Work out storage overprovisioning instantly with clear inputs, formula shown and shareable results.
Raw capacity shrinks at three stages: SSD overprovisioning held back for garbage collection and wear levelling, RAID parity, and filesystem reserved blocks. Each is multiplicative, so 28 percent overprovisioning plus two parity disks in twelve plus 5 percent reserve leaves well under 60 percent of the raw figure as usable.
Usable capacity
usable = raw x (1 - overprovisioning) x (data disks / total disks) x (1 - filesystem reserve)
Spare blocks let the controller perform garbage collection and wear levelling without waiting for host trim, which keeps write latency stable and write amplification low.
On ext4 the default 5 percent can be lowered with tune2fs. It exists to prevent fragmentation and to leave root room to fix a full filesystem, so reduce it cautiously.