I'm about to adopt MongoDB for a new project and I've chosen it for flexibility, not scalability so will be running it on one machine. From the documentation and web posts I keep reading that all indexes are in RAM. This just isn't making sense to me as my indexes will easily be larger than the amount of available RAM.

Can anyone share some insight on the index/RAM relationship and what happens when both an individual index and all of my indexes exceed the size of available RAM?