Re: grub2 and Linux software RAID devices

[Sam Morris <sam@robots.org.uk>]

On Mon, 04 Feb 2008 23:43:38 +0100, Robert Millan wrote:
> On Mon, Feb 04, 2008 at 09:36:45PM +0000, Sam Morris wrote:
>> Hi there,
>> 
>> A while ago, I tried grub2 on my Debian system, which has my root
>> filesystem on a Linux software RAID-1 array. I ran into some problems,
>> and while they were raised here, nothing really came of them. Robert
>> Millan suggested I post my problem again to see if anything can be done
>> to fix it.
>> 
>> I think my problems stem from the Promise IDE controller that my second
>> disk is connected to. It does not support 48-bit LBA addressing, and so
>> any attempt to read the end of the disk using BIOS calls will fail. Of
>> course, once an operating system has loaded its own driver for the
>> controller, the disk can be read correctly.
>> 
>> Here's what the two disks look like:
>> 
>>         Model: Maxtor 6L300R0 (ide)
>>         Disk /dev/hdb: 300GB
>>         Sector size (logical/physical): 512B/512B Partition Table:
>>         msdos
>>         
>>         Number  Start   End    Size   Type     File system  Flags
>>          1      32.3kB  543MB  543MB  primary
>>          2      543MB   300GB  300GB  primary  ext3         raid
>> 
>> The first partition is swap, the second is the root filesystem.
>> 
>> The first problem is the operation of the grub-probe partition.
>> Debian's post-install script runs the following command to determine
>> which modules to include in the generated core.img file:
>> 
>>         grub-probe --target=partmap --device-map=/boot/grub/device.map
>>         /boot/grub
>> 
>> Which fails with the error:
>> 
>>         grub-probe: error: Cannot detect partition map for md0
>> 
>> It appears that grub-probe expects to find a partition table inside the
>> RAID device, when of course, it is really in its containing device,
>> /dev/hdb.
> 
> This is a known problem, and I roughly have a solution in mind, but I
> haven't been able to reproduce it.  When I try to install Debian with
> /boot inside an LVM, the installer hangs.  This option doesn't seem to
> be supported at all.

Ouch... I have never tried /boot on LVM myself (since grub legacy can't 
handle it). However /boot on RAID works fine, I suggest you try that 
instead.

> 
> Furthermore, how do you boot that system with GRUB Legacy?

As far as grub1 is concerned, /dev/hdb2 is a normal partition containing 
an ext3 filesystem.

> If you give me some details on how to reproduce the scheme in which
> /boot is behind a lvm/raid abstraction, I could try to get this fixed.

It's pretty simple, assuming you are using d-i. When partitioning, 
configure two disks with identical partition layouts (a single partition 
on each is sufficient). Then, tell partman that you want to use them as 
'physical volumes for RAID'. A new option should appear, 'configure 
RAID' (or something similar). Here you can create a RAID1 array using 
both the partitions, which you can use as an ext3 filesystem, mounted 
at /.

>> Debian's post-install script has actually been written to substitute
>> 'pc gpt' if the partmap probing fails,
> 
> Actually, this was reverted a while ago.
> 
>> manually. However, I now hit the second problem: the menu that grub
>> presents has no text! It seems to have an entry, however, as there is a
>> highlighted line.
> 
> Which version did you try?  Is it more recent than 1.96 ?  We fixed bugs
> producing this result recently.

Damn, I installed the version from testing by mistake. I will try again 
with version 1.96+20080203-1.

>> Jeroen Dekkers previously suggested a patch to suppress the 'out of
>> disk' error, at
>> <http://www.mail-archive.com/grub-devel@gnu.org/msg02873.html> but no
>> one ever committed it.
>
> Ah, I see.  The patch looks correct to me;  only the description you
> gave before isn't.

Hmm, I don't understand the difference between your changelog entry and 
my description, but ok. :)

-- 
Sam Morris
http://robots.org.uk/
 
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