Not on a raidz vdev you can't. You can't make mirrors out of anything other than existing mirrors or single disk vdevs. (You _can_ run zpool replace on more d

Not on a raidz vdev you can't. You can't make mirrors out of anything other than existing mirrors or single disk vdevs. (You _can_ run zpool replace on more disks than the pool redundancy has, assuming you don't need to disconnect the old disks to put the new ones in...running zpool replace on two disks in a four-disk raidz1 is perfectly legal, as long as the old disks are still there.)