An enterprise is planning to deploy a new vSAN ESA enabled cluster to their existing...

Four hosts are required to meet the 12 TB usable-capacity requirement after applying the protection policy, data reduction, reserves, and overhead. Each host contributes 8 TB raw capacity from two 4 TB NVMe devices. Three hosts provide 24 TB raw capacity, but once host rebuild reservation, 10% operational reserve, RAID-5 protection overhead, and 25% filesystem/object overhead are considered, the remaining effective usable capacity falls below the required 12 TB target. Four hosts provide 32 TB raw capacity. With vSAN ESA and FTT=1/RAID-5, vSAN uses erasure coding to improve capacity efficiency compared with mirroring. The vSAN sizing guidance states that RAID-5 with FTT=1 provides approximately 75% usable capacity before other reserves and overhead. Host Rebuild Reserve must also be planned so the cluster has enough free capacity to rebuild components after a host failure. After applying the expected 1.5 data reduction ratio and the required reserves, four hosts provide enough usable capacity while three hosts do not. Reference topics: vSAN ESA Capacity Sizing, RAID-5 Erasure Coding, Host Rebuild Reserve, Operations Reserve, vSAN Object Overhead.