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On ZFS, a pool is resized by replacing all the drives in the pool. When autoexpand=on and the last drive is replaced, the pool will have expanded capacity. And drives are cheap.

When another RAID device is added to the pool instead, it is used as a stripe. Different types of RAID can be combined in this way, for example one can add a mirrored stripe to an already existing RAID-Z. Whether this is desired or not is a different discussion, since the point here is that both scenarios are possible.

Unlike other volume managers, ZFS expands on the disk boundary, rather than physical or logical elements; it's a larger boundary, but since no arcane or complex procedures or knowledge are required and drives are cheap, it's very practical, as well as elegant.