Doubt in disk quota


Hi. I have a small doubt. 37signals, a company where the creator of
Rails works has a project management application called basecamp. It is
subscription based and its plans are based on the number of users and
disk space allotted for each plan. My doubt is how they allocate the
disk space instantly for the plan we select. Do they do with Rails or
with some other method? Can anyone explain about this? Thanks