Last week we talked about the memory hierarchy — the stack from CPU registers down to spinning metal, and the enormous latency gaps between each layer. This week we go one level deeper into the bottom two layers of that stack: hard disk drives and solid-state drives. Not because database engineers need to know how to manufacture storage hardware, but because the physical geometry of these devices creates constraints that travel all the way up through the OS, through the database engine, and into every schema and index design decision you’ll ever make.