Examined the syslog after starting mongod with the --syslog option. That didn't work and noticed a bunch of other errors as well:
# tail -f /var/log/syslog
Jul 31 23:36:00 localhost systemd[5552]: Started Stop ureadahead data collection 45s after completed startup.
Jul 31 23:36:00 localhost systemd[5552]: Starting Update UTMP about System Runlevel Changes...
Jul 31 23:36:00 localhost systemd[5552]: Started Update UTMP about System Runlevel Changes.
Jul 31 23:36:00 localhost systemd[5552]: Startup finished in 28.347s.
Jul 31 23:49:34 localhost systemd[5552]: Starting Cleanup of Temporary Directories...
Jul 31 23:49:34 localhost systemd[20099]: Failed at step IOPRIO spawning /bin/systemd-tmpfiles: Function not implemented
Jul 31 23:49:34 localhost systemd[5552]: systemd-tmpfiles-clean.service: main process exited, code=exited, status=211/IOPRIO
Jul 31 23:49:34 localhost systemd[5552]: Failed to start Cleanup of Temporary Directories.
Jul 31 23:49:34 localhost systemd[5552]: Unit systemd-tmpfiles-clean.service entered failed state.
Jul 31 23:49:34 localhost systemd[5552]: systemd-tmpfiles-clean.service failed.
Former user commented on 2015-07-31T22:58:25.000-0400:
Sounds like it might be related to OS-4002.
Former user commented on 2015-08-04T11:41:47.000-0400:
illumos-joyent commit a6312f3 (branch master, by Jerry Jelinek)
OS-4002#icft=OS-4002 ionice requirements not met on lx ubuntu64 14.04.003
OS-4587#icft=OS-4587 lx: ubuntu 15.04 - Failed at step IOPRIO spawning /bin/systemd-tmpfiles: Function not implemented