A storage architect has been called into a meeting with the accounting team who is...

The two major contributors are RAID overhead and Operational Reserve. With the default policy set to FTT=1 using RAID-1 mirroring, vSAN stores two copies of protected data. The capacity planning guidance states that when Failures to tolerate is set to 1 failure with RAID-1 mirroring, virtual machines can use about 50 percent of raw capacity because each object consumes mirrored space. This is the largest capacity-impacting factor. The second contributor is Operational Reserve, which reserves capacity for vSAN internal operations such as policy changes, rebalancing, object movement, and other transient storage activities. Even when Host Rebuild Reserve is not activated, vSAN capacity monitoring still accounts for the operations threshold/reserve needed to keep the cluster functional during internal activity. Encryption overhead is typically not the primary reason for a 20% capacity difference. Host upgrade overhead is not a vSAN capacity category. VM swap can consume capacity, but it does not normally account for most unavailable raw capacity compared with RAID mirroring and operational reservation. Reference topics: vSAN Capacity Planning, RAID-1 FTT=1 Capacity Impact, Operations Reserve, Reserved Capacity.