The Top 5 Metrics to Watch in MongoDB | MongoDB Blog` Monitoring is essential for assuring performance and reliability in MongoDB installations, regardless of their size. The MongoDB Management Service (MMS) provides a wide range of metrics, but the most valuable ones are lock percentage, replication lag, journal writes, page faults, and non-mapped virtual memory. A high lock percentage can impact reads, while high replication lag can lead to using stale data and decreased performance in case of primary node failure. Journal writes that overwhelm the journal file can also have a negative impact on performance. Page faults indicate an issue with the working set fitting in memory, and non-mapped virtual memory growth often points to a memory leak that needs to be addressed proactively. These metrics are crucial for maintaining a healthy MongoDB environment and should be closely monitored.