Overview
Start with measured usable data rather than the raw capacity printed on disks. Forecast growth from observed history where possible, then add operational headroom so the array does not run permanently near capacity.
Engineering considerations
The Storage Capacity & Growth Planner compounds annual growth and adds a configurable free-space reserve before converting the requirement into an approximate raw capacity using an efficiency factor. Snapshots, metadata, compression and deduplication should be modelled separately when they are material.
How to plan it
Capacity planning should include production data, snapshots, temporary working space, backup staging and any retention requirement that lives on the primary platform. Keep backup capacity as a separate planning exercise when the repository has different retention or deduplication behaviour.
Design implications
Revisit the forecast regularly. Growth is rarely a smooth percentage, and new applications, CCTV, virtual machines and cloud sync can change the trajectory quickly.
Operational considerations
Treat the output as a planning envelope, not a procurement specification.