hesela.devDisk defect skipping
Disk defect skipping formats a record around a known defective medium region so that normal reads bypass a gap instead of storing customer data there.
In the IBM 3340 design, factory tests located defects and the track home address recorded their displacement. Formatting either split a record field or enlarged an existing gap. This is distinct from correcting corrupted bits or relocating a logical sector to spare storage. Mulvany (1974, pp. 502-504) reports 128 bytes reserved on each 3348 track and a maximum allowed defect length of 16 bytes. Formatting the affected record required an extra revolution; subsequent normal reads and update writes did not incur that formatting step. These are constraints of that design, not specifications for current HDDs or a field failure-rate measurement.
Relations
- related
- winchester-architecture, bad-block-management, count-key-data
Advantages
- The controller attachment microprogram hid the gap handling from system and user software. [1]
Limitations
- Factory-known defect avoidance did not eliminate ECC or provide an unlimited mechanism for newly developing defects. [1]