Re: Boot Disks

[Tom Pfeifer <tplists@optonline.net>]

Keith O'Connell wrote:
> 
> Hi,
> 
> I have clearly misunderstood the making of boot disks and would like some guidance. I made some for each machine here in case or emergency, and thought I would test them, and each one halted with a kernel panic.
> 
> I assumed that a floppy in the drive of a working machine followed by the command "mkboot" as root would create a boot disk from the currently running kernel. The disk booted to a panic.
> 
> I read over the rather small man page and tried the full complement of switches of "mkboot -r /dev/hda3 -i /vmlinuz" but still there is a panic
> 
> I have 2.2.20 and 2.4.28 on each machine, and want to be able to use a floppy to boot one kernel and another floppy to boot another kernel. How do I make these disks?
> 
> I have tried the man pages and the books on the shelf, but I am not getting it. What is the right way to create the floppy disks I want?
> 
> Anyone?
> 
> Keith
> --

There's several ways other than mkboot....

One very simple method is to just copy your kernel to the floppy disk
(the raw device), and then set the kernel's root device. Insert the
floppy, *don't* mount it, and then:

cp /boot/your_kernel /dev/fd0
rdev /dev/fd0 /dev/your_root_partition

To check the root device:  rdev /dev/fd0

The kernel will bootstrap itself, mount the root partition and go from
there.

The disadvantage of that method is that it's slow on boot, and you need
one disk for each kernel you might want to boot. It's slow because the
kernel has to uncompress and load itself from the floppy disk.

Another way is if you have Lilo installed, and already have a valid
/etc/lilo/conf for booting off the hard drive. Take that /etc/lilo.conf
and change the boot= parameter to boot=/dev/fd0. Then run 'lilo' with a
floppy disk inserted in the drive. It will write the Lilo boot sector to
the floppy disk. On boot, it will read just the one sector from the
floppy, and do the rest from the hard drive - much faster. And you will
have the same menu as when booting normally.

If you use grub, you can do a similar thing, and can also just make a
generic grub boot disk where you specify the kernel and root partition
interactively from grub's shell. The command to make the generic grub
disk is 'grub-floppy /dev/fd0'. If you like grub, it's not a bad idea
to have one of those around.

Tom


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