今天，啟動MySQL服務器失敗，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]
根據提示，分別使用systemctl status mysqld.service和journalctl -xe查看服務啟動失敗的原因
[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，可惜的時，這些信息並不能提供服務啟動失敗的真正原因。
這時候，不妨打開MySQL的告警日志，畢竟，只要MySQL服務啟動，告警日志都會有輸出信息的，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服務在啟動的時候，不能創建pid文件。
在終端看一下該目錄是否存在，果然，不存在。
於是，創建了/var/run/mysqld/目錄，重啟MySQL服務
[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]
依舊報錯，重新查看告警日志，有以下輸出
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
原來，/var/run/mysqld/的屬主和屬組還是root，mysql並不能在其中創建文件，後修改該目錄的屬主和屬組，啟動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 ]
總結：
以前在玩Kubernetes的時候，常遇到啟動失敗的情況，根據systemctl的提示，通過systemctl status mysqld.service和journalctl -xe命令查看服務啟動失敗的原因往往並不如人意，反而給了一種錯誤的暗示，以為這個跟系統有關。其實，通過查看服務的日志，往往更能清晰的知道服務啟動失敗的原因。
