On Mon, 1 Oct 2012 16:15:54 +0600 Roman Mamedov [off-list ref] wrote:

On Tue, 03 Apr 2012 15:53:01 +1000
NeilBrown [off-list ref] wrote:

quoted

To reverse a partial reshape which changes the number of devices
you can stop the array and re-assemble with the raid-disks numbers
reversed and it will undo.

Unfortunately this does not seem to work:

# mdadm --assemble --raid-disks=5 --backup-file=/mnt/scratch/backup-file /dev/md0 /dev/sdb1 /dev/sdc1 /dev/sde1 /dev/sdd1 /dev/sdf1
mdadm:option --raid-disks not valid in assemble mode

No, mdadm doesn't support this yet.  The patch you identified is part of
preparing the way for this functionality, but it isn't ready yet.

For now, you'll just need to let the reshape complete.

NeilBrown