RAIDZ2 Capacity Calculator
Estimate raw capacity, usable capacity, and expected real-world space for a ZFS RAIDZ2 pool.
How this RAIDZ2 calculator works
RAIDZ2 is a ZFS virtual device layout that provides double parity. In simple terms, each vdev can lose any two drives without losing pool data. The tradeoff is capacity: two drives per vdev are effectively used for parity.
This calculator uses the common planning model:
- Raw capacity = total drives × per-drive size
- Usable before overhead = (disks per vdev − 2) × drive size × number of vdevs
- Estimated practical capacity = usable before overhead × (1 − overhead%)
Why your “usable” number is not your “safe to fill” number
In ZFS, filling a pool too close to 100% can cause severe performance drops. Many admins keep at least 10%–20% free, sometimes more for write-heavy workloads or dedup-heavy datasets. That is why this calculator includes an overhead/reserved-space field.
Use lower overhead for lightweight home media workloads and higher overhead for virtualization, databases, or frequent snapshots.
RAIDZ2 design guidance
1) Pick a sensible vdev width
RAIDZ2 vdevs are often built with 6, 8, or 10 drives. Wider vdevs give better capacity efficiency, but resilvers can take longer and I/O behavior may vary by workload.
- 6-wide RAIDZ2: strong safety, moderate efficiency
- 8-wide RAIDZ2: common balance point
- 10-wide RAIDZ2: high efficiency, larger fault domain per vdev
2) Match drive sizes in each vdev
ZFS effectively treats each vdev by its smallest member size. Mixing sizes can leave capacity stranded. For predictable outcomes, keep drive models and sizes consistent within a vdev.
3) Understand fault tolerance correctly
RAIDZ2 provides two-drive redundancy per vdev, not globally. If your pool has multiple vdevs, each vdev independently tolerates up to two failures. Failure distribution matters.
Example planning scenarios
Example A: 6 × 12 TB, one vdev
- Raw: 72 TB
- Usable before overhead: (6 − 2) × 12 = 48 TB
- At 20% overhead: 38.4 TB estimated practical space
Example B: 2 vdevs of 8 × 16 TB
- Total drives: 16
- Raw: 256 TB
- Usable before overhead: (8 − 2) × 16 × 2 = 192 TB
- At 15% overhead: 163.2 TB estimated practical space
Performance and resilver notes
Capacity is only one part of pool design. You should also consider:
- Resilver time: larger disks take longer to rebuild, extending risk windows.
- IOPS scaling: more vdevs generally increase random I/O performance.
- Workload profile: sequential media storage behaves differently from VM datasets.
- Snapshots/replication: retention policy can materially reduce free space.
Important reminders
- RAIDZ2 is not a backup. Keep independent backups.
- Real reported space differs between TB and TiB due to decimal vs binary units.
- Metadata, compression behavior, snapshots, and recordsize choices can change effective capacity.
If you are finalizing a build, run multiple scenarios in this calculator and compare vdev counts, widths, and free-space targets before you buy hardware. Small design choices now can save major migration work later.