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 2017-04-20 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)
Apr 20 18:26:56 spark01 systemd[1]: Starting SYSV: MySQL database server....
Apr 20 18:26:57 spark01 mysqld[2979]: MySQL Daemon failed to start.
Apr 20 18:26:57 spark01 mysqld[2979]: Starting mysqld: [FAILED]
Apr 20 18:26:57 spark01 systemd[1]: mysqld.service: control process exited, code=exited status=1
Apr 20 18:26:57 spark01 systemd[1]: Failed to start SYSV: MySQL database server..
Apr 20 18:26:57 spark01 systemd[1]: Unit mysqld.service entered failed state.
Apr 20 18:26:57 spark01 systemd[1]: mysqld.service failed.
[root@spark01 ~]# journalctl -xe
--
-- Unit session-2.scope has begun starting up.
Apr 20 18:26:48 spark01 sshd[2916]: pam_unix(sshd:session): session opened for user spark by (uid=0)
Apr 20 18:26:52 spark01 su[2944]: (to root) spark on pts/1
Apr 20 18:26:52 spark01 su[2944]: pam_unix(su-l:session): session opened for user root by spark(uid=1000)
Apr 20 18:26:56 spark01 polkitd[909]: Registered Authentication Agent for unix-process:2974:117137 (system bus name :1.25
Apr 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.
Apr 20 18:26:57 spark01 mysqld[2979]: MySQL Daemon failed to start.
Apr 20 18:26:57 spark01 mysqld[2979]: Starting mysqld: [FAILED]
Apr 20 18:26:57 spark01 systemd[1]: mysqld.service: control process exited, code=exited status=1
Apr 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.
Apr 20 18:26:57 spark01 systemd[1]: Unit mysqld.service entered failed state.
Apr 20 18:26:57 spark01 systemd[1]: mysqld.service failed.
Apr 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：
2017-04-20T10:00:19.935771Z 0 [ERROR] /usr/sbin/mysqld: Can't create/write to file '/var/run/mysqld/mysqld.pid' (Errcode: 2 - No such file or directory)
2017-04-20T10:00:19.935795Z 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：
2017-04-20T10:28:37.183387Z 0 [ERROR] /usr/sbin/mysqld: Can't create/write to file '/var/run/mysqld/mysqld.pid' (Errcode: 13 - Permission denied)
2017-04-20T10:28:37.183431Z 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 Apr 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。