{
  "slug": "disk-head-time",
  "term": "Disk-head time",
  "aliases": [
    "HDD disk-head time"
  ],
  "definition": "Disk-head time is an estimate of HDD backend work combining per-fetch positioning cost with the transfer cost of bytes actually fetched.",
  "note": "The author explanation treats setup and transfer costs together; reducing request misses alone can spend more bandwidth. Demand hits and extra prefetched bytes must therefore be accounted for separately. [baleen-author]\n\nBaleen reports the maximum backend utilization over ten-minute windows. Its admission and prefetching policies optimize average disk-head time, not an explicit peak-admission schedule. A pooled average cannot recover a maximum without the window history. [baleen-author]\n\nA Hesela constructed ledger gives 90% demand hits with 150 ms of modeled work and 95% hits with 250 ms when expanded fetches transfer more bytes. This is exact hypothetical arithmetic, not a device measurement or a Baleen performance result. [hesela-ledger]",
  "related": [
    "baleen",
    "byte-hit-ratio",
    "request-hit-ratio",
    "prefetch-wastage"
  ],
  "status": "research",
  "advantages": [],
  "limitations": [],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "updated": "2026-11-14",
  "citations": [
    {
      "id": "baleen-author",
      "title": "Daniel Lin-Kit Wong. Baleen: ML admission & prefetching for flash caches. CMU CSD PhD Blog, 12 January 2024; design and optimization objective.",
      "url": "https://www.cs.cmu.edu/~csd-phd-blog/2023/baleen-ml-flash-caching/",
      "accessed": "2026-11-14"
    },
    {
      "id": "hesela-ledger",
      "title": "Hesela. Constructed disk-head-time ledgers: assumptions, integer arithmetic and window examples.",
      "url": "https://hesela.dev/research/disk-head-time/results.json",
      "accessed": "2026-11-14"
    }
  ]
}