Now I want to start mysql use `service mysqld start` on centos7,but I got the follow error:  

OS: centos64 bit

```
Redirecting to /bin/systemctl start mysqld.service Job for mysqld.service failed.   
See 'systemctl status mysqld.service andjournalctl -xn for details.

```

the follow had been installed without any error and warning:

```
mysql-community-bench-5.7.5-0.6.m15.el7.x86_64.rpm
mysql-community-client-5.7.5-0.6.m15.el7.x86_64.rpm
mysql-community-common-5.7.5-0.6.m15.el7.x86_64.rpm
mysql-community-devel-5.7.5-0.6.m15.el7.x86_64.rpm
mysql-community-embedded-5.7.5-0.6.m15.el7.x86_64.rpm
mysql-community-embedded-devel-5.7.5-0.6.m15.el7.x86_64.rpm
mysql-community-libs-5.7.5-0.6.m15.el7.x86_64.rpm
mysql-community-release-el7-5.noarch.rpm
mysql-community-server-5.7.5-0.6.m15.el7.x86_64.rpm
mysql-community-test-5.7.5-0.6.m15.el7.x86_64.rpm

```

systemctl -l status mysqld.service

```
[root@www mysql]# systemctl -l status mysqld.service
mysqld.service - MySQL Community Server
   Loaded: loaded (/usr/lib/systemd/system/mysqld.service; disabled)
   Active: activating (start-post) since Sun 2015-02-15 03:25:32 EST; 59s ago
  Process: 29844 ExecStart=/usr/bin/mysqld_safe (code=exited, status=0/SUCCESS)
  Process: 29833 ExecStartPre=/usr/bin/mysql-systemd-start pre (code=exited, status=0/SUCCESS)
 Main PID: 29844 (code=exited, status=0/SUCCESS);         : 29845 (mysql-systemd-s)
   CGroup: /system.slice/mysqld.service
           `-control
             |-29845 /bin/bash /usr/bin/mysql-systemd-start post
             `-30168 sleep 1

Feb 15 03:25:32 www mysqld_safe[29844]: 150215 03:25:32 mysqld_safe Logging to '/var/log/mysqld.log'.
Feb 15 03:25:32 www mysqld_safe[29844]: 150215 03:25:32 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
Feb 15 03:25:32 www mysqld_safe[29844]: /usr/bin/mysqld_safe: line 164: 29987 Killed                  nohup /usr/sbin/mysqld --basedir=/usr --datadir=/var/lib/mysql --plugin-dir=/usr/lib64/mysql/plugin --log-error=/var/log/mysqld.log --pid-file=/var/run/mysqld/mysqld.pid --socket=/var/lib/mysql/mysql.sock < /dev/null >> /var/log/mysqld.log 2>&1
Feb 15 03:25:32 www mysqld_safe[29844]: 150215 03:25:32 mysqld_safe mysqld from pid file /var/run/mysqld/mysqld.pid ended

```

journalctl -xn

```
[root@www mysql]# journalctl -xn
-- Logs begin at Sat 2015-02-14 22:04:13 EST, end at Sun 2015-02-15 03:46:01 EST. --
Feb 15 03:45:32 www systemd[1]: mysqld.service holdoff time over, scheduling restart.
Feb 15 03:45:32 www systemd[1]: Stopping MySQL Community Server...
-- Subject: Unit mysqld.service has begun shutting down
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit mysqld.service has begun shutting down.
Feb 15 03:45:32 www systemd[1]: Starting MySQL Community Server...
-- Subject: Unit mysqld.service has begun with start-up
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit mysqld.service has begun starting up.
Feb 15 03:45:32 www mysqld_safe[1286]: 150215 03:45:32 mysqld_safe Logging to '/var/log/mysqld.log'.
Feb 15 03:45:32 www mysqld_safe[1286]: 150215 03:45:32 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
Feb 15 03:45:33 www mysqld_safe[1286]: /usr/bin/mysqld_safe: line 164:  1429 Killed                  nohup /usr/sbin/mysqld --basedir=/usr --datadir=/var/lib/mysql --plugin-dir=/usr/lib64/mysql/plugin --log-error=/var/log/mysqld.log --pid-file=/var/run/mysqld/mysqld.pid --socket=/var/lib/mysql/mysql.sock < /dev/nu
Feb 15 03:45:33 www mysqld_safe[1286]: 150215 03:45:33 mysqld_safe mysqld from pid file /var/run/mysqld/mysqld.pid ended
Feb 15 03:46:01 www systemd[1]: Starting Session 2660120 of user root.
-- Subject: Unit session-2660120.scope has begun with start-up
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit session-2660120.scope has begun starting up.
Feb 15 03:46:01 www systemd[1]: Started Session 2660120 of user root.
-- Subject: Unit session-2660120.scope has finished start-up
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit session-2660120.scope has finished starting up.
-- 
-- The start-up result is done.
Feb 15 03:46:01 www CROND[1522]: (root) CMD (run-parts /etc/cron.hourly)

```

cat /var/log/mysqld.log

```
[root@www mysql]# cat /var/log/mysqld.log
150213 03:00:56 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
150213 03:00:57 mysqld_safe mysqld from pid file /var/run/mysqld/mysqld.pid ended
150213 03:10:55 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
150213 03:10:56 mysqld_safe mysqld from pid file /var/run/mysqld/mysqld.pid ended
150213 03:20:55 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
150213 03:20:55 mysqld_safe mysqld from pid file /var/run/mysqld/mysqld.pid ended
150213 03:30:55 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
150213 03:30:55 mysqld_safe mysqld from pid file /var/run/mysqld/mysqld.pid ended

```

Whats wrong?