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The Answer Guy

By James T. Dennis, tag@lists.linuxgazette.net
Starshine Technical Services, https://www.starshine.org/

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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