Convert vSAN raw disk capacity into effective usable capacity after RAID overhead, slack space and dedupe or compression.
vSAN applies overheads in a strict order: the storage policy's redundancy comes first, then slack space is held back from what remains, and dedupe and compression multiply only the data actually written. RAID-5 and RAID-6 erasure coding are far more space-efficient than mirroring but need at least 4 and 6 hosts respectively and cost more write amplification. Sizing vSAN on raw terabytes overstates real capacity by two to four times, and a cluster that hits the slack threshold stops rebuilding objects — which is precisely when a second disk failure becomes data loss.
vSAN Storage
effective = hosts × disks × disk size × RAID factor × (1 − slack) × dedupe ratio, where the RAID factor is 0.50 for FTT=1 mirroring, 0.33 for FTT=2 mirroring, 0.75 for RAID-5 and 0.67 for RAID-6.
effective = hosts × disks × disk size × RAID factor × (1 − slack) × dedupe ratio, where the RAID factor is 0.50 for FTT=1 mirroring, 0.33 for FTT=2 mirroring, 0.75 for RAID-5 and 0.67 for RAID-6. vSAN applies overheads in a strict order: the storage policy's redundancy comes first, then slack space is held back from what remains, and dedupe and compression multiply only the data actually written. RAID-5 and RAID-6 erasure coding are far more space-efficient than mirroring but need at least 4 and 6 hosts respectively and cost more write amplification.
Sizing vSAN on raw terabytes overstates real capacity by two to four times, and a cluster that hits the slack threshold stops rebuilding objects — which is precisely when a second disk failure becomes data loss.
This calculator takes 7 inputs: Hosts contributing storage, Capacity disks per host, Size of each capacity disk, Storage policy, Slack space reserved, Dedupe and compression ratio, Storage per VM. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
vSAN stores two data copies plus a witness component, and all three must live on separate hosts so no single failure can take out a majority. FTT=2 mirroring needs five hosts for the same quorum reason with three copies and two witnesses.