FOLLOW-UP: RAID-1 (mirroring) disk failed; now what? [failed]

[Paul Howarth]

Will Partain wrote:
> * A _big mistake_ that I made! --
> 
>   At some point, with my new disk (/dev/sda) in and booted off the
>   rescue CD, I did something like...
> 
>   dd if=/dev/sdb2 of=/dev/sda2
> 
>   ... i.e. brute-force copy all of a raid-1 partition onto its
>   presently-empty cousin.  Theory: "/dev/sda is empty and not in play;
>   what harm can it do?"
> 
>   Answer (I think): Lots.  The RAID software snoops around on these
>   (type 'fd') partitions and silently decides what to make of the
>   situation.  This is really not what you want in this delicate
>   state.  Information is OK ("I've spotted a degraded array on
>   /dev/sdb2, which seems odd"), and doing nothing is OK, but "being
>   helpful" isn't.  (Is there a kernel boot parameter to turn off RAID
>   cleverness?)

Doing this copies the UUIDs that the RAID software uses to identify each 
partition of the RAID. It's the RAID equivalent of having identical 
filesystem labels on two partitions so that mount doesn't know which one 
to use when you use the "LABEL=" syntax in fstab. You're best off just 
creating the partitions and adding them to the array; the RAID software 
will then restore the contents from the other mirror in the background 
whilst the system is running.

Paul.