I've logged this under docs, but I think core is probably more appropriate:

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Hi,
I'm running mongo server on rhel7 and systemd: # rpm -q mongodb-org-server
mongodb-org-server-2.6.4-1.x86_64

I'm trying to run the daemon: # /etc/rc.d/init.d/mongod start
Starting mongod (via systemctl): Job for mongod.service failed. See 'systemctl status mongod.service' and 'journalctl -xn' for details.
[FAILED]

when I look at the log, I'm seeing:
2014-10-20T11:36:28.085+0530 ***** SERVER RESTARTED *****
2014-10-20T11:36:28.087+0530 ERROR: Cannot write pid file to /var/run/mongodb/mongod.pid: No such file or directory

The way around it is to create mongodb.conf in /lib/tmpfiles.d/ with following content:
d /var/run/mongodb 0755 mongod mongod

Please add /lib/tmpfiles.d/mongodb.conf to mongodb-org-server package

http://stackoverflow.com/questions/23086655/mongodb-service-will-not-start-after-initial-setup

Many thanks,
Tomas