On: MongoDB Performance for more data than memory

Right, makes sense and experimenting with different systems is the only sane way to make a solid idea about the different solutions... instead of asking on twitter what NoSQL database to use ;)

Btw it's worth noting that there are on-disk databases that will behave pathologically in scenarios where the available memory starts to get seriously smaller than dataset. For instance I saw instances of MySQL that once the data set size reached a given threshold the performance started degradating exponentially. This can probably be avoided.

Exactly, knowing that Mongo isn't one of these (or at least doesn't appear to be so far) is a comfort :)

I assure you MongoDB is just as subject to memory pressure as any other database.