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Re: booting from a raid1

From: lee
Subject: Re: booting from a raid1
Date: Fri, 8 Oct 2010 18:34:29 +0200
User-agent: Mutt/1.5.20 (2009-06-14)

On Fri, Oct 08, 2010 at 10:30:23AM -0500, Chris Weber wrote:
> >> Now I can boot from /dev/sdd, but the kernel panics because it can't
> >> mount the root filesystem (/dev/md0p2). Entries in /etc/fstab on md0p2
> >> are correct. I take it the md-devices aren't up/accessible in time.
> >>
> >> Any idea what's missing? Like there's no RAID support in grub?
> >>
> >> _______________________________________________
> >> Help-grub mailing list
> >> address@hidden
> >>
> >>
> >
> > Have you made a new initrd that has md and/or raid support?  I ran
> I'm guessing that you might need an extra module for raid support and
> maybe some extra lines in init, but maybe this will help get you
> started.

All that's needed for booting is compiled into the kernel, I'm not
using initrd at all. Not using initrd makes things a lot easier ...

The question probably is why the kernel doesn't seem to know about the
RAID devices. If I understand things correctly, those might be started
only after the root partition has been mounted. If that is true,
having the root partition on a software RAID device would be generally
impossible --- unless grub (or whatever else) does something to make
them available to the kernel so that the root partition can be

Now I can speculate that when having a root partition on a software
RAID device that is not partitioned, the boot process is cheating in
that the kernel first mounts the root partition from one of the
physical disks the RAID device is made from and later somehow changes
to the actual RAID device. That might explain why it's not possible to
boot from a RAID5.

Does anyone know how this works?

There have been Debian installers that asked the user who created RAID
devices during the installation which devices would have to be brought
up at boot time. Recent installers don't seem to ask that
anymore. This leads to wondering why the partition type "raid
autodetect" is deprecated and wheather it is nevertheless required
when the root partition is on a RAID device or not.

In any case, before the root partition is mounted,
/etc/mdadm/mdadm.conf cannot be read. So how can the software RAID
devices be brought up before the root partition is mounted?

How does one tell grub to bring up the software RAID devices? It seems
that the modules "raid" and "mdraid" are required, and I've put them
into the grub.cnf. Perhaps I also need to put some information into
grub.cnf about what physical devices/partitions to use to bring up the
RAID devices. But how do I do that?

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