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 2016-01-20 18:26:57 CST; 40s ago
     Docs: man:systemd-sysv-generator(8)
  Process: 2979 ExecStart=/etc/rc.d/init.d/MysqLd start (code=exited,status=1/FAILURE)

Jan 20 18:26:56 spark01 systemd[1]: Starting SYSV: MysqL database server....
Jan 20 18:26:57 spark01 MysqLd[2979]: MysqL Daemon Failed to start.
Jan 20 18:26:57 spark01 MysqLd[2979]: Starting MysqLd:  [Failed]
Jan 20 18:26:57 spark01 systemd[1]: MysqLd.service: control process exited,code=exited status=1
Jan 20 18:26:57 spark01 systemd[1]: Failed to start SYSV: MysqL database server..
Jan 20 18:26:57 spark01 systemd[1]: Unit MysqLd.service entered Failed state.
Jan 20 18:26:57 spark01 systemd[1]: MysqLd.service Failed.

[root@spark01 ~]# journalctl -xe

-- 
-- Unit session-2.scope has begun starting up.
Jan 20 18:26:48 spark01 sshd[2916]: pam_unix(sshd:session): session opened for user spark by (uid=0)
Jan 20 18:26:52 spark01 su[2944]: (to root) spark on pts/1
Jan 20 18:26:52 spark01 su[2944]: pam_unix(su-l:session): session opened for user root by spark(uid=1000)
Jan 20 18:26:56 spark01 polkitd[909]: Registered Authentication Agent for unix-process:2974:117137 (system bus name :1.25
Jan 20 18:26:56 spark01 systemd[1]: 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 20 18:26:57 spark01 MysqLd[2979]: MysqL Daemon Failed to start.
Jan 20 18:26:57 spark01 MysqLd[2979]: Starting MysqLd:  [Failed]
Jan 20 18:26:57 spark01 systemd[1]: MysqLd.service: control process exited,code=exited status=1
Jan 20 18:26:57 spark01 systemd[1]: 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 20 18:26:57 spark01 systemd[1]: Unit MysqLd.service entered Failed state.
Jan 20 18:26:57 spark01 systemd[1]: MysqLd.service Failed.
Jan 20 18:26:57 spark01 polkitd[909]: Unregistered Authentication Agent for unix-process:2974:117137 (system bus name :1.

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

2016-01-20T10:00:19.935771Z 0 [ERROR] /usr/sbin/MysqLd: Can't create/write to file '/var/run/MysqLd/MysqLd.pid' (Errcode: 2 - No such file or directory)
2016-01-20T10:00:19.935795Z 0 [ERROR] Can't start server: can't create PID file: No such file or directory
160120 18:00:20 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

2016-01-20T10:28:37.183387Z 0 [ERROR] /usr/sbin/MysqLd: Can't create/write to file '/var/run/MysqLd/MysqLd.pid' (Errcode: 13 - Permission denied)
2016-01-20T10:28:37.183431Z 0 [ERROR] Can't start server: can't create PID file: Permission denied
160120 18:28:37 MysqLd_safe MysqLd from pid file /var/run/MysqLd/MysqLd.pid ended
160120 18:32:06 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 2 root root 40 Jan 20 18:28 /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。