zfs device removal

using currently zfs-0.6.5.6

according to the oracle docs They must have some good way to copy the data safely back to the remaining vdevs.

Background is that ZFS branched off into the Oracle proprietary ZFS and the various OpenZFS implementations. The Oracle documentation is actually great (at least I think so, it's easy to understand and provides good examples) but sometimes you can run into a feature that is documented there but not actually implemented in the OpenZFS variants. OpenZFS cannot use code from Oracle since it is proprietary, so they have to code new features themselves.

I just checked the status and zfsonlinux will support this in the next upcoming major release 0.8.0 (currently zfsonlinux is on 0.7.x and 0.8.0 is not yet released) as follows: 

    Only single device vdevs can be removed, neither mirrors nor RAID-Z vdevs. That means you can remove all devices from a mirror vdev until it is a single device vdev and then you can remove it. At least this is how I understand it will work. Since the mirror has to be broken up first, this means that there might be a small risk of pool destruction if that single device fails during the removal process. I might be wrong but if I am not, then I don't really see the point of this feature, it would be a last resort kinda thing.