effective-spare-factor · research · also effective over-provisioning (physical-capacity basis)

Effective spare factor

Effective spare factor is the fraction of a modeled SSD's physical page capacity not occupied by valid logical data at a specified time.

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. [1]

Hesela'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. [2]

Relations

related

over-provisioning, trim, discard, garbage-collection, write-amplification

Limitations

• A workload-model quantity, not a standardized SMART counter or a direct measurement of erased blocks. [1, 2]

Citations

1. Tasha Christine Frankie. Model and Analysis of Trim Commands in Solid State Drives. UC San Diego doctoral dissertation (2012), chapter 3. (accessed 2026-10-18)
2. Hesela: TRIM target selection and finite-state occupancy, model comparison and code (accessed 2026-10-18)

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