Changing boot disk bus connections/settings?

David Marsh drmarsh at
Sat Jan 29 13:21:44 GMT 2000

On 29-Jan-00 Richard Smith wrote:
> David Marsh wrote:
[Boot manager/booting off 'alternative' disk]
>> Is there anywhere that I can have the 'set root_disk_unit=2' stored
>> permanently?
> It's possible that you also need to change the `config' line in your
> kernel configuration file... From LINT:
> __________________________________________________________________________
># This directive defines a number of things:
>#  - The compiled kernel is to be called `kernel'
>#  - The root filesystem might be on partition wd0a
>#  - Crash dumps will be written to wd0b, if possible.  Specifying the
>#    dump device here is not recommended.  Use dumpon(8).
> config          kernel  root on wd0 dumps on wd0
> __________________________________________________________________________

This looks worth a look..

> Normally the default allows for booting from IDE or SCSI, but I suspect
> that your's is a special case.

I read somewhere (I think it was the built-in help for the boot manager)
that the boot manager expects hard disks to be installed in sequence: ie
primary master, primary slave, etc.., and gets confused if there is a 
'gap' (which is why it was looking for wd1 if left unaided - and failing,
that's my CD drive ;-). 

I could always swap the secondary master drive (ie, the drive in
question) and the CD around, but then I'd really have to dig into the case
to set the slave jumper.. (these things are always inaccessible) :-(
(..and then I'd have to change fstab again!)

>> The second issue was that upon loading the kernel, and starting up, the
>> system then found that the hard disks had moved (surprise!). Yep,
>> /etc/fstab still had the old disk partition names in it, of course.
>> Easy to fix, I thought. But no. Because the boot process hadn't
>> completed properly, / was mounted read-only, and I couldn't edit the
>> file. The only
> mount -u -w /

Duh. I suspected it was a fairly simple option to mount, but unfortunately,
because the system couldn't access the other partitions, the man page wasn't
available.. ;-(
> when you _do_ have a working fstab, `mount -a' is quite useful too.

I think I had to reboot enough times _anyway_ that mounting the rest while
running probably wasn't an issue ;-)  But I really must get out of the
"Hmm, if I can't get it to work now, a reboot _will_ sort it" mindset. I do
try, though.

>> CDs, boot with the boot floppies and then crank up the 'emergency
>> holographic shell' (ROTFL!) from the fixit floppy, and mount the root
> I've often wondered about that one, and just realised where it comes
> from :-)

It's less helpful, but less all-knowing, than the original EHDr ;-)
Running off floppy is a 'fun' experience. It reminds me of my very first
Amiga, before I got a hard disk.. <grind, grind, crunch, crunch>

>> (And a final idle-curiousity question: what files are actually on the
>> root partition, and which are on /usr? How can you tell where one
>> stops and the other starts? Since everything is placed under "/", I'm not
>> sure how I can tell what actually really lives where..)

> The default configuration has /, /usr and /var, so any other root
> directory is actually on the 30-50M root partition. 

D'oh! That was a silly question of mine! So basically, if it's not not
[sic] on root (ie, not /usr or /var (or /home, if present, or symlinked to
another partition)) then it _is_ on root!

> Except for things like /compat, /home, and /sys which are symbolic links
> to directories on /usr. 

Noted. I can see the symlinks for these if I look in /.
In fact, if I'd thought about this ;-), I'd have realised that anything
that wasn't on root would have had to have a symlink to the other partition
otherwise the filesystem wouldn't be able to find it. So it's the presence
of symlinks that lets you see what files are on which partitions. I should
have known this if I'd thought about it! Sorry..!

Many thanks for your help,


David Marsh,drmarsh at |          |
Glasgow/Glaschu, Scotland.      | If urgent, phone: +44 77-121-848-90 |
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