What RAID does
RAID (redundant array of independent disks) combines several drives into one volume for more speed, more capacity, protection against drive failure, or a mix. The level you choose decides how much of the raw capacity you can actually use.
RAID levels compared
| Level | Minimum drives | Usable capacity | Failures survived |
|---|---|---|---|
| RAID 0 | 2 | n × size | 0 |
| RAID 1 | 2 | 1 × size | n − 1 |
| RAID 5 | 3 | (n − 1) × size | 1 |
| RAID 6 | 4 | (n − 2) × size | 2 |
| RAID 10 | 4 (even) | n ÷ 2 × size | 1 guaranteed, up to n ÷ 2 |
- RAID 0 stripes data for speed, with no redundancy: one failure loses everything.
- RAID 1 mirrors the same data on every drive.
- RAID 5 spreads parity across drives; one drive’s worth of capacity goes to parity.
- RAID 6 uses double parity and survives two failures.
- RAID 10 stripes across mirrored pairs; it survives one failure per pair.
Example
Four 4 TB drives in RAID 5:
- raw capacity: 16 TB;
- usable: 3 × 4 = 12 TB, or 75% efficiency;
- the operating system shows about 10.9 TiB;
- it survives one drive failure.
TB versus TiB
Drive makers use decimal terabytes (10¹² bytes), while many operating systems report binary tebibytes (2⁴⁰ bytes). A “4 TB” drive shows as about 3.64 TiB, which is why a new array always looks smaller than expected. File system overhead takes a little more.
Rebuild risk with large drives
When a drive fails, the array must read every other drive in full to rebuild. With large drives, this takes many hours or days, and a second failure or an unreadable sector during the rebuild can lose data in RAID 5. For drives of 8 TB and more, RAID 6 or RAID 10 is usually recommended.
RAID is not a backup
RAID protects against a drive failure, not against deletion, ransomware, fire, theft or a controller fault. Follow the 3-2-1 rule: three copies of your data, on two kinds of media, with one copy off-site.
Other options
Many NAS systems offer their own schemes, such as Synology SHR or ZFS RAID-Z, which behave like RAID 5 or 6 but can mix drive sizes or add checksums. Usable capacity is similar for drives of equal size.