Take Squid for instance, a 1975 program if I ever saw one: You tell it how much RAM it can use and how much disk it can use. It will then spend inordinate amounts of time keeping track of what HTTP objects are in RAM and which are on disk and it will move them forth and back depending on traffic patterns. Well, today computers really only have one kind of storage, and it is usually some sort of disk, the operating system and the virtual memory management hardware has converted the RAM to a cache for the disk storage.
You should read the whole thing, it's a nice summary of technological changes that still haven't made it into everyone's head. I feel guilty myself: I recently taught our OS course, but I don't think I ever made this sufficiently clear when we talked about virtual memory. I'll try to add a relevant assignment to the Unix course next semester... :-D