- See the initial commit on Github.
- See the comment in the mailing list.
- See the video explaining the new feature.
- See the slides explaining it.
That said, the feature will continue to be extended so that rearranging arrays is possible (for example, the feature doesn't let you remove a leg of a RAIDZ array, because that would require reformatting the entire vdev). But, with this feature in ZFS — which will no doubt migrate to other open source ZFS implementations, as it's been developed in the open and under the same ZFS license — Matthew Ahrens and the gang have finally brought to us what so many sysadmins clamored for.
Oh, and, for the btrfs fanbois: sorry, ZFS trumped you again, on the one thing that btrfs allegedly could do.
Thank you, Matthew and gang!