- Secondary storage device of choice.
- Main advantage over tapes: random access vs. sequential.
- Data is stored and retrieved in units called disk blocks or pages.
- Unlike RAM, time to retrieve a disk page varies depending upon location on disk.
- Therefore, relative placement of pages on disk has major impact on DBMS performance!
Components of a Disk
- The platters spin (say, 90rps).
- The arm assembly is moved in or out to position a head on a desired track. Tracks under heads make a cylinder (imaginary!).
- Only one head reads/writes at any one time.
- Block size is a multiple of sector size (which is fixed).