{
  "slug": "effective-spare-factor",
  "term": "Effective spare factor",
  "aliases": [
    "effective over-provisioning (physical-capacity basis)"
  ],
  "definition": "Effective spare factor is the fraction of a modeled SSD's physical page capacity not occupied by valid logical data at a specified time.",
  "note": "With physical pages P and valid logical pages X, the fraction is (P-X)/P; it can change without changing host-addressable capacity. In Frankie's TRIM model, one logical address represents one page. [frankie-thesis]\n\nHesela's finite-state comparison holds the TRIM attempt probability fixed while changing its target set. The resulting mean occupancy differs; neither this fraction nor filesystem free space alone predicts write amplification. Invalid pages are not necessarily erased and immediately writable. [hesela-trim]",
  "status": "research",
  "advantages": [],
  "limitations": [
    {
      "text": "A workload-model quantity, not a standardized SMART counter or a direct measurement of erased blocks.",
      "citationIds": [
        "frankie-thesis",
        "hesela-trim"
      ]
    }
  ],
  "related": [
    "over-provisioning",
    "trim",
    "discard",
    "garbage-collection",
    "write-amplification"
  ],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "citations": [
    {
      "id": "frankie-thesis",
      "title": "Tasha Christine Frankie. Model and Analysis of Trim Commands in Solid State Drives. UC San Diego doctoral dissertation (2012), chapter 3.",
      "url": "https://escholarship.org/uc/item/8gn63800",
      "accessed": "2026-10-18"
    },
    {
      "id": "hesela-trim",
      "title": "Hesela: TRIM target selection and finite-state occupancy, model comparison and code",
      "url": "https://hesela.com/analyses/ssd-overprovisioning-denominators/#trim-occupancy",
      "accessed": "2026-10-18"
    }
  ],
  "updated": "2026-10-18"
}