ZFS Calculators

Nov 18, 2024
Revised May 31, 2025

A collection of calculators from around the web, for the ZFS filesystem. This was mainly done to preserve them in one place, and to experiment with using PyScript (allows python to be used for browser scripting instead of javascript).

If you don’t know much about ZFS, or need to brush up on your knowledge, “OpenZFS: the final word in file systems” by Jason Rose is a fantastic introduction.

RAIDZ Expansion

The latest TrueNAS (Electric-Eel / 24.10) now supports RAIDZ expansion, which allows you to append disks to an existing vdev. This allows you to extend you’re vdevs over time, rather than having to erase the vdev, create a new one, and restore from backup. It has one small caveat however, which is that the existing data still uses the data-to-parity ratio and block width of the old vdev size. This effectively means that you don’t get the full capacity of your drives until that existing data is rewritten. If you’re interested in recovering that capacity, check out ZFS In-Place Rebalancing by Markus Ressel.

This calculator, based on the TrueNAS RAIDZ extension calculator, allows you to determine how much space is lost compared to creating a new vdev from scratch.

Drive Size (TB): RAIDZ Level / Number of Parity Disks: Initial VDEV Capacity (%): Initial Number of Drives: Final Number of Drives:

Extend VDEV to final width all at once Capacity lost (%): 0% Capacity lost (bytes): 0 TB Capacity lost (drives): 0 Drives Extend VDEV one drive at a time Capacity lost (%): 0% Capacity lost (bytes): 0 TB Capacity lost (drives): 0 Drives