RAID 6 for NAS: Double Parity, Usable Capacity, and the RAID 5 Trade-off
RAID 6 adds a second set of parity to RAID 5, so the pool survives two failed drives. It gives two drives' worth of capacity to parity, so it needs at least four drives and leaves less space than RAID 5 with the same drives.
See exactly how much usable storage your configuration delivers.
How RAID 6 Works
RAID 6 keeps two independent sets of parity spread across all drives. Any two drives can fail without losing data. The practical difference from RAID 5 is the rebuild: after one drive fails, RAID 6 still has redundancy while the replacement rebuilds, where RAID 5 has none.
Capacity Formula
Usable layout = (n − 2) × the smallest drive, before the NAS platform's own reserves.
- 4 × 4 TB in RAID 6: 2 × 4 TB = 8 TB
- 6 × 4 TB in RAID 6: 4 × 4 TB = 16 TB
- 8 × 4 TB in RAID 6: 6 × 4 TB = 24 TB
Your operating system may show these in TiB, which is the same space as a smaller number. The RAID calculator adds your platform's reserves.
RAID 6 vs RAID 5
| RAID 5 | RAID 6 | |
|---|---|---|
| Parity drives | 1 | 2 |
| Drive failures survived | 1 | 2 |
| Usable share, 4 identical drives | 75% | 50% |
| Usable share, 6 identical drives | 83% | 67% |
| During a rebuild | No redundancy left | One more failure survived |
When RAID 6 Makes Sense
- Long rebuilds: the bigger and busier the drives, the longer a rebuild runs and the longer RAID 5 runs without protection. The RAID Rebuild Time Estimator estimates it for your drives.
- More drives: more drives that can fail, while RAID 6's parity share shrinks as the drive count grows.
- Slow replacement: if a spare drive is days away, a second failure has longer to happen.
The Drive Failure Risk Estimator shows how the risk during a rebuild changes between RAID 5 and RAID 6 for your drive count and replacement time.
RAID keeps a NAS running through a drive failure. It is not a backup: deletion, ransomware, a failed rebuild or a dead NAS can still take everything, so keep a separate copy.
Frequently Asked Questions
Is RAID 6 worth it on a 4-bay NAS?
On four drives RAID 6 gives half the capacity to parity, against a quarter for RAID 5. Whether that is worth it depends on how long a rebuild takes, how quickly you can replace a drive and whether you have a backup, not on a fixed drive size. The rebuild and risk estimators above show those numbers for your setup.
Can I change RAID 5 to RAID 6 later?
Some NAS platforms can convert a RAID 5 pool to RAID 6 by adding a drive, and some cannot. Check your platform's own documentation before you plan on it.