RAID 6 for NAS: Double Parity, Usable Capacity, and the RAID 5 Trade-off
RAID 6 extends RAID 5 with a second parity drive, giving two-drive fault tolerance. It uses two drives' worth of capacity for parity. So you need at least four drives, and the capacity efficiency is lower than RAID 5.
See exactly how much usable storage your configuration delivers.
How RAID 6 Works
RAID 6 uses two independent parity calculations (P+Q parity) distributed across all drives. Any two drives can fail simultaneously without data loss. The two parity sets allow reconstruction of the missing data. This makes RAID 6 significantly safer than RAID 5 during the high-risk period of a drive rebuild.
Capacity Formula
Usable = (n − 2) × smallest drive capacity
- 4 × 4TB in RAID 6: 2 × 3.64 TiB = 7.28 TiB raw
- 6 × 4TB in RAID 6: 4 × 3.64 TiB = 14.55 TiB raw
- 8 × 4TB in RAID 6: 6 × 3.64 TiB = 21.82 TiB raw
RAID 6 vs RAID 5: When to Choose Each
| RAID 5 | RAID 6 | |
|---|---|---|
| Parity drives | 1 | 2 |
| Drive failures tolerated | 1 | 2 |
| Capacity efficiency (4 drives) | 75% | 50% |
| Capacity efficiency (6 drives) | 83% | 67% |
| Rebuild safety | No protection during rebuild | Protected during rebuild |
| Write performance | Moderate | Slightly lower (double parity) |
When RAID 6 Makes Sense
- Large drives (8TB+): Rebuilds take longer with larger drives. A second failure during a multi-day rebuild is a real risk.
- 6+ drive arrays: More drives = more failure risk = more value from double parity. RAID 6 efficiency improves as drive count increases.
- Critical data: When the data cannot be recovered from another source, the extra parity drive is worth the capacity cost.
Frequently Asked Questions
Is RAID 6 worth the extra capacity cost on a 4-bay NAS?
On a 4-bay NAS, RAID 6 uses half your capacity for parity (2 of 4 drives). This is a significant trade-off. For most home users with 4-bay NAS and drives under 10TB, RAID 5 (or SHR) is the typical choice. RAID 6 becomes more compelling as drive sizes and array counts grow.
Can I migrate from RAID 5 to RAID 6 without data loss?
Synology DSM, QNAP QTS, and most modern NAS platforms support online migration from RAID 5 to RAID 6 by adding a drive. The process rebuilds parity while the NAS remains accessible. This requires a free drive bay and an empty drive.