NAS pSEO Landing
Estimate usable TB, parity overhead, and fault tolerance for 4x 4TB in RAID-Z2. Includes reserve planning for NAS and homelab arrays.
Raw Capacity
16.00 TB
Usable Capacity
7.20 TB
Fault Tolerance
2 drives
Efficiency
50.0%
Popular TrueNAS default for medium arrays; dual-parity with good safety margin. This scenario applies a 10% filesystem reserve.
Run Interactive Calculator →| Mode | Usable | Tolerance | Efficiency |
|---|---|---|---|
| RAID 5 | 10.80 TB | 1 drive | 75.0% |
| RAID 6 | 7.20 TB | 2 drives | 50.0% |
| RAID 10 | 7.20 TB | 1 drive per mirror pair* | 50.0% |
| RAID-Z1 | 10.80 TB | 1 drive | 75.0% |
| RAID-Z2 | 7.20 TB | 2 drives | 50.0% |
Keeping free space improves filesystem behavior for snapshots, metadata, and write performance. Full arrays often perform worse and rebuild more slowly.
This setup can tolerate 2 drives. Real-world survivability depends on mirror placement, rebuild stress, and drive health.
This NAS planning scenario estimates 7.20 TB usable after a 10% reserve from 16.00 TB raw.
For long-lived NAS pools, resiliency first is usually safer. Capacity can be expanded later, while a risky parity choice can force migration sooner.