重啟前好好的系統；重啟後，web頁面打不開。查tomcat日誌發現償試連線資料庫3次失敗。登入伺服器，shell命令也不能正常連線。檢視系統程序沒有mysql。
啟動mysql服務報錯
[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]
[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:
-- -- 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:
-- -- 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
[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
建立目錄/var/run/mysqld，並授權，再次啟動mysql成功
drwxr-xr-x. 2 root root 40 jul 6 13:54 mysqld
drwxr-xr-x. 2 mysql mysql 40 jul 6 13:54 mysqld
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