Understanding RAID Levels: Choosing the Right Array for Your Storage

RAID — Redundant Array of Independent Disks — combines multiple physical drives into one logical volume, but “RAID” isn’t one thing. Each numbered level makes a different trade-off between usable capacity, performance, and how many drives can fail before you lose data, and picking the wrong one for your situation is a common, expensive mistake.

RAID 0: striping, all speed, zero redundancy

RAID 0 splits data across all drives in the array with no duplication at all — a “stripe” of each file spreads across every disk. Usable capacity is the full sum of every drive: 4 drives of 4TB each give you the full 16TB. Performance is excellent, since reads and writes happen across all drives simultaneously.

The catch is right there in the name it doesn’t have: no redundancy. If any single drive in a RAID 0 array fails, the entire array’s data is gone — not just that drive’s share, all of it, because every file is striped across all disks. RAID 0 is appropriate only for data you can afford to lose entirely, like a scratch/cache volume or a video-editing working directory backed up elsewhere.

RAID 1: mirroring, safety through duplication

RAID 1 writes the same data to two (or more) drives simultaneously — a live, real-time mirror. Usable capacity is just one drive’s worth, regardless of how many drives are in the mirror: two 4TB drives in RAID 1 give you 4TB usable, not 8TB, because the second drive is a complete duplicate, not additional space.

The upside is straightforward: any one drive can fail completely and the array keeps running on the surviving mirror, with zero data loss. This makes RAID 1 the simplest, most robust option when capacity efficiency matters less than reliability — common for boot drives and small, critical volumes.

RAID 5: single parity, a practical middle ground

RAID 5 stripes data across all drives like RAID 0, but also calculates and distributes a parity block across the array — extra data that can reconstruct any single missing drive’s contents mathematically. Usable capacity is (N − 1) drives’ worth: 4 drives of 4TB give you (4−1) × 4 = 12TB usable, with the “missing” drive’s worth of space spent on distributed parity instead.

RAID 5 tolerates exactly one drive failure without data loss — a second failure before the first is repaired means total data loss. It requires a minimum of 3 drives, and needs meaningfully more computation than RAID 0 or 1 to calculate parity on every write, which can affect write performance on older or software-based controllers.

RAID 6: double parity, for larger arrays

RAID 6 extends RAID 5’s idea with a second independent parity block, tolerating two simultaneous drive failures instead of one. Usable capacity is (N − 2) drives’ worth: 6 drives of 4TB give you (6−2) × 4 = 16TB usable.

The extra parity matters more than it might seem, for a specific reason: larger drives take longer to rebuild after a failure, and during that rebuild window, the array is vulnerable — a RAID 5 array rebuilding after one failure has zero redundancy left until the rebuild finishes, so a second failure during that window is catastrophic. RAID 6 keeps one level of protection even mid-rebuild, which is why it’s the more common choice as individual drive sizes (and therefore rebuild times) have grown. It requires a minimum of 4 drives.

RAID 10: striped mirrors, speed and safety together

RAID 10 (sometimes written 1+0) combines both ideas: drives are first mirrored in pairs (RAID 1), then those mirrored pairs are striped together (RAID 0). Usable capacity is half the total raw capacity: 4 drives of 4TB give you (4÷2) × 4 = 8TB usable — the same 50% overhead as plain mirroring, but spread across more drives for better performance.

RAID 10 tolerates multiple drive failures, as long as both drives in the same mirrored pair don’t fail simultaneously — and it rebuilds faster than RAID 5/6 after a failure, since it only needs to copy from that drive’s mirror partner, not recalculate parity across the whole array. It requires a minimum of 4 drives (always an even number) and gives up the most capacity of any level here in exchange for the best combination of speed and resilience.

Choosing between them

• Need maximum speed, don’t care about data loss? RAID 0.
• Need simple, bulletproof redundancy on a small volume? RAID 1.
• Need reasonable redundancy without giving up much capacity, on a smaller array? RAID 5.
• Same as RAID 5, but on a larger array where rebuild time is a real risk? RAID 6.
• Performance-critical and redundancy both matter, and you can afford the capacity trade-off? RAID 10.

None of these levels are a substitute for backups — RAID protects against a drive failing, not against accidental deletion, ransomware, or a fire in the server room. The RAID Calculator on this site computes exact usable capacity for any drive count, drive size, and level combination above, so you can compare options before committing to a configuration.