[crote]

The linked pull request says "After the expansion completes, old blocks remain with their old data-to-parity ratio (e.g. 5-wide RAIDZ2, has 3 data to 2 parity), but distributed among the larger set of disks". That'd mean that the disks do not contain the same data, but it is getting moved around?

Regardless, my entire point is that you still lose a significant amount of capacity due to the old data remaining as 3+2 rather than being rewritten to 4+2, which heavily disincentives the expansion of arrays reaching capacity - but that is the only time people would want to expand their array.

It just seems to me like they are spending a lot of effort on a feature which you frankly should not ever want to use.

[rincebrain]

I don't think that's true.

I don't use raidz for my personal pools because it has the wrong set of tradeoffs for my usage, but if I did, I'd absolutely use this.

Yes, your data has the old data:parity ratio for older data, but you now have more total storage available, which is the entire goal. Sure, it'd be more space-efficient to go rewrite your data, piecemeal or entirely, afterward, but you now have more storage to work with, rather than having to remake the pool or replace every disk in the vdev with a larger one.