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Re: Use RAID1 mirror as backup during dist-upgrade?

Reiner Buehl <reiner@buehl.net> wrote:
> I have a Debian Etch system that runs on a RAID 1 software raid system. 
> now I would like to upgrade it to Lenny by splitting the mirror off and 
> keep one mirror as a backup. Alternatively I could add a third disk as a 
> second mirror and split off this one.
> If the upgrade works, I then add in the old disk again. If it fails I'd 
> like to be able to rebuild the array using the OLD mirror that I kept.

Pretty reasonable.

> Does anybody have a more detailed description on how to do this? I know 
> I can fail a drive with mdadm, but my understanding is that the data on 
> this this drive can't then be used again after that.

I don't have details to hand but the general principles may help. Be
aware that this is from memory, although I have successfully done it.

    *	When you fail the mirror, do it in single-user mode after a sync
    *	Note which physical disk is live (eg /dev/sdl) and failed (sdf)
    *	Boot your rescue CD and ensure you can differentiate sdl and sdf
    *	BACKUP your system. Or at the very least, the important bits

    *	Fail the mirror (/dev/sdl == live, sdf == failed)
    *	Use mdadm to zero the RAID superblock on sdf ONLY
    *	Trash/upgrade/reinstall using RAID on /dev/sdl. Ignore sdf
    *	When you're happy, bring in sdf (possibly with --force)

If it all goes badly...
    *	Boot from the rescue CD
    *	Use mdadm to zero the RAID superblocks on sdl AND sdf
    *	Use mdadm to create a RAID1 wrapper around sdf
    *	Bring in sdl as a new disk for the RAID1 array
    *	Update fstab, menu.lst, etc. to reference the new /dev/md* device


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