 <p>You have 17GB of RAM available for about 20GB of data plus 4GB of indexes (and that's just on this DB - I don't know if there is more in your system, but mongostat says you have total datafile size of about 28GB).</p> <p>If you are querying over your entire dataset, wouldn't you expect that eventually new data records being read can no longer fit into RAM without displaying some of the previously read (and loaded into RAM) records? That's what you're seeing with page faulting.</p> <p>It's not data storage that you are concerned about, it's data being used or queries or accessed. If it all can't stay in RAM then it will get swapped out and then will need to be swapped back in when accessed again.</p>