In-Memory Databases

How expensive would it be to have all of your production data in an in-memory database as opposed to slow-ass SSD or HDD ones? Or can you do some sort of caching where it puts certain parts in RAM but not all of it? How would that work?

I only have basic experience working with SQLite and MongoDB, but the ideas of super-fast IMDBs is appealing. I've vaguely heard of redis, but I don't know how you'd use it.

>>14
That's the opposite of what I mean.
You're talking about turning RAM into a hard drive.

I mean a PCIe card that lets you install additional RAM in your computer (more than what fits in the DIMM slots). Then you can use it as a RAM drive.

Let's say your motherboard only supports 64GB of RAM, but you actually want way more. But you don't want to buy a super expensive enterprise server board that will cost thousands of bucks. But your consumer grade (cheap/affordable) motherboard has some unused expansion slots. So you get some more RAM and some RAM drive cards to put in the expansion slots. At least that's how it's supposed to be, anyway. The last time I heard of a RAM drive was years ago, for like DDR2 or DDR3 or something. Really old stuff.