How to size usable RAID capacity

⌚ About 2 min read

Compare raw capacity, usable capacity, fault tolerance, and smallest-disk impact before purchasing storage.

Before you start

Work on a copy or a controlled test when the change can affect production. Keep timestamps, screenshots and the previous configuration so the result can be compared.

Step by step

List the number and capacity of all disks.
Choose a RAID level matching availability and performance needs.
Calculate usable capacity based on the smallest disk in the set.
Plan space for spares, snapshots, reserves, and growth.
Confirm that backups remain independent from RAID.

Validation

Repeat the original test after the change and confirm that the expected service works without creating a new regression. Document the final state.

Technical deep dive

RAID: usable capacity, fault tolerance and performance are linked

Technical checkpoints

Usable capacity is calculated from the smallest drive in the set; extra capacity on larger drives is unused in a classic homogeneous RAID.
RAID 1 ≈ 1× one disk, RAID 5 ≈ (N-1)×, RAID 6 ≈ (N-2)×, RAID 10
RAID improves availability but does not replace independent backup against deletion, ransomware or logical corruption.

Example

Six 4 TB drives in RAID 6 provide about 16 TB usable raw before formatting/controller reserve: (6-2) × 4 TB.

RAID6: (N - 2) × smallest_drive_size
(6 - 2) × 4 TB = 16 TB

Topic-specific pitfalls

Sizing only capacity without considering rebuild time and URE risk can make a large RAID 5 unsuitable.
A hot spare improves reaction time but does not count toward usable capacity.

How to validate

Usable capacity after formatting/reserves covers data, snapshots and growth with headroom.
RAID level meets simultaneous failure tolerance and backup remains independent.