{
  "slug": "reconstruct-write",
  "term": "Reconstruct-write",
  "aliases": [
    "reconstruction write"
  ],
  "definition": "A parity-array update method that computes new parity from incoming replacement data and unchanged data from the same stripe instead of updating old parity from old overwritten data.",
  "note": "In the healthy-array accounting model, replacing u complete equal-size data blocks among k data blocks with p parity blocks requires k-u old-data reads and u+p writes. The name describes parity regeneration during an update, not a failed-drive rebuild. Cached data, partial blocks, missing members and implementation policy change the required device work.",
  "advantages": [
    "Avoids reading the overwritten data and old parity",
    "Can require fewer prereads as more of a stripe is replaced"
  ],
  "limitations": [
    "Reading the unchanged data can cost more than RMW for small updates in wider stripes",
    "Fewer block transfers do not necessarily mean lower latency",
    "This definition does not specify a degraded-array update algorithm"
  ],
  "related": [
    "read-modify-write",
    "full-stripe-write",
    "parity-raid",
    "raid-5",
    "raid-6"
  ],
  "status": "shipping",
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "updated": "2026-10-11",
  "citations": [
    {
      "id": "williams2006",
      "title": "Dan J. Williams. MD RAID Acceleration: Support for Asynchronous DMA/XOR Engines. Linux Symposium 2006, Volume Two, printed pp. 409-414; sections 3-5. The PDF filename uses different pagination.",
      "url": "https://www.kernel.org/doc/ols/2006/ols2006v2-pages-417-422.pdf",
      "accessed": "2026-10-11"
    },
    {
      "id": "chen1994",
      "title": "Peter M. Chen, Edward K. Lee, Garth A. Gibson, Randy H. Katz and David A. Patterson. RAID: High-Performance, Reliable Secondary Storage. ACM Computing Surveys 26(2), 145-185 (1994). Author manuscript: sections 3.2.5-3.3.2, Table 3 and 4.1.",
      "url": "https://pdl.cmu.edu/PDL-FTP/RAID/computin.pdf",
      "accessed": "2026-10-11"
    },
    {
      "id": "hesela-cost",
      "title": "Hesela: RAID write penalty is not a speed ratio; model and limitations",
      "url": "https://hesela.com/analyses/raid-write-penalty-io-not-speed/",
      "accessed": "2026-10-11"
    }
  ]
}