Re: Requesting assistance recovering RAID-5 array

Phil Turmel
2020-04-01T18:32:18Z

Hi Daniel,

On 4/1/20 2:07 PM, Daniel Jones wrote:
> Hi Phil,
>
> So far so good.

Yes.

> # mdadm --manage /dev/md0 --add /dev/mapper/sdb1
> mdadm: added /dev/mapper/sdb1

Don't do this. Overlays can't really handle the amount of data that
would be involved, and you definitely don't want to rebuild yet.

> 4: Summary
>
> The drives have had physical partitions written.
> I think I've found the correct offset and device order to use --create
> to restore the array to the degraded state it was in before the
> superblocks were overwritten.

Yes.

> I'm not sure why the --add doesn't work.

Don't do the --add operation until you've copied anything critical in
the array to external backups (while running with 3 of 4). The reason
is that any not-yet-discovered URE on those three will certainly crash
the array during rebuild. It could still crash copying critical stuff,
but you can repeatedly --assemble --force to keep going with the next
items to backup.

Only when you've backed up everything possible do you --add the fourth
drive back into the array.

> Thanks so much for your help this far.

You're welcome.

> Regards,
> DJ

Phil