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The Answer Guy 31: Lilo not working on SCSI when IDE drives installed
"Linux Gazette...making Linux just a little more fun!"
 Lilo not working on SCSI when IDE drives installed
From Timm Gleason on Wed, 08 Jul 1998
in the
comp.unix.questions
newsgroup
We have several Debian/Linux
workstations around the office that
contain both SCSI and IDE devices. Now we almost exclusively use
SCSI hard drives but occasionally we will stick an extra IDE drive
into a box when more disk space is needed. My problem is this, when
we recompile a new kernel after a security patch of fix, and then
run lilo it gives the error `Warning:/dev/sda2 is not the first
disk`. So then I have to halt the machine, physically remove the IDE
device, boot with a rescue disk, rerun lilo, halt the machine,
reinstall the IDE device and reboot. Now while I admit that this
does work, it can be a real pain to have to do that to every
workstation we have that has mixed devices installed.
Thanks
Timm Gleason
Normally there is no problem with integrating SCSI and
IDE drives in a system (under Linux).
However I think I have a hint as to your problem.
A normal PC BIOS will boot off of the first IDE drive
on a system. It will only look at the MBR (master
boot record) on the first hard drive (usually after
looking for a boot record on only the first floppy).
The normal consequence of this is that you usually have
to install your OS on the first hard drive. IDE drives
conform to an interface which is built into the BIOS.
With SCSI there is a BIOS extension on the controller's
ROM that allows the system to boot.
BIOS extensions are code that's contained on the ROM's of any
expansion card -- so long as it contains a specified header
and calling convention. Part of a normal PC boot process is
to scan the reserved address space between A0000 and E0000 (or
so -- it might be from B0000 through D0000, I don't have a
BIOS reference handy) for this "BIOS extension signature" ---
and then to call setup routines an give offsets from any of
these signature blocks that it finds. That is how SCSI
controllers and ethernet boot PROMS (and other "bootable"
devices) work.
The problem is that most SCSI controllers and PC BIOS' will
give the IDE drives precedence over any SCSI drives.
One approach would be to let the system have "its" way
and let lilo write the MBR to the IDE drive. So long
as your BIOS (with extensions) can see the SCSI drives
it doesn't matter that the lilo boot block refers to another
drive.
An odd and confusing thing about Linux is that the boot
block, the kernel, and the root filesystem can all be
on different devices. In fact you don't need a "boot block"
at all --- you can use LOADLIN.EXE to start a Linux kernel
(I've heard it can be used to load a FreeBSD kernel, too ---
but they have their own program for that anyway).
So, if you have a /etc/lilo.conf that looks like:
boot=/dev/hda
read-only
prompt
timeout=200
image = /vmlinuz
root = /dev/sda1
label = example
... and you run /sbin/lilo to "compile" this set of
configuration directives into a boot block here's what
it will do:
Write the boot block to /dev/hda
note: we use the raw device, so it goes to the MBR,
if we use /dev/hda1, it goes to the "logical boot
record" or "superblock" of the specified (first)
partition.
The 'read-only' directive is passed to any Linux
kernels that you load --- it specifies that the
root filesystem will be initially mounted read-only
to allow the fsck to work properly on it.
The 'prompt' directive is used to control the behavior
of the LILO boot program at it's runtime (it forces it
to print an interactive prompt to the console during boot)
and the 'timeout' directive specifies how long the
prompt will wait before continuing to the default
boot "stanza" (the first, and in this case, only one).
The image directive points /sbin/lilo to the location
of a kernel image (file). Note the distinction between
/sbin/lilo's "compile time" and the lilo boot code's
"run time" (that's why I push this analogy of treating
the lilo package as a "boot block compiler" --- it
helps put these distinctions in familiar terms).
This directive causes /sbin/liloi to "map" the device
number and address of the image file. That address can
be in different formats depending on whether the
"linear" directive is used to inform /sbin/lilo that the
device is being accessed through "sector translation."
