重启前好好的系统；重启后，web页面打不开。查tomcat日志发现偿试连接数据库3次失败。登录服务器，shell命令也不能正常连接。查看系统进程没有mysql。

启动mysql服务报错

[root@zhangcool-cooler01 init.d]# systemctl start mysqld
Job for mysqld.service failed because the control process exited with error code. See "systemctl status mysqld.service" and "journalctl -xe" for details.

[root@zhangcool-cooler01 init.d]# service 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]

根据提示检查服务状态

[root@zhangcool-cooler01 init.d]# systemctl status mysqld.service
● mysqld.service - SYSV: MySQL database server.
   Loaded: loaded (/etc/rc.d/init.d/mysqld; bad; vendor preset: disabled)
   Active: failed (Result: exit-code) since Fri 2018-07-06 13:43:52 CST; 1min 2s ago
     Docs: man:systemd-sysv-generator(8)
  Process: 3886 ExecStart=/etc/rc.d/init.d/mysqld start (code=exited, status=1/FAILURE)

Jul 06 13:43:51 zhangcool-cooler01 systemd[1]: Starting SYSV: MySQL database server....
Jul 06 13:43:52 zhangcool-cooler01 mysqld[3886]: MySQL Daemon failed to start.
Jul 06 13:43:52 zhangcool-cooler01 mysqld[3886]: Starting mysqld:  [FAILED]
Jul 06 13:43:52 zhangcool-cooler01 systemd[1]: mysqld.service: control process exited, code=exited status=1
Jul 06 13:43:52 zhangcool-cooler01 systemd[1]: Failed to start SYSV: MySQL database server..
Jul 06 13:43:52 zhangcool-cooler01 systemd[1]: Unit mysqld.service entered failed state.
Jul 06 13:43:52 zhangcool-cooler01 systemd[1]: mysqld.service failed.

[root@zhangcool-cooler01 init.d]# journalctl -xe
Jul 06 13:42:58 zhangcool-cooler01 polkitd[477]: Unregistered Authentication Agent for unix-process:3597:8111728 (system bus name :1.355, object path
Jul 06 13:43:51 zhangcool-cooler01 polkitd[477]: Registered Authentication Agent for unix-process:3880:8117203 (system bus name :1.356 [/usr/bin/pktt
Jul 06 13:43:51 zhangcool-cooler01 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.
Jul 06 13:43:52 zhangcool-cooler01 mysqld[3886]: MySQL Daemon failed to start.
Jul 06 13:43:52 zhangcool-cooler01 mysqld[3886]: Starting mysqld:  [FAILED]
Jul 06 13:43:52 zhangcool-cooler01 systemd[1]: mysqld.service: control process exited, code=exited status=1
Jul 06 13:43:52 zhangcool-cooler01 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.
Jul 06 13:43:52 zhangcool-cooler01 systemd[1]: Unit mysqld.service entered failed state.
Jul 06 13:43:52 zhangcool-cooler01 systemd[1]: mysqld.service failed.
Jul 06 13:43:52 zhangcool-cooler01 polkitd[477]: Unregistered Authentication Agent for unix-process:3880:8117203 (system bus name :1.356, object path
 

查看mysql日志，分析mysql启动过程有何异常

[root@zhangcool-cooler01 init.d]# vi /var/log/mysqld.log
2018-07-06T05:43:52.466691Z 0 [Note] Server socket created on IP: '::'.
2018-07-06T05:43:52.469258Z 0 [ERROR] /usr/sbin/mysqld: Can't create/write to file '/var/run/mysqld/mysqld.pid' (Errcode: 2 - No such file or directory)
2018-07-06T05:43:52.469270Z 0 [ERROR] Can't start server: can't create PID file: No such file or directory

原来/var/run/mysqld目录不存在，所在mysqld.pid无法创建

创建目录/var/run/mysqld，并授权，再次启动mysql成功

[root@zhangcool-cooler01 webapps]# mkdir -p /var/run/mysqld
[root@zhangcool-cooler01 webapps]# ll /var/run/
drwxr-xr-x.  2 root root   40 Jul  6 13:54 mysqld
[root@zhangcool-cooler01 webapps]# chown -R mysql.mysql /var/run/mysqld
[root@zhangcool-cooler01 webapps]# ll /var/run/
drwxr-xr-x.  2 mysql mysql   40 Jul  6 13:54 mysqld