RAIDZ Expansion Calculator

Since OpenZFS 2.3 you can attach single disks to an existing RAIDZ vdev. But blocks written before the expansion keep their old data-to-parity ratio, so the immediately usable capacity is less than the native figure for a pool of that size — it converges as data gets rewritten. This tool shows the honest numbers.

Your expansion

RAIDZ level

Drives before expansion×

Drive sizeTB

Drives to add (one at a time)+

Pool fill before expansion%

Results

Verdict

How this is calculated

Old blocks keep their pre-expansion stripe geometry: data written on an N-disk RAIDZ-p still consumes raw space at ratio N/(N−p) after expanding to N+k disks. New writes use the wider (N+k)/(N+k−p) layout. Usable-right-after = old data + remaining raw space × new ratio. See the zpool-attach man page and the OpenZFS 2.3 release notes.

Rewriting data (zfs send/recv, or natural churn) converges the pool toward the native figure. Expansion never changes the parity level — RAIDZ1 stays RAIDZ1.

Takeaway: Expansion is real and works — just budget for less immediate space than the naive math. Full background in our RAID expansion guide →