[techtalk] SCSI/IDE/booting...the other thread

[James Sutherland]

On Thu, 24 May 2001, Brian Sweeney wrote:

> Ok, here's a situation I've witnessed but cannot explain, and I'm sure
> that's because once again there's a mack-truck sized hole in my Linux
> knowledge I'm unaware of. ;-) Here goes:

Hmm... boot-related major hole, and your e-mail address includes the word
"physics"... deja vu, anybody? :)

> Now, near as I can tell, lilo has always pointed to the SCSI device, and
> therefore it has always been the boot device.  Indeed, the boot info is on a
> scsi device, as is the root partition.  I thought, however, that Linux
> *ALWAYS* had to boot off the first device present, which if you have an ide
> drive on your system, with most bioses it (the ide) will be the first
> device.  Yet, this thing had both, and seems to have booted off the SCSI?

Quite possible: booting is controlled by the BIOS. You can set the BIOS to
try booting from SCSI first, which is what your friend has probably done.

Ignore the IDE drive entirely: you're booting from the SCSI drive.

> I became aware of this after he upgraded the kernel, rebooted the machine
> and got "LI".  Being a good user, he of course did not have a boot disk for
> the machine before doing this.  So, we booted off a 6.2CD into rescue mode,
> mounted the SCSI root partition (there is no separate boot partition),
> chrooted to it and ran lilo.  It rebooted just fine (with his old kernel)
> off the scsi device with the ide device attached.  I created a boot disk
> with the old kernel, and we tested it out; it worked fine.  So at that point
> I decided to edit lilo.conf to include the new kernel, and re-installed
> lilo.  This time, on reboot, I got the infamous "LI".
>
> Clearly, I have no idea what's going on.

LILO loads your kernel by calling the BIOS disk SWI (software interrupt).
This identifies disks by number (0x80 = first HDD). With only a SCSI HDD
present, 0x80 is clearly the SCSI HDD; adding an IDE HDD probably moved
the SCSI disk to 0x81 and the IDE disk took over 0x80.

You need to tell LILO about this change; adding the following lines to
lilo.conf should do that:

disk=/dev/sda
	bios=0x81
disk=/dev/hda
	bios=0x80

(from "man lilo.conf")

and re-run lilo.


James.