RAIDZ1 Capacity Calculator
Estimate usable capacity for a single RAIDZ1 vdev in ZFS. RAIDZ1 uses one disk worth of parity.
What this RAIDZ1 calculator tells you
This calculator gives a practical estimate of how much space you can use in a RAIDZ1 pool. It starts with raw disk capacity, subtracts one drive for parity, then applies optional overhead and free-space targets. The result is closer to what you can safely plan for in real-world operation.
Core RAIDZ1 formula
For equal-sized drives in one RAIDZ1 vdev:
- Raw capacity = number of drives × drive size
- Parity cost = 1 × drive size
- Usable before overhead = (number of drives - 1) × drive size
If your drives are mixed sizes, ZFS generally treats all drives as if they were the size of the smallest drive in that vdev.
RAIDZ1 vs RAIDZ2 vs mirrors
| Layout | Fault Tolerance | Capacity Efficiency | Best Use Case |
|---|---|---|---|
| RAIDZ1 | 1 drive failure | High | Smaller pools, backups, non-critical workloads |
| RAIDZ2 | 2 drive failures | Medium | Large disks, primary storage, safer rebuild window |
| Mirrors | Depends on mirror layout | Lower | IOPS-heavy VMs/databases, simpler expansion path |
Important planning notes for RAIDZ1
1) Large drives increase rebuild exposure
As drive capacities grow, rebuilds take longer. Longer rebuilds mean more time where a second fault could cause data loss in RAIDZ1. For 14TB, 18TB, or 22TB drives, RAIDZ2 is often the conservative choice.
2) Keep healthy free space
ZFS can slow down as pools approach full utilization, especially for random writes. Keeping around 20% free is a common rule of thumb for smoother performance and easier housekeeping.
3) Capacity numbers differ between TB and TiB
Drive vendors market in decimal TB, while many operating systems report binary TiB. That is why your observed capacity appears smaller than the label. This calculator shows both units.
Example
Suppose you have 6 drives at 16TB each in RAIDZ1:
- Raw = 96TB
- Usable before overhead = 80TB
- After 3% overhead ≈ 77.6TB
- At 20% reserve target ≈ 62.08TB practical fill target
This gives you a realistic working figure for planning media libraries, backups, or home lab storage.
Best practices checklist
- Use same model and size drives when possible.
- Scrub pools regularly and monitor SMART health.
- Keep backups; RAID is availability, not backup.
- Avoid running near 100% utilization.
- Choose RAIDZ2 when uptime and data safety are top priority.