{
  "slug": "defect-skipping",
  "term": "Disk defect skipping",
  "aliases": [
    "IBM 3340 defect skipping"
  ],
  "definition": "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.",
  "note": "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.",
  "status": "unknown",
  "advantages": [
    {
      "text": "The controller attachment microprogram hid the gap handling from system and user software.",
      "citationIds": [
        "mulvany1974"
      ]
    }
  ],
  "limitations": [
    {
      "text": "Factory-known defect avoidance did not eliminate ECC or provide an unlimited mechanism for newly developing defects.",
      "citationIds": [
        "mulvany1974"
      ]
    }
  ],
  "related": [
    "winchester-architecture",
    "bad-block-management",
    "count-key-data"
  ],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "updated": "2026-10-11",
  "citations": [
    {
      "id": "mulvany1974",
      "title": "R. B. Mulvany, Engineering Design of a Disk Storage Facility with Data Modules, IBM J. Res. Dev. 18(6), 489-505 (1974); pp. 489-494, 502-504",
      "url": "https://www.mirrorservice.org/sites/www.bitsavers.org/pdf/ibm/IBM_Journal_of_Research_and_Development/186/ibmrd1806D.pdf",
      "accessed": "2026-10-11"
    }
  ]
}