And when people say "won't fit in anything else", do explore your options before! RAM storage can be very, very, very big and the cost for using it is minuscule compared to what it used to be!
On that note, there is this great website for seeing if there are servers that can handle fitting all your data in RAM: https://yourdatafitsinram.net/
Systems like Power System E980 can handle up to 64TB RAM, which is a lot of data. Just like parent said here, do try to fit things on machines like this before even trying out large-scale storage clusters, because they are a hassle to deal with and generally not worth the cost.
Edit: For the Cloud-hosting crowd out there, I took a quick look at https://instances.vantage.sh/ and sorted it by memory. Largest instances you can get at AWS is "U-24TB1 Metal", which comes with 24TiB of RAM (and 448 vCPU so the computation itself gets as fast as possible too [granted you can parallelize it]), which should fit most cases of "big data" I've seen in the wild today. Unsure about running costs though, but I'm 90% sure it's cheaper to have that instance for a couple of hours than the cost for having to deal with storage clusters, which are also slower.