{
  "slug": "armorcache",
  "term": "ArmorCache",
  "aliases": [],
  "definition": "ArmorCache is an HDD cache-protection feature that uses OptiNAND flash to preserve buffered user writes during emergency power loss on supported drives.",
  "note": "Spindle rotational energy powers transfer of buffered writes into iNAND after external power fails, in WCE and WCD modes. Host queue depth 1 can coexist with a larger internal backlog after command completions. [wd-armor]\n\nA SAS drive specification permits SYNCHRONIZE CACHE completion after persistence in iNAND when write caching and protection are enabled; that need not mean a platter commit. [hc570-sas] Device protection is not a transaction protocol and cannot preserve data still in upstream caches. [linux-cache]",
  "status": "shipping",
  "advantages": [],
  "limitations": [],
  "related": [
    "optinand",
    "power-loss-protection",
    "write-back-cache",
    "cache-flush",
    "force-unit-access"
  ],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "citations": [
    {
      "id": "wd-armor",
      "title": "Western Digital. ArmorCache Write Cache Data Protection, May 2022, pp. 1-3; host and internal queue depth on p. 3.",
      "url": "https://documents.westerndigital.com/content/dam/doc-library/en_us/assets/public/western-digital/collateral/tech-brief/tech-brief-write-cache-data-safety.pdf",
      "accessed": "2026-10-16"
    },
    {
      "id": "hc570-sas",
      "title": "Western Digital. Ultrastar DC HC570 SAS specification, revision 1.2, 21 September 2023, section 6.5.2, p. 36.",
      "url": "https://documents.westerndigital.com/content/dam/doc-library/en_us/assets/public/western-digital/product/data-center-drives/ultrastar-dc-hc500-series/product-manual-ultrastar-dc-hc570-sas-oem-spec.pdf",
      "accessed": "2026-10-16"
    },
    {
      "id": "linux-cache",
      "title": "Linux kernel documentation. Explicit volatile write back cache control: REQ_PREFLUSH, REQ_FUA and driver responsibilities.",
      "url": "https://docs.kernel.org/block/writeback_cache_control.html",
      "accessed": "2026-10-16"
    },
    {
      "id": "hesela-boundary",
      "title": "Hesela. OptiNAND metadata is not the ArmorCache guarantee: model and system boundaries.",
      "url": "https://hesela.com/analyses/hdd-adjacent-track-interference-refresh/#optinand-armorcache",
      "accessed": "2026-10-16"
    }
  ],
  "updated": "2026-10-16",
  "provenance": [
    "ArmorCache is a Western Digital feature; support and performance depend on model, firmware, configuration and workload.",
    "The HC570 SAS revision 1.2 example specifies up to 128 MiB of protected user data. It is a model-scoped protection capacity, not total DRAM or a transaction limit. [hc570-sas]"
  ]
}