ~]# systemctl status mysqld.service

?.mysqld.service - SYSV: MySQL database server.
   Loaded: loaded (/etc/rc.d/init.d/mysqld)
   Active: failed (Result: exit-code) since Wed -- :: CST; 40s ago
     Docs: man:systemd-sysv-generator()
  Process:  ExecStart=/etc/rc.d/init.d/mysqld start (code=exited, status=/FAILURE)

Jan  :: spark01 systemd[]: Starting SYSV: MySQL database server....
Jan  :: spark01 mysqld[]: MySQL Daemon failed to start.
Jan  :: spark01 mysqld[]: Starting mysqld:  [FAILED]
Jan  :: spark01 systemd[]: mysqld.service: control process exited, code=exited status=
Jan  :: spark01 systemd[]: Failed to start SYSV: MySQL database server..
Jan  :: spark01 systemd[]: Unit mysqld.service entered failed state.
Jan  :: spark01 systemd[]: mysqld.service failed.

[root@spark01 ~]# journalctl -xe

--
-- Unit session-.scope has begun starting up.
Jan  :: spark01 sshd[]: pam_unix(sshd:session): session opened for user spark by (uid=)
Jan  :: spark01 su[]: (to root) spark on pts/
Jan  :: spark01 su[]: pam_unix(su-l:session): session opened for user root by spark(uid=)
Jan  :: spark01 polkitd[]: Registered Authentication Agent for unix-process:: (system bus name :1.25
Jan  :: spark01 systemd[]: Starting SYSV: MySQL database server....
-- Subject: Unit mysqld.service has begun start-up
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
--
-- Unit mysqld.service has begun starting up.
Jan  :: spark01 mysqld[]: MySQL Daemon failed to start.
Jan  :: spark01 mysqld[]: Starting mysqld:  [FAILED]
Jan  :: spark01 systemd[]: mysqld.service: control process exited, code=exited status=
Jan  :: spark01 systemd[]: Failed to start SYSV: MySQL database server..
-- Subject: Unit mysqld.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
--
-- Unit mysqld.service has failed.
--
-- The result is failed.
Jan  :: spark01 systemd[]: Unit mysqld.service entered failed state.
Jan  :: spark01 systemd[]: mysqld.service failed.
Jan  :: spark01 polkitd[]: Unregistered Authentication Agent for unix-process:: (system bus name :.

但，可惜的时，这些信息并不能提供服务启动失败的真正原因。

这时候，不妨打开MySQL的告警日志，毕竟，只要MySQL服务启动，告警日志都会有输出信息的，果然

--20T10::.935771Z  [ERROR] /usr/sbin/mysqld: Can't create/write to file '/var/run/mysqld/mysqld.pid' (Errcode: 2 - No such file or directory)
--20T10::.935795Z  [ERROR] Can't start server: can't create PID file: No such file or directory
 :: mysqld_safe mysqld from pid file /var/run/mysqld/mysqld.pid ended

MySQL服务在启动的时候，不能创建pid文件。

在终端看一下该目录是否存在，果然，不存在。

于是，创建了/var/run/mysqld/目录，重启MySQL服务

[root@spark01 ~]# mkdir -p /var/run/mysqld/

[root@spark01 ~]# /etc/init.d/mysqld start

Starting mysqld (via systemctl):  Job for mysqld.service failed because the control process exited with error code. See "systemctl status mysqld.service" and "journalctl -xe" for details.
                                                           [FAILED]

依旧报错，重新查看告警日志，有以下输出

--20T10::.183387Z  [ERROR] /usr/sbin/mysqld: Can't create/write to file '/var/run/mysqld/mysqld.pid' (Errcode: 13 - Permission denied)
--20T10::.183431Z  [ERROR] Can't start server: can't create PID file: Permission denied
 :: mysqld_safe mysqld from pid file /var/run/mysqld/mysqld.pid ended
 :: mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql

原来，/var/run/mysqld/的属主和属组还是root，mysql并不能在其中创建文件，后修改该目录的属主和属组，启动OK。

[root@spark01 ~]# ls -ld /var/run/mysqld/
drwxr-xr-x  root root  Jan  : /var/run/mysqld/
[root@spark01 ~]# chown mysql.mysql /var/run/mysqld/
[root@spark01 ~]# /etc/init.d/mysqld start
Starting mysqld (via systemctl):                           [  OK  ]

总结：

以前在玩Kubernetes的时候，常遇到启动失败的情况，根据systemctl的提示，通过systemctl status mysqld.service和journalctl -xe命令查看服务启动失败的原因往往并不如人意，反而给了一种错误的暗示，以为这个跟系统有关。其实，通过查看服务的日志，往往更能清晰的知道服务启动失败的原因。