{
  "slug": "tail-hour",
  "term": "RAID tail hour",
  "aliases": [
    "tail hour"
  ],
  "definition": "In The Tail at Store, a RAID tail hour is a group-hour in which at least one data drive has an hourly average I/O latency at least twice its group's median.",
  "note": "The event is defined at hourly resolution and does not measure the fraction of individual slow requests or establish simultaneous stalls. A relative baseline can miss a shared absolute slowdown. [hao] The sample boundary differs from Perseus's workload-dependent slowdown ratio.",
  "related": [
    "tail-latency",
    "slowdown-ratio",
    "fail-slow"
  ],
  "citations": [
    {
      "id": "hao",
      "title": "Hao et al., The Tail at Store, FAST 2016, sections 3.3 and 6",
      "url": "https://www.usenix.org/system/files/conference/fast16/fast16-papers-hao.pdf",
      "accessed": "2026-10-15"
    },
    {
      "id": "hesela-tail",
      "title": "Hesela: Slow RAID hours are not request tail latency; source audit and reproducible counterexamples",
      "url": "https://hesela.com/analyses/storage-tail-slowdowns-raid-fanout/",
      "accessed": "2026-10-15"
    }
  ],
  "status": "research",
  "advantages": [],
  "limitations": [],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "updated": "2026-10-15"
}