Re: Upgraded grub, now confused about mirrored /boot

[John Robinson <john.robinson@anonymous.org.uk>]

On 02/11/2010 06:11, Guy Watkins wrote:
> Hello,
>
> 	I upgraded my system from Red Hat FC10 to FC11.  The instructions
> say to run this command:
> /sbin/grub-install BOOTDEVICE
>
> And if it fails, run this:
> /sbin/grub-install --recheck /dev/sda
>
> However, my boot disk (/boot) is mirrored on 4 disks and I think (or hope)
> all 4 are bootable.  The mirrors were created at install time many years ago
> when I installed FC5.  No idea if it really made more than 1 bootable.  I
> have assumed that if sda failed, I could still boot from sdb, sdc or sdd.
> And I do understand that I might need to remove sda first, depending on the
> type of failure.  Lucky for me, no drive has failed yet and I don't recall
> if I tested booting off of any other disks.
>
> I do have this on the kernel line:
> md-mod.start_dirty_degraded=1
>
> So, what do I do now?  Run that command on all 4 disks?  Or run it on
> /dev/md0?

I don't know which grub version you get in FC10/11, but in CentOS 5 
(with grub 0.97), grub-install is a clever little script which does all 
the work for you, so you just
   /sbin/grub-install /dev/md0
and it will install grub on all of /dev/md0's constituent drives and 
generally get everything right.

You should be fine with metadata 0.90 or 1.0 as both store the 
superblock at the end of the device. 1.1 and 1.2 probably won't work 
because their constituent partitions don't look like bare filesystems.

What actually happens is that the BIOS boots off the first live disc, 
and so does grub 0.97, neither has any inherent support for RAID but 
they don't have to if it's a RAID-1 mirror because both (or all) parts 
of the mirror can be used on their own, at least for reading enough to 
boot with.

You should probably test booting up with your first drive disconnected, 
just to be sure!

Cheers,

John.