1 · Choose a RAID level
Double-parity ZFS vdev. Min 3 drives; 4+ typical.
2 · Configure drives
3 · Drive class
3.5" nearline SAS/SATA capacity HDD — indicative figures.
Advanced — read/write mix, URE rate, ZFS tuning
Calculated for planning. We don't publish prices — a 24-year UK reseller, Servnet confirms the exact drives, array and pricing on quote. IOPS, throughput & rebuild are indicative estimates.
What RAIDZ2 is
RAIDZ2 is ZFS’s double-parity vdev: usable capacity is (drives-per-vdev − 2) × drive size per vdev, and each vdev survives two failures. It is the recommended default for wide vdevs of large drives because, like RAID 6, it keeps a parity in reserve during a resilver — a bad block on a surviving disk is repaired rather than fatal.
This calculator models the full ZFS picture: dual-parity capacity, the ~3.2% slop reservation, the (parity+1)-sector padding (negligible at 128 KiB, significant for small zvols), and the per-vdev random-IOPS rule. A 6 × 8 TB RAIDZ2 vdev gives exactly 32 TB usable at the default recordsize.
A single RAIDZ2 vdev of six 8 TB drives gives exactly 32 TB usable at 128 KiB recordsize — confirmed against ZFS’s own figures — and tolerates two failures with a parity still held in reserve during a resilver.
Advantages
- Survives two failures per vdev
- Redundancy retained during a resilver
- Strong balance of capacity and resilience
- ZFS checksums, scrubs, snapshots, compression
Trade-offs
- Two drives of parity per vdev
- Random IOPS ≈ one drive per vdev
- Padding overhead at small recordsize
- Resilvers can be long on very large drives
Best for
- TrueNAS / OpenZFS NAS pools
- Wide vdevs (6–12 drives) of large nearline drives
- Bulk, media and backup-target storage
- Anywhere RAIDZ1 rebuild risk is too high
Consider another level when
- Random-IOPS-heavy databases (use mirrors)
- Very small drive counts
- Latency-critical block workloads
RAIDZ2 — common questions
How is RAIDZ2 usable capacity calculated?
Per vdev it is (drives − 2) × drive size, less the ~3.2% ZFS slop and (parity+1)-sector padding. A 6 × 8 TB RAIDZ2 vdev gives exactly 32 TB usable (66.67% efficiency) at the default 128 KiB recordsize — matching ZFS itself.
Why is RAIDZ2 the recommended default?
Because it keeps a second parity during a resilver. On large drives, the chance of hitting a bad block while rebuilding a single failed drive is real; RAIDZ2 repairs it, whereas RAIDZ1 (single parity) would lose the vdev.
How much does small recordsize cost on RAIDZ2?
A lot. ZFS rounds each allocation up to a multiple of (parity+1) sectors, so small volblocksize (e.g. 8 KiB zvols for VMs) on a wide RAIDZ2 vdev can waste a large fraction of capacity to padding. Switch the recordsize in the calculator to see the effect for your layout.