The common RAID levels
| Level | How it works | Min disks | Fault tolerance | Usable capacity |
|---|---|---|---|---|
| RAID 0 | Striping — splits data across disks for speed | 2 | None | 100% |
| RAID 1 | Mirroring — identical copy on each disk | 2 | 1 disk | 50% |
| RAID 5 | Striping + distributed parity | 3 | 1 disk | (n−1)/n |
| RAID 6 | Striping + double parity | 4 | 2 disks | (n−2)/n |
| RAID 10 | Mirrored pairs, then striped (1+0) | 4 | 1 per mirror | 50% |
How to choose
- Speed, no safety: RAID 0 (scratch/cache only).
- Simple redundancy: RAID 1.
- Balance of capacity + safety: RAID 5 (one disk) or RAID 6 (two disks; better for large drives).
- Performance + redundancy: RAID 10 (databases, heavy write).
Exam traps
- RAID is not backup. It protects against disk failure, not deletion, corruption or disaster.
- RAID 5 with very large disks. Long rebuilds risk a second failure — RAID 6 is safer.
- Confusing RAID 10 and 0+1. 10 = mirror then stripe; both need ≥ 4 disks.
Test yourself
4 quick questions — tap an answer to check it instantly. Nothing is sent anywhere.
1. Which RAID level has NO fault tolerance?
Answer: A. RAID 0 is striping only — any disk failure loses all data.
2. RAID 1 usable capacity is…
Answer: B. Mirroring keeps two copies, so 50% of raw capacity is usable.
3. How many disk failures can RAID 6 tolerate?
Answer: C. RAID 6 uses double parity and survives two simultaneous failures.
4. Minimum disks for RAID 10?
Answer: C. RAID 10 (mirror then stripe) needs at least 4 disks.
Sources
- CompTIA Network+ / Security+ exam objectives.
- Relevant RFCs and vendor documentation.
Reference summary only.