⌚ 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.
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 TBTopic-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.