We start by defining what the working set is. It is a subset of the total data frequently accessed by the application. In an application, which stores information over a period of time, the working set is mostly the recently accessed data. The word ''recently'' is subjective; for some it might be a day or two, for others it might be a couple of months. This is mostly something that needs to be thought of while designing the application and sizing the database. The working set is something that needs to be in the RAM of the database server to minimize the page faults and get the optimum performance.

In this recipe, we will see a way that gives the estimate of your working set and is a feature introduced in Mongo 2.4. The word ''estimator'' is slightly misleading, as the initial sizing still is a manual activity, and the system designers need to be judicious about the server configuration. The working set estimator utility we will see now is more of a reactive approach...