Nice, this is a feature that'll really improve ZFS's usability for me, as a home user. I'd like to be able to redo my Zpool with a different architecture (6xDisks in RaidZ2 rather than 4x in RaidZ1). Doing it online rather than as a copy would be awesome. The next best thing would be online ZVol restructuring, though.

I don't think this will do what you want. One limitation is:

   Currently, device removal only works if all top
   level vdevs are plain disks / files. At the very
   least, we want to make device removal interact
   intelligently with mirrored disks.

I think that statement means that this feature will not work with mirrored or RaidZ* configurations.

It's a start, from here on out this can only mature and other use cases can start being considered.

yup, hopefully removal of any top-level vdev and rebalancing data in a pool.

Good to see that progress is being made toward catching up with btrfs in this really useful area. For those of us who don't buy hard drives by the dozen, it's really nice to have absolute flexibility about adding and removing drives and changing RAID modes non-destructively with btrfs, but ZFS still has some pretty enticing features, too.

This is good; perfect for that time you fat-finger it and end up adding drives as top-level vdevs instead of adding them as a mirror pair.