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

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Date: Tue, 06 Sep 2005 11:44:40 +0100
To: For users of Fedora Core releases <fedora-list@redhat.com>

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.

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