RAID in Plain English
RAID stands for Redundant Array of Independent Disks. It's a technology that combines multiple physical hard drives or SSDs into a single logical unit, with the goal of improving performance, adding redundancy (protection against drive failure), or both.
RAID is widely used in servers, NAS devices, workstations, and anywhere that data availability or speed is critical. Understanding the most common RAID levels helps you decide which configuration fits your needs.
The Core Concepts: Striping, Mirroring, and Parity
Before diving into RAID levels, it helps to understand the three building blocks:
- Striping: Data is split across multiple drives, so reads and writes happen in parallel — boosting speed. No redundancy.
- Mirroring: Identical data is written to two or more drives simultaneously. If one fails, the mirror takes over. Slower writes, but strong redundancy.
- Parity: A calculated value derived from your data is stored separately. If a drive fails, the missing data can be mathematically reconstructed from the remaining drives and parity data.
Common RAID Levels Explained
RAID 0 — Striping (Speed, No Redundancy)
Data is striped across all drives for maximum read/write performance. With two drives, you get roughly double the speed and double the capacity.
The catch: If any single drive fails, all data is lost. RAID 0 is a zero-redundancy setup — despite the "RAID" name, it offers no protection.
Best for: Temporary scratch disks, video editing caches, or performance-critical tasks where data loss is acceptable.
RAID 1 — Mirroring (Redundancy, No Extra Speed)
Every drive is an exact mirror of the other. With two 4TB drives in RAID 1, you get 4TB of usable space — the other 4TB is the mirror copy.
The catch: You lose half your total capacity to redundancy, and write speeds don't improve.
Best for: Critical data storage, operating system drives, or small setups where data survival is the top priority.
RAID 5 — Striping with Parity (Balance of Speed and Redundancy)
Requires at least 3 drives. Data and parity information are striped across all drives. You can lose one drive without losing any data — the array rebuilds from parity.
Usable capacity = (N − 1) × drive size, where N is the number of drives.
Best for: File servers, NAS devices, and environments that need a good balance of capacity, speed, and fault tolerance.
RAID 6 — Striping with Double Parity
Like RAID 5, but with two parity blocks. This means you can survive the simultaneous failure of two drives. Requires at least 4 drives.
Best for: Large storage arrays where the risk of a second drive failing during a RAID 5 rebuild is a concern.
RAID 10 (1+0) — Mirroring + Striping
A combination of RAID 1 and RAID 0. Drives are mirrored in pairs, and those pairs are striped. Requires at least 4 drives. Offers excellent performance and redundancy — you can survive one drive failure per mirrored pair.
The catch: Only 50% of total capacity is usable — the same trade-off as RAID 1.
Best for: Databases, virtualization hosts, and high-performance servers where both speed and resilience matter.
Quick Comparison Table
| RAID Level | Min. Drives | Redundancy | Usable Capacity | Best Use Case |
|---|---|---|---|---|
| RAID 0 | 2 | None | 100% | Speed-critical, non-critical data |
| RAID 1 | 2 | 1 drive failure | 50% | Critical OS or data drives |
| RAID 5 | 3 | 1 drive failure | (N−1)/N | File servers, NAS |
| RAID 6 | 4 | 2 drive failures | (N−2)/N | Large arrays, high availability |
| RAID 10 | 4 | 1 per mirror pair | 50% | Databases, high-performance servers |
Important: RAID Is Not a Backup
A common misconception is that RAID replaces backups. It does not. RAID protects against hardware failure, but it offers no protection against accidental deletion, ransomware, file corruption, or physical disasters. Always maintain separate backups, regardless of your RAID configuration.