By far, the most important metric to monitor in MongoDB is memory usage. MongoDB (and every database system, for what it's worth) uses system memory extensively to increase performance. No matter whether we use MMAPv1 or WiredTiger storage engines, the memory that's used is the first thing that we should keep our eyes on.

Understanding how computer memory works can help us to evaluate metrics from our monitoring system. These are the most important concepts related to computer memory.