Raw, usable and effective: calculating storage capacity

From drive capacity to application space

Raw capacity is the sum of the drives’ nominal capacities. Usable space is calculated after RAID protection, reserves and system allocations. This distinction helps select a configuration that can accommodate the required data. Compare proposals in the same units and with the protection layout stated.

Effective capacity estimates the logical data volume that can be accommodated with deduplication and compression. Its calculation combines usable space, the data class and the assumed reduction ratio. Licensed capacity is a separate quantity governed by the product’s licensing rules.

Compare proposals on a common basis

Calculating units and free-space headroom

For a worked example, six 7.68 TB drives provide 46.08 TB raw. Since TB = 10¹² bytes and TiB = 2⁴⁰ bytes, this is approximately 41.910 TiB. Both figures describe the same number of bytes. Usable capacity is then calculated for the selected protection layout, with licensed capacity calculated separately.

Free-space headroom also needs a precise calculation. If occupied space is U and 20% of the pool must remain free, the required pool is U/0.8. Multiplying U by 1.2 describes a different condition: adding 20% to the current volume. Account for snapshot and temporary migration space separately from operational headroom.

Connect capacity to the expansion plan

Current data volume, growth and the reduction ratio together shape the expansion plan. Conservative and expected scenarios show when additional drives may be needed if data grows faster or reduction is lower. This helps balance the initial purchase against later expansion.

The application owner supplies volume and growth requirements, engineering calculates usable capacity, and procurement checks licensing rules. Retaining the calculation date, software version and expansion limits makes it easier to update the configuration and budget as workloads change.