在CentOs7x64下，安装mongodb3.2.6版本，无法启动成功。

报以下错误

# systemctl status mongod.service
● mongod.service - Mongodb
   Loaded: loaded (/usr/lib/systemd/system/mongod.service; enabled; vendor preset: disabled)
   Active: failed (Result: exit-code) since 六 2016-05-21 18:39:18 CST; 27s ago
  Process: 3813 ExecStart=/bin/su - mongodb -c /www/server/mongodb/bin/mongod --config /www/server/mongodb/mongodb.conf (code=exited, status=1/FAILURE)
  Process: 3812 ExecStartPre=/bin/rm -rf /www/data/mongodb/mongod.lock (code=exited, status=0/SUCCESS)
 Main PID: 3510 (code=exited, status=1/FAILURE)

systemd[1]: Starting Mongodb...
su[3813]: (to mongodb) root on none
systemd[1]: mongod.service: control process exited, code=exited status=1
systemd[1]: Failed to start Mongodb.
systemd[1]: Unit mongod.service entered failed state.
systemd[1]: mongod.service failed.

-------------------------------
# vi /etc/systemd/system/multi-user.target.wants/mongod.service 

[Unit]
Description=Mongodb
After=network.target remote-fs.target nss-lookup.target

[Service]
Type=forking
PIDFile=/www/server/mongodb/mongodb.pid
ExecStartPre=/bin/rm -rf /www/data/mongodb/mongod.lock
ExecStart=/bin/su - mongodb -c "/www/server/mongodb/bin/mongod --config /www/server/mongodb/mongodb.conf"
ExecStop=/bin/kill -2 $MAINPID
KillMode=process
KillSignal=SIGQUIT
TimeoutStopSec=5
PrivateTmp=true

[Install]
WantedBy=multi-user.target