{
  "slug": "io-request-size",
  "term": "I/O request size",
  "aliases": [
    "Benchmark block size"
  ],
  "definition": "I/O request size is the number of bytes requested by one input/output operation at a specified system boundary.",
  "note": "fio's blocksize configures bytes per I/O; bssplit permits a mixture of sizes [fio]. This is distinct from a device's logical sector or filesystem allocation unit. Kernel request merging can change counts and sizes below the application boundary [linux-iostats]. When relating successful data-transfer IOPS to bytes/s, use the actual completed bytes and matching operation population, especially if sizes vary or transfers are short [hesela-accounting].",
  "status": "shipping",
  "advantages": [],
  "limitations": [
    {
      "text": "Equal IOPS at different request sizes does not imply equal data throughput; a byte-weighted size average is not the required operation-weighted mean.",
      "citationIds": [
        "hesela-accounting"
      ]
    }
  ],
  "related": [
    "iops",
    "throughput-storage",
    "queue-depth",
    "logical-sector",
    "physical-sector",
    "partition-alignment"
  ],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "citations": [
    {
      "id": "fio",
      "title": "fio documentation: blocksize, bssplit, I/O depth and interpreting the output; online manual reviewed 13 October 2026.",
      "url": "https://fio.readthedocs.io/en/master/fio_doc.html",
      "accessed": "2026-10-13"
    },
    {
      "id": "linux-iostats",
      "title": "Linux kernel documentation: I/O statistics fields; completed requests, merged requests and sectors.",
      "url": "https://docs.kernel.org/admin-guide/iostats.html",
      "accessed": "2026-10-13"
    },
    {
      "id": "hesela-accounting",
      "title": "Hesela: IOPS, bytes and average occupancy; synthetic accounting examples.",
      "url": "https://hesela.com/analyses/storage-latency-coordinated-omission/#io-accounting",
      "accessed": "2026-10-13"
    }
  ],
  "updated": "2026-10-13"
}