RAID Storage Calculator

RAID Storage Calculator

RAID (Redundant Array of Independent Disks) combines multiple drives into one logical unit. Different RAID levels trade usable storage against redundancy and performance.

RAID Level Summary:

Formulas:

RAID 0: Usable = N × Capacity

RAID 1: Usable = Capacity, however many drives you add, because every one of them is a mirror

RAID 5: Usable = (N − 1) × Capacity

RAID 6: Usable = (N − 2) × Capacity

RAID 10: Usable = (N / 2) × Capacity

Overhead percentage:

Overhead % = (1 − Usable / Total Raw) × 100

Choosing a RAID level:

Use RAID 0 only for non-critical data where speed matters. Use RAID 1 for simple two-disk mirroring (boot drives, critical single servers). Use RAID 5 for NAS (Network Attached Storage) boxes and small servers, where storage efficiency and one-drive tolerance are both worth having. Use RAID 6 when downtime during a rebuild is unacceptable. Use RAID 10 for high-performance databases, since it gives fast reads and writes with full redundancy.

Why the rebuild is the part that bites

The storage efficiency table is the easy half. The hard half is what happens after a drive dies, and it is the reason experienced admins stopped putting large drives in RAID 5.

When a drive fails, the array runs degraded and the controller reads every sector of every surviving drive to reconstruct the missing one onto the replacement. On a 6-drive array of 4 TB disks that is 20 TB of sustained reading, which takes somewhere between 8 and 30 hours depending on the controller and whether the array is also serving users. Throughout that window a RAID 5 array has no redundancy left at all. A second failure, or a single unrecoverable read error on any of the surviving drives, loses the whole array.

That second-failure window is the entire argument for RAID 6, and it is why the crossover point is drive size rather than drive count. With 1 TB drives a rebuild is a few hours and RAID 5 is defensible. With 12 TB drives the rebuild runs for days on hardware that has just proved it can fail, and RAID 6 stops being paranoid.

RAID 10 sidesteps this differently: rebuilding a mirror copies one drive to one drive, so it reads 4 TB rather than 20 TB and finishes in a fraction of the time. You pay half your raw capacity for that.

RAID is not a backup:

RAID protects against a drive failure. That is the whole of what it does. It does not protect against accidental deletion, ransomware, a failed controller writing garbage to every member at once, or the building catching fire. Always maintain a separate backup copy of important data, and ideally one that is not in the same room.