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Re: grub floppy snafu

From: Haines Brown
Subject: Re: grub floppy snafu
Date: Sun, 3 Oct 2004 09:35:36 -0400 (EDT)

> > When I upgraded from debian woody to sarge, grub (both new and old
> > versions) began to map my drives incorrectly. My "work-around" has
> > been to change the root command to compensate for the mapping
> > error:
> > 
> >      sda drive 0       root (hd0,0)
> >      sdb drive 1       root (hd2,0)
> >      sdc drive 2       root (hd1,0)
> > 
> > Will this work-around cause me any problems? 
> Hard to say . sometimes stuf just works for months on end without
> problem . and then the next boot and you are in trouble.

Sorry, what I meant was trouble _after_ a successful boot, not getting
the boot itself to complete. But you do seem to hint that mapping can
change for no reason at all, and so I guess I should be less concerned
that a simple upgrade to debian sarge should cause drives to be mapped

> Here are some examples of mappings while I was running 3 HD's sometime
> ago. May be of some help to you.
> title windows xp
> root (hd2,0)
> map (hd0) (hd2)
> map (hd2) (hd0)
> makeactive
> chainloader +1

I tried this approach again, but still without any luck. For example,
a hard disk that is set in BIOS to be the third disk is seen by grub
as the second disk. So I tried to boot the third disk:

  title ...
  root (hd2,0)
  map (hd1) (hd2)
  kernel ...

This had no effect. Grub is still seeing the third disk as hd1 and
tries to boot the BIOS' second disk.

> > My "work-around" has
> > been to change the root command to compensate for the mapping error:  
> >
> >      sda drive 0       root (hd0,0)
> >      sdb drive 1       root (hd2,0)
> >      sdc drive 2       root (hd1,0)
> This is not incorrect, 

OK, so I gather I can remap in this way without risk and without
having to use a device.map file or the map command in the menu. 

> In order to change those device numbers you have to change things in 
> the BIOS.

Understood, but my instinct is to adapt grub to BIOS, not the BIOS to

> > Instead of using the root command, would I do better changing the
> > map, either the map file in /boot or placing a map command in the
> > grub menu? I tried doing both, but without any effect.
> What do you want to do?  If that is changing the mapping in
> GNU/Linux (or whatever you use), change device.map.

I tried putting this device.map file in the floppy /boot/grub

  (hd0)   /dev/sda
  (hd2)   /dev/sdb
  (hd1)   /dev/sdc

This looks like it should take the disk defined as third in BIOS (sdc)
and remap it to be grub (hd1). However, this does not work (I'm not
sure one can add a device.map file to the grub boot floppy).

What do I want to do? I gather there are four methods to take care of
a mapping issue: 

  a) reorder the drives in BIOS (but then grub would just get them
     wrong again, I suppose),

  b) use a map command in the grub menu stanza (tried, but had no

  c) add a device.map file to /boot/grub (tried, but had no effect),

  d) the method I used successfully is simply to point the
     root command at what grub incorrectly thinks is the correct
     disk (so, for the third disk, I use `root (hd1,0)').

The problem is that I've never seen mention of my method (d), and so
I'm concerned that I may not get a clean boot. The reason for my
concern is that I see peculiar message during boot (although dmesg at
the end looks OK) and, more seriously, I'm have problems installing an
X Windows system. I skip specifics about these things here, but only
need some assurance that my method (d) for remaping disks is not a
possible source of difficulty.

Haines Brown

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