Calculate usable SAN capacity after RAID overhead, thin provisioning and reserved space.
SAN usable capacity is the raw capacity minus RAID overhead (parity/mirror space), minus space reserved for hot spares, metadata, and snapshots. Thin provisioning allows allocating more logical capacity than physical — beneficial for sparse workloads but risky if actual consumption exceeds physical capacity without monitoring.
Usable capacity
usable = raw × (1 - raid_overhead%) × (1 - reserved%)
Thin provisioning allocates storage on-demand rather than reserving the full logical size upfront, allowing you to present more capacity to hosts than physically exists — at the risk of running out if actual writes exceed physical capacity.
Typically 1-2 disks worth of capacity per shelf (roughly 3-5% of total), plus additional space for snapshots and controller metadata depending on the SAN vendor's recommendations.