When your MongoDB becomes unresponsive, it’s imperative that you can quickly identify the cause.
Although there can be many reasons for unresponsiveness, we sometimes find that particularly long-running and/or blocking operations (either initiated by a human or an application) are the culprit. Some examples of common operations that can bog down the database are:
operations on unindexed fields
expensive map-reduce jobs
One way to quickly see if one or more operations are particularly long-running is to use db.currentOp().