NAS pSEO Landing
Estimate usable TB, parity overhead, and fault tolerance for 4x 12TB in RAID-Z2. Includes reserve planning for NAS and homelab arrays.
Raw Capacity
48.00 TB
Usable Capacity
21.60 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 | 32.40 TB | 1 drive | 75.0% |
| RAID 6 | 21.60 TB | 2 drives | 50.0% |
| RAID 10 | 21.60 TB | 1 drive per mirror pair* | 50.0% |
| RAID-Z1 | 32.40 TB | 1 drive | 75.0% |
| RAID-Z2 | 21.60 TB | 2 drives | 50.0% |
No. Use this page for pre-purchase sizing, then validate with workload benchmarks, SMART health policy, and a tested restore plan.
It can be practical, but larger drives increase rebuild windows. Validate parity choice and backup policy before committing to the final layout.
This setup can tolerate 2 drives. Real-world survivability depends on mirror placement, rebuild stress, and drive health.
Keeping free space improves filesystem behavior for snapshots, metadata, and write performance. Full arrays often perform worse and rebuild more slowly.