πfs (or PiFS) is the "content-free file system". On it the data is actually not stored anywhere on the drive, but rather the FS stores the offset in π constant.
The argument is that π is thought to be transcedental, and it is hoped that at least in theory every single pattern of digits can be found somewhere in it. In reality it may take too long to find the necessary pattern as the whole, so data is split into chunks and those chunks are then found in π.
The reading is done via Bailey–Borwein–Plouffe formula that is able to produce any digit of π in base 16 if the offset is known without having to produce any previous digits.