Safely swapping a disk in a RAID456

Dear list,

I would like to swap out a drive in a RAID6 (4 drives), but ideally
without letting the RAID degrade. I was thinking that it should be
possible to declare the new disk a copy of the old one, let it sync,
then remove the old one and let the new one take over, but MD does
not seem to support that.

Next, I thought that I could add the new disk as a spare, grow the
array to 5 disks, and somehow shrink it back to 4 again, but of
course that does not work either, since 4→5 is a reshape, and there
seems to be no way to control which disk to remove from the RAID6 on
a shrink (making it a spare), without degrading the array in the
process.

Does anyone have any ideas? This should not be so hard…

-- 
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"courage is not the absence of fear, but the decision
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