APC Trivia

From: John Gray <johng@sodapop.org>
Newsgroups: comp.unix.sco.misc
Subject: Re: Can I boot from secondary Hard Disk?
Date: Fri, 01 Sep 2000 21:20:43 -0700

Getting SCO Unix to boot if it isn't the active partition.

Contrary to popular opinion SCO Unix can and will boot if it isn't the active partition, but it does take some tweaking.

First some background info. The SCO disk driver has a concept of the active partition and the default kernel/installation configuration uses this when accessing the disk. This allows the correct kernel and partition to be accessed no matter how many changes are made to the boot drive fdisk table as long as it is active. If you have multiple SCO installs on the same drive, just make the one you want active, it just works, but that's problem. The disk driver also has the concept of absolute minors too. These minor numbers point to partitions/divisions on the drive that aren't affected by what is active.


  1. The minor numbers 40 - 47 are floaters, that is to say that those numbers will track with the active partition.

  2. The concept of the active partition is divided into 3 major groups, boot, kernel and utilities.

    1. Boot:

      The boot program also knows about the driver minor numbers. If you want "boot to do what you want and not what your tell it" you need to give it the correct instructions. When the /boot program is loaded it looks in /etc/default/boot for instructions to load the next program. By default this is hd(40)unix. If you want the boot program to always look for and load the unix kernel on the boot division of the second partition, the number 40 needs to change to 16. The /etc/default/boot file will now contain a line that says hd(16)unix instead of hd(40)unix. This will load the correct kernel.

    2. Kernel:

      The kernel has compiled in defaults for the root filesystem, swap and dump. The dump device is where the panic image is written. These can be changed with a boot string option but why take the risk. The file /etc/conf/cf.d/sassign contains the kernel defaults for where it will look for these devices.

      swap hd 41
      dump hd 41
      root hd 42
      swap hd 17
      dump hd 17
      root hd 18

      Edit this file and then relink your kernel. ./link_unix.
      Don't reboot the new kernel yet.

    3. Utilities:

      So far boot will find the correct kernel and the kernel will load. That's about it though. Remember that all the utilities that use the various devices that point to /dev/swap, /dev/root, /dev/home and /dev/data won't be able to find them. They are looking for devices that still have minor numbers that aren't any good, they point somewhere else. The driver doesn't care where the minor number points. It's job is to look where it's told and get blocks.

      To change this there are two places that need to change, the place that controls how device node get made and the existing device nodes.

      The /etc/conf/node.d/hd file has a list of the disk nodes and the minor numbers to use when creating them. The entries that have numbers 40 - 47 will need to change to 16 - 23. Changing these will insure that a kernel relink won't mess up the changes you are about to make.

      The /dev, /dev/dsk and /dev/rdsk directories have the current device nodes. carefully look for the nodes which have a major number of 1 and a minor in the range 40 - 47 and remove them one at a time. When you remove one recreate it with the correct (new) minor number. Remember to use the correct b or c flag to mknod too. For example

                       rm /dev/home
                       mknod /dev/home  b 1 19
                       rm /dev/rhome
                       mknod /dev/rhome c 1 19
      It take a while but once you are done you can test your work with the mount command, divvy and sar. If you relink a new kernel and something doesn't work you know it is in the node.d directory and you will also need to track the node down in /dev. Remember that the changes you made to the device nodes are valid with the old kernel. A floating minor pointing to the same place as the absolute minor is still the same place.

    Once you have smoke tested the settings reboot.

Check out SCO TA (103327) or use man HW hd locally for more detail on minor numbers.

Comments and suggestions always welcome - tom@tkrh.demon.co.uk