Our server does not show right value on the "resident" field in the "mem" object of the server status, since the beginning. We started using MongoDB with 2.0. We use only one database and one collections. Is is about 120 GB now.

I whink it is wrong because of several reasons. First of all MongoDB cannot respond some queries in out peak times. It gets 30 - 50 queries per seconds and shows 200 - 500 page faults per seconds in the peak times. We solved that problem two times by increasing the memory from 16 GB to 32 GB and from 32 GB to 64 GB. Now, It is happening again. In all of these cases server status was showing less than half of the available memory as resident.

My second experiment is monitoring caches from the operating system. I try to drop caches, restart the MongoDB server and it gets full again in 30 - 60 seconds but the server status does not show it. For example after 30 minutes after dropping caches, Linux free command shows 62 GB as caches but MongoDB server status show only 7 GB.

My third experiment is with the tool named Mongomem from ContextLogic, that I found lately. I think it shows the right value which is nearly all of the memory.

We have tested it with different servers and with different configurations but the situation did not changed. I have tried to "compact" the collections which we have not done before. Server status started show even less resident memory after that.

It is impossible that we are reading from 1 / 10 of the whole database. So we did not want to believe that MongoDB was using so much memory, for a long time.

https://github.com/ContextLogic/mongotools