Well, I found my error thanks to your comments about dm. While it's true that 
the /dev/mapper/sdxx devices are the same as /dev/sdxx , in my case this 
wasn't entirely true, as the disks i added had a primary partition on them, 
but i deleted them and made that primary partition a little smaller. However, 
dm mappings are only loaded on boot normally, so these were not reloaded . 
Result : the reshape took place on the older "larger" partitions instead of 
the new, slightly smaller ones. Ofcourse, the superblock was in that area too 
ofcourse. I restored the partitions to their original larger size and was able 
to get the superblocks read, and have succesfully recreated a degraded 7 disk 
array and all data is still intact :)

I guess i really should be more carefull using mapper and real devices when 
changing partition layout.

Thanks for the assistance.

On Saturday 23 May 2009 23:05:35 you wrote:

On Sun, May 24, 2009 1:03 am, rdc wrote:

quoted

Yesterday i grew my 4*500GB raid5 array to 7*500GB. The reshape operation
ended succesfully according to the logs :

May 22 22:24:56 [kernel] md: md2: reshape done.
May 22 22:24:56 [kernel] RAID5 conf printout:
May 22 22:24:56 [kernel]  --- rd:7 wd:7
May 22 22:24:56 [kernel]  disk 0, o:1, dev:dm-3
May 22 22:24:56 [kernel]  disk 1, o:1, dev:dm-0
May 22 22:24:56 [kernel]  disk 2, o:1, dev:dm-2
May 22 22:24:56 [kernel]  disk 3, o:1, dev:dm-1
May 22 22:24:56 [kernel]  disk 4, o:1, dev:dm-8
May 22 22:24:56 [kernel]  disk 5, o:1, dev:dm-6
May 22 22:24:56 [kernel]  disk 6, o:1, dev:dm-5

The devices in your array are dm-xx devices, but....

quoted

However, upon restarting this morning the grown array was no longer
detected,
and my investigation revealed that the 3 device that were added do not
contain
an md superblock at all !

One of the 3 new devices :
mdadm --examine -v /dev/sdb1
mdadm: No md superblock detected on /dev/sdb1.

... you are looking on sdxx devices for the superblock.  This
seem to me to be a strange thing to do.
What does
   mdadm --example /dev/mapper/whatever
show?

What exactly are the dm-xx devices?

NeilBrown