VSAN 6.2 - Capacity Planning for Overheads

While creating VSAN Disk group all the disks are formatted with an On-Disk File System.
Like any other File system "VSAN-FS (format type)" has some overheads. This should be considered while capacity planning in VSAN environment.

VSAN 6.2 introduced VSAN-FS version3 (V3) on-disk file system format. This format has an overhead of 1% + Deduplication Metadata

So in practicality 1% of disk capacity + additional Deduplication Metadata overhead should also be factored while capacity calculation.
Here Deduplication metadata overhead refers to the Translation tables and Hash mapping tables created to achieve Deduplication and Compression.

Deduplication metadata is highly variable and depends on the configured data-set (configured during Storage Policy creation)

This can be observed in the Cluster -> Monitor -> Virtual SAN -> Capacity View.

Besides the 1% File System + Deduplication metadata overheads, VMware also recommends considering additional 30% Slack space capacity of the raw storage post disk formatting.

Slack space is to accommodate automatic rebalancing of data when a disk reaches 80% threshold of its capacity. When rebalancing starts it adds to the rebuild traffic on the cluster.
To avoid the traffic surge, VMware recommends to have capacity limited to 10% lesser than the threshold limit...hence 30% additional Slack space capacity.


In summary, besides the FTT/Erasure Coding overheads,  we should also consider 1% File System overhead, Deduplication Metadata overhead and 30% Slack Space Capacity Overhead.