On Wed, May 24, 2017 at 12:12:32PM +1000, NeilBrown wrote:

What does work is:
 # start with a degraded array, device 0 missing
 mdadm --grow /dev/md42 --array-size=.....
 echo frozen > /sys/block/md42/md/sync_action
 mdadm /dev/md42 --add /dev/loop0
 echo 0 > /sys/block/md42/md/dev-loop0/slot
 mdadm --grow /dev/md42 --backup-file=... --raid-devices=4

Wow. Thanks.

This should merge Step 1+2, but not Step 3, right?

I really need to take a closer look at the things in /sys/.../md/...

Seems like you can do great things with it... terrible, yes, but great.

(
  Step 3 would be turning slot 4 to-be-spare into slot 0, 
  without --add ing another device at all.

  That's what would happen if /dev/loop0 was actually backed 
  by and thus identical with the slot4 device.

  But that's playing dirty.
)

Regards
Andreas Klauer