howto change MDADM UUID / superblock superblock on /dev/n

After switching my server from 6.06 > 7.04 ( i'm skipping 6.10 because of broken sata_uli with M5281 chipset ) to support my new hardware ( sata_mv + fixed sata_uli on 2.6.20 kernel )

To be on the safe side I removed my RAID
clean installation of 7.04 +complete update + mdadm install ( before attaching any raid disk )
halt
attached raid array
boot

array is raid 5 with 4 disks, previously I removed 1 disk
array was temporary running in degraded mode ( which should have been fine )

array refuses to assemble

root@ubuntu:/# mdadm --assemble /dev/md0 /dev/hdg1 /dev/hdh1 /dev/sda1
mdadm: superblock on /dev/sda1 doesn't match others - assembly aborted


brought array back to 6.06 server, same result.... so this means :evil: 7.04 wrote something on /dev/sda1, without noticing or consulting me - it wasn't even mounted, doesn't exist in fstab, I always mount manually -

it's similar ( yet different ) from my previous post
http://ubuntuforums.org/showthread.php?t=405782&highlight=mdadm


root@ubuntu:/# mdadm --examine /dev/sda1
/dev/sda1:
Magic : a92b4efc
Version : 00.90.03
UUID : 8d754c1d:5895bb70:b1e8e808:894665ea
Creation Time : Sun Sep 10 22:51:43 2006
Raid Level : raid5
Device Size : 156288256 (149.05 GiB 160.04 GB)
Array Size : 468864768 (447.14 GiB 480.12 GB)
--------snipsnip
-
/dev/hdg1:
Magic : a92b4efc
Version : 00.90.03
UUID : 8d754c1d:5895bb70:c89ffdee:815a6cef
Creation Time : Sun Sep 10 22:51:43 2006
Raid Level : raid5
Device Size : 156288256 (149.05 GiB 160.04 GB)
Array Size : 468864768 (447.14 GiB 480.12 GB)
-------snipsnip
-
/dev/hdh1:
Magic : a92b4efc
Version : 00.90.03
UUID : 8d754c1d:5895bb70:c89ffdee:815a6cef
Creation Time : Sun Sep 10 22:51:43 2006
Raid Level : raid5
Device Size : 156288256 (149.05 GiB 160.04 GB)
Array Size : 468864768 (447.14 GiB 480.12 GB)

root@ubuntu:/# mdadm --assemble /dev/md0 /dev/hdg1 /dev/hdh1 /dev/sda1
mdadm: superblock on /dev/sda1 doesn't match others - assembly aborted


while the hdh1 and hdg1 drive are ok, the UUID / superblock for sda ( and the removed sdb ) changed.

How do I fix this ?

guess here goes my sunday afternoon.

Thanks alot !

OK. resolved this,

Don't know ( yet ) how this happened, but seems like you can re-create your array
( tested this first with dummy arrays on vmware )
I used one missing device when defining the array to make sure it didn't start (possibly wrong ) resyncing.
It's very important to define the EXACT sequence your array previously was in ( make sense as it then finds it's spare blocks where they where before ),
do:
mdadm -E /dev/sda1 ( or whatever device + partition your using ) and this for any device in the array

root@feisty-server:/# mdadm -E /dev/sda1

----snip

Number Major Minor RaidDevice State
this 0 8 1 0 active sync /dev/sda1

0 0 8 1 0 active sync /dev/sda1
1 1 0 0 1 faulty removed
2 2 22 1 2 active sync /dev/hdc1
3 3 22 65 3 active sync /dev/hdd1


and then do

mdadm --create /dev/md0 --assume-clean --level=5 --raid-devices=4 /dev/sda1 missing /dev/hdc1 /dev/hdd1


it gave me some info about the previous state of the array, and asked me if I really wanted to continue