FC2 - Booting from remaining raid disk

[Ed Wilts]

On Tue, Oct 12, 2004 at 04:05:45PM +0100, Matthew Claridge wrote:
> I've got a basic FC2 installation, using software raid 1 across two IDE 
> hard disks. Each hard disk is the master drive on its controller (so one 
> is Primary master and the other is secondary master).
> 
> If I remove the secondary master drive and boot the machine, all is 
> well. If I remove the primary master, linux fails to boot (the bios 
> cannot find an OS to boot). Even if I connect the secondary master hard 
> disk to the primary master controller, it still cannot boot. So its as 
> though linux hasn't mirrored the boot image successfully across the two 
> disks.

Exactly.  There are 2 types of RAID 1 out there - one way is to mirror a
physical volume (like VMS does) which mirrors everything.  The other way
(like Linux does) is to mirror data partitions.  There are pros and cons
to both, and you're getting hit by the con of the Linux approach.
 
> There are three partitions on these disks, /dev/md[0-2]. /boot is on md0 
> and / is on md1. Grub is installed on /dev/md0.

The boot block is what you're missing, and /boot isn't the same as the
boot block.  The BIOS reads and executes the first physical sector of
the chosen boot media on the systme.  Usually, this is contained in the
first 512 bytes of the hard disk.  This is your 1st stage loader.  The
boot loader is not part of any file system so software mirroring will
not duplicate the data.  When you tell grub to install on /dev/md0, it
doesn't actually write a boot block to both mirror members.

> There's an easy way around this, which is to use a boot disk, which 
> should boot regardless of which disk has failed, and I know that linux 
> will carry on working happily if either disk fails, but I'd like to find 
> out why its misbehaving and correct it so that it can boot normally 
> after a disk failure, making the whole thing a bit more resilient, 
> especially if there's a delay obtaining a new hard disk or something.

What you need to do is duplicate the boot block.

http://www.redhat.com/docs/manuals/enterprise/RHEL-3-Manual/ref-guide/s1-grub-installing.html

        .../Ed

-- 
Ed Wilts, RHCE
Mounds View, MN, USA
mailto:ewilts at ewilts.org
Member #1, Red Hat Community Ambassador Program