Open-ZFS Allocation classes are a new vdev type to hold dedup tables, metadata, small io or single filesystems. They should offer a comparable redundany as the pool. Beside basic vdevs (not suggested as a disk lost=pool lost) you can use n-way mirrors. With several mirrors load is distributed over them. I have made some performance benchmarks, http://napp-it.org/doc/downloads/special-vdev.pdf I am really impressed about the result as this allows to use a slow disk pool where you can decide per ZFS filesystem based on the "recsize" vs "special_small_blocks" settings that data of this filesystem land on the special vdev ex an Intel Optane.