Why is 'disklabel'ng a new drive so difficult?

Antony Mawer fbsd-questions at mawer.org
Thu Mar 29 23:56:00 UTC 2007

On 30/03/2007 9:22 AM, Jerry McAllister wrote:
> On Fri, Mar 30, 2007 at 08:07:23AM +1000, Antony Mawer wrote:
>> Is it important to use 16 as the offset still, or is this a historical 
>> piece of information that is no longer relevant? Or is this is a bug in 
>> disklabel that should be fixed?
> As I indicated in another post in this thread, it appears to
> be vestigial.    I have never used it for a bsdlabel(disklabel)
> being done on a slice - since 1998.

I just went back and re-read your other messages in the thread. I must 
have glossed over that part of them - my apologies! I too looked at my 
sysinstall-created labels, and they were all at offset of 0.

I actually started writing my own partitioning/labelling tool based on 
libdisk, as part of a custom install CD I was building, but discovered 
that it did not support non-disk devices (eg. gmirror)... I started 
looking at trying to hack support into libdisk to do so (and made some 
success), but in the end decided that it was probably a task better 
suited for someone that knows libdisk better than I...

As a result I went back to looking at fdisk/bsdlabel to see what I could 
do using them instead...

> There seems to be a lot of left over stuff in the documentation and 
> man pages for fdisk and bsdlabel (and disk formatting, partitioning
> and booting in general).   Someone made a pass at cleaning them up 
> about 6 years ago and that helped, but it could stand to be done some 
> more.  If I felt knowledgeable enough, I would take a whack at it.  
> But there are too many holes (not wholes) in my knowledge.   I would
> guess from posts in the list that a lot of people are in that position - 
> knowing a bunch of it, but not quite enough to be authoratative about it.
> I have written several long replies to questions on this list that
> could be the basis for FAQs or HowTo-s, but they still leave a lot
> of things out and generalize or slide over lots of other things for
> the sake of convenience, avoiding confusing a newbie and/or not being 
> sure about all the details.

I can attest to that -- I would love to see a clear, newbie friendly 
explanation on disk geometry, and why it is/isn't relevant in this day 
and age. The big scary warnings sysinstall likes to throw up made me 
think it must have some significance, but from days of 
searching/reading, the general gist I came up with is that geometry was 
a largely obsolete concept (as most things use LBA for addressing, 
including /boot/mbr from what I could tell), largely only relevant if 
you have other operating systems on the drive, in which case all OSes 
needed to agree on the drive geometry in order for the fdisk slice table 
to make any sense to all of them...

In that case, can anyone comment with any knowledge if geometry 
"fix-ups" are only necessary if the drive is shared with non-FreeBSD 
operating systems? Or are they important for a drive (non-dangerously 
dedicated) with just a single FreeBSD slice on it?

If they are needed, should some of the sysinstall magic be added to the 
command line fdisk tool as well (as an option), so it can perform the 
same modifications if it detects non-sane BIOS C/H/S values?


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