Monitor disk usage with MongoDB Ops Manager
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.

Prior versions of Ops Manager did not include hardware monitoring and required the use of an external tool (munin-node) for CPU &amp; disk metrics. If you are still using an older version of Ops Manager and have installed Munin-node for monitoring CPU &amp; I/O stats you could look into using the Munin df plugin to track free disk space. Munin supports sending alerts via email as well as through Nagios and external scripts.