Mongo Memory management I had a few questions 


If there is sufficient RAM, does Mongo load entire collection into memory?
Suppose I had two collections A &amp; B and I know that there is not enough RAM to hold both - and if 90% of my queries are to collection A - is there a way to configure Mongo to prefer to keep A in RAM as much as possible?
No, MongoDB don't "read whole collection to memory" (automatically).
If you don't read/write collection B, then yes, MongoDB keeps collection A in the memory. 


So, basically MongoDB keeps (as long as there is enough memory to keep) all read and write data (indexes, documents) in the memory. Of course, you can "warm up" collection, by reading all documents. When memory is "full", the oldest documents are thrown away when newer data comes in.