But Redis should have at least as much RAM as you have data and will only use the data from RAM. You cannot swap or store it on a slower disk, while Mongo can use a disk file.
Redis can backup or journal to disk, but this is just so the data is persistent between restarts, not used in queries or runtime.
* Updated based on comments: I originally wrote that redis requires as much ram as data (technically more) but I'm correcting this to state that Redis CAN use disk, but Salvatore, the man behind redis, publicly states that he does not recommend it as a solution.
