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Re: booting from a raid1

From: Michael Evans
Subject: Re: booting from a raid1
Date: Fri, 8 Oct 2010 13:17:10 -0700

On Fri, Oct 8, 2010 at 9:34 AM, lee <address@hidden> wrote:
> 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
> mounted.
> 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?
> _______________________________________________
> Help-grub mailing list
> address@hidden

If you're doing root off of software raid under Linux one of two
things is happening:

Thing 1: Kernel + initrd; md can be module or compiled in; some method
for adding devices to raid arrays (I typically pull in a full mdadm
for my custom initrds, but more minimal options exist that don't
provide as many disaster recovery tools).  Part of the pre-root
environment will setup the basic system devices and then transfer
control to them when the initrd/initramfs is complete.

Thing 2: Older deprecated; using md 0.90 labels in kernel (not as
module) auto assembly.

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