disk geometry

Think of how you would describe a seat in a giant stack of round amphitheatres. You could say which building floor (which platter surface), which ring of seats counting out from the centre (which track), and which seat along that ring (which sector). Together those three numbers pin down exactly one place. Disk geometry is that addressing scheme for a hard drive: a way to name any tiny spot on the spinning platters by where it physically sits.

Concretely, the classic scheme is called CHS — cylinder, head, sector. The platters are stacked rings of tracks; the matching tracks at the same radius across all platter surfaces form a cylinder (so 'cylinder' picks how far out the arm is). The head number picks which platter surface (top or bottom of which platter). The sector number picks which arc along that track. So a triple like (cylinder 200, head 3, sector 17) names one exact block of data. The reason cylinder is the natural grouping is that all heads move together on one arm: every track of a cylinder is reachable without any new seek, so storing related data within a cylinder avoids the slow arm movement.

Why it matters historically: the OS once issued reads and writes in CHS terms and had to understand the disk's physical layout. But this exposed too much detail and broke down as drives got cleverer — outer tracks are longer and hold more sectors than inner ones (zoned recording), and drives began hiding bad sectors and remapping internally. So modern drives present a flat numbering called logical block addressing instead, and the OS no longer needs the true geometry. The geometry still exists physically and still explains performance (seek and rotation), even though software no longer addresses it directly.