The 'root' directive specifies where the root filesystem
for this "stanza" is located. That is passed to the
kernel unless over-ridden by parameters that are entered
interactively at the prompt.
Obviously the 'label' directive allows the user to
select different boot stanzas by name at the lilo
prompt. (Not that the "lilo prompt" refers to run-time
rather than "compile-time" --- it is the "boot block code"
that is doing the prompting and /sbin/lilo is utterly
uninvolved at that point).
I've played with a number of configurations of lilo.conf over
the years. I've written boot blocks to floppies, specified
image files that were on mounted (non-root) filesystems,
and done other silly things. Lilo's flexibility can be
very confusing and I've tried many things that didn't work
(usually for good reason --- Lilo is constrained by the
limitations and conventions employed by your BIOS).
If you try this and it doesn't work you might need to
put a tiny partition on that drive. You can put a
very small minix filesystem on that (conceptually you
could put a kernel and no fs in a single track ---
but /sbin/lilo doesn't have a way to "find" that).
For that matter you can put a Linux kernel file on
any fs that you put on that IDE drive. If you
put a kernel image on an MS-DOS filesystem and
run /sbin/lilo while that fs is mounted (it will
complain if it cant find the kernel images at
"compile time") you should be fine (until you run a
DOS defrag utility!).
A wholly different approach is to to lie to your CMOS
setup and tell it that you have no IDE drives
installed.
Linux doesn't rely on your CMOS settings or BIOS to
detect or use IDE drives. So, once Linux is loaded
you should be able to see your IDE drives even if they
are not listed in your CMOS configuration.
(I'm pretty sure I did that once and I don't remember
having to do any tweaking to get it to work. It might
be dependent on your kernel version and compilation
options, though).
Obviously, you're doing something a little different
--- most people who mix IDE and SCSI drives start with
the IDE's and always let them be the boot devices. That's
the path of least resistance. However, you can try
the tricks I've described --- let me know if removing
the CMOS parameters for the drives works (or if I'm
suffering from delusional memories).
Lilo not working on SCSI when IDE drives installed
From Timm Gleason on Fri, 10 Jul 1998
Also posted in Newsgroups:
comp.unix.questions
Thanks for your prompt reply. Many of the things you mentioned we have tried
at one time or another with limited results. We are suing Asus P2L97
motherboards which have the ability to set in the BIOS which drive type you
want it to look at first. We have even removed the IDE drive from the BIOS
settings. The problem is that since Linux does not rely on BIOS settings for
IDE drives it always detects them first. And typically we are adding old
extra IDE drives for expanding storage or the CD-ROM drive in the machine is
an IDE.
It's true that Linux doesn't rely on BIOS settings.
However, the lilo boot loader does. It is the boot loader
that is looking in the wrong place. This is probably a
bug in your BIOS (or still in your configuration of that
BIOS, as it sits in the CMOS registers).
I have yet to try setting boot=/dev/hda and root=/dev/sda in the lilo.conf.
I was under the impression (mistakenly I guess) that the MBR and root disks
had to be on the same partition.
The fact that this is a very common misconception explains
why I take such pains to explain it in so much detail.
I must have gone over this about twenty times in LG and
about a hundred in the newsgroups.
Also, a nitpick, don't think of them as "root disks" ---
think of them as root filesystems.
An MBR consists of a boot record and a partition table. The
boot loader points to a kernel (or several) --- or it might
point to a secondary boot loader. The Linux kernel contains a
pointer to the default root fs. The LILO boot loader (from
the MBR or from a floppy or any logical boot record) may pass
parameters (such as video mode and root fs parameters) to the
kernel to over-ride the one that's built-in. The 'rdev'
command can write a new rootfs string into a kernel (and it
can tweak other values like "video mode" --- look at the
'rdev' man page for details).
Thanks Again
You're welcome.
Copyright © 1998, James T. Dennis
Published in Linux Gazette Issue 31 August 1998
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