The version I've heard is that small data fits on an average developer workstation, medium data fits on a commodity 2U server, and "big data" needs a bigger footprint than that single commodity server offers.
I like that better than bringing racks into it, because once you have multiple machines in a rack you've got distributed systems problems, and there's a significant overlap between "big data" and the problems that a distributed system introduces.
I like that better than bringing racks into it, because once you have multiple machines in a rack you've got distributed systems problems, and there's a significant overlap between "big data" and the problems that a distributed system introduces.