On Sun, Mar 13, 2011 at 1:33 PM, Urs Beyerle <[log in to unmask]> wrote:
>>
>> As part of upgrade planning for my main workstation at home (an
>> updated SL 5.2 system), I booted from the live CD just to see if all
>> the major devices worked. Everything worked :). Unfortunately, that
>> act rendered my existing system unbootable.
>>
>> When I booted from the live CD it found and started my md RAID1
>> partitions, including my system root partition. Nice, I had all my
>> data there to play with. I noticed that where I had previously used
>> /dev/md0, /dev/md1, etc..., the live CD system had created them as
>> /dev/md122, /dev/md123, and so on, and not in the original order. I
>> didn't think much of this, figuring my regular system would start them
>> back in the original configuration, but apparently some metadata
>> somewhere got changed by the live CD system and now the original
>> system would not get past switching to the root file system with a 'no
>> such device' error. I could see, just before going off the screen,
>> where it had started my root partition as /dev/md125 rather than
>> /dev/md0.
>>
>> I tried a couple of things to recover from this: I tried stopping and
>> reassembling the RAID sets with the desired device names from the live
>> CD system, and repackaging my initrd with device files for the
>> /dev/md12* devices, but no luck. At that point I decided that, since I
>> had already done my backups and was planning on eventually going to
>> SL6, that I'd just push forward with a fresh install of SL6. It took a
>> few hours that I hadn't planned on to get everything back up to speed,
>> and I have one issue yet to work and a couple of minor things to
>> configure.
>>
>> I have another retired box with the same RAID setup and it too, got
>> hosed by the live CD. I'm going to play with this box to see if I can
>> find a way to recover from this.
>>
>> Beware.
>
> Thanks for letting us know!
>
> I would be very interesting in a recovery procedure.
>
> I guess that it should be possible to re-assemble the raid device. So in
> case /dev/md0 was renamed to /dev/125 and was setup with /dev/sda1
> /dev/sdb1, you may get /dev/md0 back, if you do
>
> mdadm --stop /dev/125
> mdadm --assemble /dev/md0 /dev/sda1 /dev/sdb1
>
> Are your raid partition of the type 0xfd (Linux RAID autodetect) or 0x83
> (Linux)?
>
> If this is a general problem with the LiveCD and software raids, I would
> like to fix it on the LiveCD.

Usually md devices are renamed/renumbered when "/etc/mdadm.conf" has
"HOMEHOST <system>" set and the metadata in the superblock has a
different homehost value.

You can change the homehost "setting" in the superblock at assembly
time with "--update=homehost" - assuming "hostname -s" returns the
value that you want.