I know that on ZFS, the only way to grow a RAID-Z (or RAID-Z2 or RAID-Z3) vdev is to replace the disks with larger ones; there is no way to change the geometry. However, is it possible to do so without degrading the array in the process?
As an example, suppose I have a RAID-Z array with 4 disks: 1x2TB and 3x1TB makes 3 TB usable space. With all drives working, I buy 3 more 2TBs in order to grow the array (to 6 TB usable). If I remove and replace each of the 1 TBs with a 2 TB, one at a time, to force each resilver, then I'd be unnecessarily and repeatedly degrading the array, and increasing risk of failure in the process.
What I hope is possible is to mirror each disk before replacing it. i.e. Add the 1st 2 TB to a spare bay, mirror the first 1 TB disk to it, then remove the 1 TB, and put the 2 TB in the removed drive's slot; then repeat for the 2nd and 3rd 1 TB disks. This could potentially even obviate the need to resilver - or to recalculate parity, anyway.
Is such a thing possible?