On Wed, 9 Dec 2015, George Rapp wrote:

# mdadm --assemble --verbose --invalid-backup --force
--update=revert-reshape /dev/md4 $OVERLAYS
mdadm: looking for devices for /dev/md4
mdadm: /dev/mapper/sdg4 is identified as a member of /dev/md4, slot 2.
mdadm: /dev/mapper/sdi1 is identified as a member of /dev/md4, slot 5.
mdadm: /dev/mapper/sdc4 is identified as a member of /dev/md4, slot 1.
mdadm: /dev/mapper/sdh1 is identified as a member of /dev/md4, slot 3.
mdadm: /dev/mapper/sdj1 is identified as a member of /dev/md4, slot 4.
mdadm: /dev/mapper/sdd4 is identified as a member of /dev/md4, slot 0.
mdadm: /dev/md4 has an active reshape - checking if critical section
needs to be restored
mdadm: Failed to find backup of critical section
mdadm: continuing without restoring backup
mdadm: added /dev/mapper/sdc4 to /dev/md4 as 1
mdadm: added /dev/mapper/sdg4 to /dev/md4 as 2
mdadm: added /dev/mapper/sdh1 to /dev/md4 as 3
mdadm: added /dev/mapper/sdj1 to /dev/md4 as 4
mdadm: added /dev/mapper/sdi1 to /dev/md4 as 5
mdadm: added /dev/mapper/sdd4 to /dev/md4 as 0
mdadm: /dev/md4: Need a backup file to complete reshape of this array.
mdadm: Please provided one with "--backup-file=..."
mdadm: (Don't specify --update=revert-reshape again, that part succeeded.)

How can the array have an active reshape if the reshape pos'n is 0 on
all devices? Doesn't that mean that the reshape never actually
started? If so, can I just revert -- somehow -- to a 5-device RAID 6
array to recover my data?

Just a shot in the dark, what happens if you add a backup file to the 
above command, but without revert-reshape? Ie state both --invalid-backup 
but also supply a backup-file. The text above could indicate that this 
might help.

If you get the array up and running again but it's still reshaping but at 
position 0, issue a --continue to it (this has worked for others). Also, I 
would get the latest git version of mdadm and try with that one if you're 
still using v3.2.2 as per the link to your august original email.

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se