{
  "slug": "sis-link",
  "term": "SIS link",
  "aliases": [
    "Single Instance Storage link"
  ],
  "definition": "A SIS link is a sparse NTFS file with a reparse point through which Single Instance Storage presents private file contents backed by shared common-store data.",
  "note": "The reparse payload is opaque to backup software and is handled through SIS APIs. A link is not a hard link: its user-visible file can diverge while shared backing data remain unchanged. [sis-links]\n\nDuring restore, SisRestoredLink requires unallocated ranges to remain unallocated, not filled with zeroes. It returns each needed backing file at most once; an empty result can mean already reported, not already restored. [sis-restore-link]\n\nAfter restoring a requested backing file, call SisRestoredCommonStoreFile to let SIS repair its internal structures and links. Track requested dependencies through completion rather than treating discovery as a data-transfer receipt. [sis-restore-store]",
  "status": "unknown",
  "advantages": [
    {
      "text": "Repeated links can share backing data, reducing duplicate backup transfers.",
      "citationIds": [
        "sis-backup-link"
      ]
    }
  ],
  "limitations": [
    {
      "text": "Restoring the link name and reparse metadata alone does not supply missing common-store contents.",
      "citationIds": [
        "sis-links",
        "sis-restore-link"
      ]
    },
    {
      "text": "The 2000 paper's deferred copy-on-close and the later overview's pre-write copying are distinct descriptions; neither establishes current Windows support.",
      "citationIds": [
        "sis2000",
        "sis-store"
      ]
    }
  ],
  "related": [
    "copy-on-close",
    "sparse-file",
    "deduplication"
  ],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "updated": "2026-11-05",
  "citations": [
    {
      "id": "sis-links",
      "title": "Microsoft: SIS Links and Reparse Points, sparse representation and opaque metadata",
      "url": "https://learn.microsoft.com/en-us/windows/win32/backup/sis-links-and-reparse-points",
      "accessed": "2026-11-05"
    },
    {
      "id": "sis-restore-link",
      "title": "Microsoft: SisRestoredLink, sparse ranges and once-per-operation dependency reporting",
      "url": "https://learn.microsoft.com/en-us/windows/win32/backup/sisrestoredlink",
      "accessed": "2026-11-05"
    },
    {
      "id": "sis-restore-store",
      "title": "Microsoft: SisRestoredCommonStoreFile, reporting restored backing files",
      "url": "https://learn.microsoft.com/en-us/windows/win32/backup/sisrestoredcommonstorefile",
      "accessed": "2026-11-05"
    },
    {
      "id": "sis-backup-link",
      "title": "Microsoft: SisCSFilesToBackupForLink, shared-file discovery",
      "url": "https://learn.microsoft.com/en-us/windows/win32/backup/siscsfilestobackupforlink",
      "accessed": "2026-11-05"
    },
    {
      "id": "sis-store",
      "title": "Microsoft: The SIS Common Store and Common-Store Files",
      "url": "https://learn.microsoft.com/en-us/windows/win32/backup/the-sis-common-store-and-common-store-files",
      "accessed": "2026-11-05"
    },
    {
      "id": "sis2000",
      "title": "Bolosky, Corbin, Goebel and Douceur (2000). Single Instance Storage in Windows 2000, USENIX Windows Systems Symposium, pp. 13-24, sections 2.3.3-2.3.5.",
      "url": "https://www.usenix.org/legacy/publications/library/proceedings/usenix-win2000/full_papers/bolosky/bolosky_html/index.html",
      "accessed": "2026-11-05"
    },
    {
      "id": "hesela-clones",
      "title": "Hesela: SIS restore and backing-file dependencies",
      "url": "https://hesela.com/analyses/file-clones-deferred-copy-cost/#sis-links",
      "accessed": "2026-11-05"
    }
  ],
  "provenance": [
    "Historical source synthesis of the Windows 2000 SIS paper and Microsoft backup API documentation, reviewed 2026-11-05. No Windows restore, mapped-I/O or failure-injection experiment was performed."
  ]
}