Over-Provisioning: Spare Space That Buys Performance
The hidden capacity that keeps garbage collection cheap.
A "512 GB" SSD usually contains more than 512 GB of flash. The extra, hidden capacity is over-provisioning (OP) — spare space the host can never use directly, reserved so the FTL always has room to maneuver during garbage collection.
OP = (physical capacity − usable capacity) / usable capacity
Typical consumer drives reserve ~7%; enterprise drives ic often 28% or more. More OP → lower write amplification, but less usable space.The spare pool means victims can be emptier on average, so GC copies fewer valid pages.
Why a little spare space helps so much
Imagine a drive that is 100% full of valid data. To reclaim any block, GC would have to copy almost all of it — catastrophic amplification. Now reserve 20% as spare: the drive only ever holds 80% valid data, so the average victim is far emptier, and each cleanup copies much less. A small percentage of reserved space produces a large drop in write amplification. The relationship is non-linear — the first few percent of OP help the most.
Analogy. Over-provisioning is the empty space you leave in a sliding tile puzzle. With one empty tile you can rearrange everything; with zero empty tiles the puzzle is frozen. That one gap makes all the movement possible.
In the EyanaSSDSim paper & simulator. On the Live Simulator, the Over-provisioning dropdown (10% / 20% / 25%) changes exactly this reserve. Run the same trace at different OP levels and watch the WAF curve drop as OP rises — one of the clearest cause-and-effect demonstrations the tool offers.