MongoDB Ops Manager 3.4.0 (released November, 2016) includes monitoring for hardware metrics (cpu, disk usage, and I/O utilization) via the Automation agent. For more information, see: [System and Disk Alerts](https://docs.opsmanager.mongodb.com/current/reference/alerts/#system-and-disk-alerts).

Prior versions of Ops Manager did not include hardware monitoring and required the use of an external tool (`munin-node`) for CPU & disk metrics. If you are still using an older version of Ops Manager and have installed [Munin-node](http://munin-monitoring.org/) for [monitoring CPU & I/O stats](https://docs.opsmanager.mongodb.com/current/tutorial/configure-monitoring-munin-node/) you could look into using the Munin `df` plugin to track free disk space. Munin supports [sending alerts](http://munin.readthedocs.org/en/latest/tutorial/alert.html) via email as well as through Nagios and external scripts.