There are three issues with Raidz Expansion
- it takes a relatively long time… this is solved by waiting patiently. You can restart etc safely.
- old data has the old storage efficiency. So if your raidz array had a storage efficiency of 2/3 (ie 66% or 2 data disks + 1 parity disk) before expansion the old data will still have that storage efficiency, although new data would have 3/4, or 75%. Rebalancing or otherwise rewriting the old data turns it into new data and will mean it’s stored more efficiently.
- Zfs Raidz Space Accounting is whacked. Which there may be a future fix. One day. Maybe.