Device removal was added in the recent ZoL 0.8 release, so with that you can remove a vdev that is either a sole drive, or a mirror. Currently it can't be used

Device removal was added in the recent ZoL 0.8 release, so with that you can remove a vdev that is either a sole drive, or a mirror. Currently it can't be used to remove a RAIDZx vdev thou. It does carry a memory overhead for a remap table , but this does shrink as old allocations are retired back to the pool. And the first alpha for RAIDZ expansion became available a week or two ago. (For going from say a 6 disk RAIDZ2 vdev, to a 7+ disk RAIDZ2 vdev). Just in case anyone decides to play with this feature, the on disk format for this feature is not stable yet, only use it on test pools.