A common issue when using MMAPv1 (less frequent with WiredTiger) is free disk space. Like the memory, we need to track the disk space usage and be proactive, rather than reactive, with it. Keep monitoring the disk space usage, with proper alerts when it reaches 40%, 60%, or 80% of the disk space, especially for datasets that grow quickly.
Disk space issues are often the ones that cause the most headaches for administrators, DevOps, and developers, because of the time it takes to move data around.
The directoryperdb option can help with data sizing, as we can split our storage into different physically mounted disks.