RAIDZ1 Calculator
ZFS RAIDZ1 is single-parity RAID built into ZFS's copy-on-write filesystem — same capacity math as RAID 5, without the write-hole risk of traditional hardware RAID.
The RAIDZ1 formula
Usable Capacity = (N − 1) × Disk Size
Same N-minus-1 math as hardware RAID 5, but implemented as part of ZFS's own copy-on-write filesystem rather than a separate RAID controller layer.
Worked example
4 disks × 4 TB = 12 TB usable out of 16 TB raw, 75% storage efficiency, tolerates 1 disk failure. This is the smallest practical RAIDZ1 vdev size.
Why RAIDZ1 avoids the classic RAID 5 write-hole problem
Traditional RAID 5 can develop an inconsistency between data and parity if power is lost mid-write. ZFS is copy-on-write — it never overwrites data in place, so a RAIDZ1 vdev structurally can't develop that specific corruption. This doesn't remove RAIDZ1's other trade-off: like RAID 5, a second disk failure during a resilver (ZFS's term for rebuild) still means data loss, so RAIDZ2 is the safer default once you have 5 or more disks.
What's the minimum number of disks for RAIDZ1?
3 disks, matching RAID 5's minimum. Below that, use ZFS mirrors (equivalent to RAID 1) instead.
Should I use RAIDZ1 or RAIDZ2?
RAIDZ1 is defensible on smaller vdevs (3-4 disks) where resilver time is short. Once you're at 5+ disks, most admins default to RAIDZ2 for the same reason hardware RAID 6 is preferred over RAID 5 at that scale — resilver windows on large drives carry real risk of a second failure.
Need a different RAID level? Use the full RAID Calculator covering RAID 0, 1, 5, 6, 10, 50, 60 and ZFS RAIDZ1/2/3, or read the RAID levels comparison guide.