[TUHS] Re: Question about BSD disklabel history

[Grant Taylor via TUHS <tuhs@tuhs.org>]

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On 12/31/23 11:38, arnold@skeeve.com wrote:
> The different overlapping partitions predates disk labels.

Okay.  That in and of itself doesn't surprise me much that a convention 
of overlapping partitions was carried forward from the driver based 
partitioning into label based partitioning.

> Up to and including 4.3 BSD, to change the size of partitions on a 
> particular disk, you had to recompile the kernel.

So I've learned over the last couple of years as I read more about Unix 
history.

> They were that way so that if you had multiple disks, you could use 
> one for root + swap + some thing small and use another whole disk 
> for a single filesystem.

I'm not understanding how /overlapping/ partitions helps make use of 
portions of disks.

Maybe I should back up and ask for clarification.  What /overlapping/ 
partitions means in this context?

My naive assumption was that partition -- I use that term loosely -- "c" 
overlaps / contains / all other partitions on the disk; "a", "b", and 
maybe "d".

I'm eliding the "c" MBR partition vs "d" entire drive" distinction for 
the moment.

I see some value in the "c" partition being the entire disk as used by 
BSD so that it's possible to point backup / restore / copy utilities at 
the entire disk.

But I don't understand value in having partitions overlap / contain each 
other's blocks, save for backup via "c".

I do see some value in extending the "c" is the entire MBR partition 
methodology to "d" is the entire disk containing multiple MBR 
partitions.  Again, the value seems to be in backup and recovery.

But I still simply do not understand why I would ever want partition "e" 
to be blocks 100-199, partition "f" to be blocks 195-299, and partition 
"g" to be blocks 295-399.  What value is there in having partitions e, 
f, and g overlap each other?

I get dd if=/dev/<something>0c of=/dev/rmt.  Or even /dev/<something>0d.

I fail to understand why I'd ever want other partitions to overlap.

> It was also helpful, if you had the drives, to nightly dd your real 
> root to the "a" partition on another, identical drive, so that you 
> could boot the backup root in an emergency.

Sure.  But I don't see what that has to do with overlapping partitions.

I'd naively think that I could do something like the following:

    dd if=/dev/<something>0a of=/dev/<something>1a

And get the same effect.

> I am guessing that the original conventions date back to V7 or 32V, 
> but one would have to go looking at code to be sure.

"a" for root makes some intuitive sense as the root file system is 
required to do anything else.

Then when you want swap, the next partition is "b".

Wanting another partition that is the entire disk (as seen by BSD) makes 
some logical sense to me too, so "c".

Were subsequent partitions sort of used as needed and had less 
consistency?  Especially when "d" because the entire disk containing 
multiple MBR partitions when "c" was restricted to the MBR partition the 
label was in?

Aside:

Would that mean that the following "d" partitions would be the same 
thing, as in the entire /dev/ad0 disk?

/dev/ad0s0d
/dev/ad0s1d

Wherein I'm borrowing the FreeBSD slice nomenclature -- as I understand 
it -- to identify the first (zero) and second (one) MBR partition on 
/dev/ad0

History and how we got to where we are today can be both very confusing 
and even more enlightening once you understand it.  What's more is that 
once you understand it, things start making more intuitive sense when 
you look at them.



-- 
Grant. . . .

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