grub on raid1 cannot mount filesystem

[Tom H]

On Wed, Jun 29, 2011 at 5:16 PM, Pavel Lisy <pali at tmapy.cz> wrote:
>
> I've found strange problem in new Fedora 15. I suppose it should work.
> It's maybe fdisk or grub problem.
>
> Normaly I have this (working) multiboot solution on my home PC:
>
> On first small (100MB) partition are grub1 files only with this config
> (grub is installed to MBR):
>
> --- cut ---
> title Fedora 15
>   root (hd0,2)
>   chainloader +1
> title Fedora 14
>   root (hd0,4)
>   chainloader +1
> title Ubuntu
>   root (hd0,5)
>   chainloader +1
> --- cut ---
>
> Grub of linux distributions is installed into their partitions
>
>
> I want make the same on mirrored environment
>
> 2 HD in mirror (raid1):
>        size    id      OS/SW           FS
> 1. part 200MB   fd      grub            ext4
> 2. part 8GB     82      SWAP            swap
> 3. part 10 GB   fd      fedora 15       ext4
> 4. part extended
> 5. part 10 GB   fd      fedora 14       ext4
> 6. part 10 GB   fd      ubuntu          ext4
> ...
> last part rest. of disk - LVM
>
> I've installed Fedora 15 on
> /dev/md0 (/dev/sda1,/dev/sdb1)          /boot
> /dev/md1 (/dev/sda3,/dev/sdb3)          /
> grub was in MBR and everything worked.
> THIS WORKS:
> grub> root (hd0,0)
> grub> setup (hd0)
>
> When I want put grub of Fedora 15 to start of /dev/sda3 grub is
> complaining:
>
> THIS DOESN'T WORK:
> grub> root (hd0,2)
> grub> setup (hd0,2)
> Error 17: cannot mount selected partition
>
> Without mirror (raid1) it works. With raid1 it works only on first
> partition. Can you explain me why?

When you install grub to a partition's boot sector, a list of the
blocks "occupied" by grub's stage1 has to be stored in the first
sector of that partition. Assuming that there's enough space there for
it when you're using mdraid (I suspect that you'd have to ensure that
you're using a metadata version that embeds the metadata elsewhere), I
don't think that you can point at a file through blocks because grub
has no knowledge of mdraid - hence the "cannot mount selected
partition" error.