CentOS - Mar 2023 - Mount removed raid disk back on same machine as? original raid

On 3/14/2023 10:03 AM, Robert Heller wrote:
> At Tue, 14 Mar 2023 09:50:33 -0400 Bowie Bailey <Bowie_Bailey at
BUC.com>,? CentOS mailing list <centos at centos.org> wrote:
>
>> I know I will have to bring the drive online as a broken array, but
I've
>> done that from other systems.???? The only question there is can I
simply
>> rebuild it with a different name.???? I assume I can just do
"mdadm -A
>> --run /dev/md0 /dev/sdc1" (possibly with "--force" due
to the broken
>> array) even if sdc1 was originally part of the existing md127 array?
> This should work.
Unfortunately, no.? The system would not mount the drive without some 
other changes.? Listing the process here for anyone else who comes 
across this thread.

Trying to start the array (with or without --force), fails with the message:

 ??? mdadm: Found some drive for an array that is already active: /dev/md127
 ??? mdadm: giving up

I found with some digging that I needed to change the UUID of the drive 
to be able to mount it separately from the existing array.? I used 
"sgdisk -G /dev/sdg1" to do this.? It worked, but gave quite a few
scary
warning messages in the process.? A better idea would have been to use 
uuidgen to generate a random uuid and then start the array like this:

 ??? mdadm --assemble /dev/md99 --update=uuid --uuid=<newuuid> /dev/sdg1
(might require --force for a broken array, I'm not sure since I didn't 
actually do it this way)

Once the array is running, there is another problem.? Attempting to 
mount the array gives another error:

 ??? mount: wrong fs type, bad option, bad superblock on /dev/md99,
 ??? ??? missing codepage or helper program, or other error

Useless error message.? You have to look in dmesg to see the actual problem:

 ??? XFS (md99): Filesystem has duplicate UUID 
7e237dbd-6c24-4781-98d1-a1ae80a3ed13 - can't mount

I would assume you would have a similar issue with any other 
filesystem.? In my case, since it is XFS, I used uuidgen to generate 
another random uuid and then updated it like this:

 ??? xfs_admin -U <newuuid> /dev/md99

After that, the filesystem mounted normally.

Hopefully that is helpful for anyone else who finds themselves in this 
situation.

-- 
Bowie