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Re: install on multiple disks


From: Andrey Borzenkov
Subject: Re: install on multiple disks
Date: Wed, 26 Dec 2012 11:19:56 +0400

On Wed, Dec 26, 2012 at 2:56 AM, Ross Boylan <address@hidden> wrote:
>> > The online manual is cryptic.  It suggests
>> > grub-install /dev/sdb
>> > might work but....
>> > My /boot is on a separate partition, also known as /dev/md0
>>
>> Well ... current GRUB2 support MD disks so in this case you do not
>> need to do anything special. I have no idea whether grub 1.96 supports
>> it. If not, you are likely out of luck.
> RAID1 should, and has been, working either way.  That is, /dev/sdb2
> looks the same as /dev/md0, except maybe for some control information at
> the end.
>

You misunderstand. When I say "current GRUB2 supports MD" it means
exactly that - GRUB2 provides native drivers, it assembles MD array
and is using it, and not individual disks. It applies not only to
RAID1 but to other RAID levels as well.

>> 3. It also stores code in core.img to search for device where
>> /boot/grub is located at runtime. It normally does it by searching for
>> filesystem UUID.
> This is the part that concerned me: getting the right disk and directory
> in this search.
>
> I assume that means it will look for the RAID, not the underlying
> physical disk.  Or maybe the filesystem on the RAID?
>

Yes, exactly. It will assemble RAID and search for filesystem. This is
similar to mount by UUID in Linux. Physical name of device does not
matter - filesystem will always be found.

> So if it's looking on a separate boot partition, it should search
> for /grub, whereas if everything's on one partition it would need to
> search for /boot/grub.
>


grub-install knows it and computes correct paths during installation.


> What I'm looking for is something that will really boot even if I change
> which disk is first;

grub2 makes it much more robust than grub legacy.

> Of course, if grub 2 uses the RAID device md0 or the filesystem on it
> that will be insensitive to the physical disk changes I'm contemplating,
> and so maybe it will just work.
>

Please understand that I always refer to current grub2, which is
version 2.00. Version 1.96 is ... I do not know, over three years old.
So you are really better off to ask on your distribution forums or
update grub2 if possible.



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