{
  "slug": "backpointer-based-consistency",
  "term": "Backpointer-based consistency",
  "aliases": [
    "NoFS consistency"
  ],
  "definition": "Backpointer-based consistency checks agreement between a filesystem reference and ownership information embedded in its target.",
  "note": "NoFS data-block backpointers encode owner inode number and logical block offset; inode backlinks identify parent directories. Allocation information is reconstructed online rather than trusted from persistent bitmaps. [nofs2012]\n\nFollowing a crash, pathname lookup may reject an entry still visible in a directory listing. Orphan recovery is possible when its data blocks validate, but neither atomic transaction recovery nor restoration of overwritten contents is guaranteed. [nofs2012]\n\nIn the prototype, mounting does not finish verification. Calling stat() may inspect all of a file's blocks, the first overwrite of an unverified block requires an ownership read, and allocation can wait for a scan to identify usable free space. [nofs2012]",
  "status": "research",
  "advantages": [
    {
      "text": "Ownership checks can expose stale references to reallocated blocks.",
      "citationIds": [
        "nofs2012"
      ]
    }
  ],
  "limitations": [
    {
      "text": "The design assumes atomic block-plus-backpointer writes. Matching identity permits stale versions; detection does not guarantee restoration of lost or overwritten contents.",
      "citationIds": [
        "nofs2012"
      ]
    }
  ],
  "related": [
    "soft-updates",
    "metadata-update-dependency",
    "optimistic-crash-consistency",
    "crash-consistency"
  ],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "citations": [
    {
      "id": "nofs2012",
      "title": "Chidambaram et al. Consistency Without Ordering. FAST 2012, sections 2.1, 3.2-3.3 and 4.2-4.4 (allocation reconstruction: 4.3).",
      "url": "https://research.cs.wisc.edu/adsl/Publications/nofs-fast12.pdf",
      "accessed": "2026-11-13"
    }
  ],
  "updated": "2026-11-13",
  "provenance": [
    "Historical research design; source review, not a Hesela implementation or hardware test."
  ]
}