{
  "slug": "copy-on-close",
  "term": "Copy-on-close",
  "aliases": [],
  "definition": "Copy-on-close defers filling an edited shared file's unchanged regions from shared storage until all users have closed it.",
  "note": "Windows 2000 Single Instance Storage (SIS) keeps edits private while unchanged regions use its common store. After users close a modified file, a worker copies the remaining clean regions. Closing therefore does not certify completed materialization. If allocation runs out, the unfinished file retains its reparse point and common-store dependency. [sis2000]\n\nFAST 2020 contrasts this whole-file unsharing policy with copy-on-abundant-write, which batches changes before copying shared nodes. The two policies have different triggers. [fast2020]",
  "status": "unknown",
  "advantages": [
    {
      "text": "A complete overwrite avoids copying the old contents during materialization.",
      "citationIds": [
        "sis2000"
      ]
    }
  ],
  "limitations": [
    {
      "text": "A small edit can leave most of the file to be copied; background work must be included when assessing lifecycle cost.",
      "citationIds": [
        "sis2000",
        "fast2020"
      ]
    },
    {
      "text": "This is a historical SIS mechanism, not a claim about current Windows support or generic close-call durability.",
      "citationIds": [
        "sis2000"
      ]
    }
  ],
  "related": [
    "copy-on-write",
    "copy-on-abundant-write",
    "reflink",
    "sis-link"
  ],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "updated": "2026-11-05",
  "citations": [
    {
      "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": "fast2020",
      "title": "Zhan et al. (2020). How to Copy Files, FAST 2020, section 6: file or directory clones.",
      "url": "https://www.usenix.org/system/files/fast20-zhan.pdf",
      "accessed": "2026-11-05"
    },
    {
      "id": "hesela-clones",
      "title": "Hesela: Fast file clones: where the copying cost goes",
      "url": "https://hesela.com/analyses/file-clones-deferred-copy-cost/#copy-on-close",
      "accessed": "2026-11-05"
    }
  ],
  "provenance": [
    "Historical source review of the SIS paper, especially sections 2.3.3-2.3.5, cross-checked against FAST 2020 section 6. No SIS workload or storage-failure experiment was run; current platform support was not assessed."
  ]
}