Re: BSD disklabel partition letters in NetBSD

[Don NetBSD <netbsd-embedded%gmx.com@localhost>]

On 9/27/2018 3:32 AM, Rocky Hotas wrote:
> For the sake of completeness, let's consider another case, hopefully
> interesting to others. If you decide to install on the same disk two or more
> BSD systems, all compatible with BSD disklabel (for example, two different
> versions of NetBSD, or NetBSD and FreeBSD), would that unique BSD disklabel
> in sector 1 of the disk be able to handle this?

Remember, the partitions have no knowledge of where they actually exist in a
FILESYSTEM!

So, in a degenerate example, put 2 partitions on a disk that each represent
an entire root filesystem TO THE OS THAT IS BOOTED.

Boot partition 1 and you get one "system" (OS and filesystem).  Boot partition
2 and you get another.

To be more exotic, fstab(5) in the filesystem represented in partition 1
can call for /home to be mounted from partition 6 with /oldhome mounted from
partition 7.  Meanwhile, the fstab(5) in the filesystem represented in
partition 2 swaps this order (/home from partition 7 and /oldhome from
partition 6).

> In all the examples I've seen, this data structure is conceived to describe
> only a single system, with one root partition (and then optional separate
> partitions as /home, /var, /usr according to the administrator's choice, but
> all referred to the same root). Multiple OSs would mean multiple root
> partitions.

Yes.

> However, it would be very odd if, in order to allow the existence of
> multiple BSD OSs, a third-party partitioning scheme as MBR would be needed.
> If these questions can be answered by reading some documentation or some
> other source and you know the link, I would check it out (unfortunately I
> found almost nothing about this).

I think you can actually get even messier if you use an MBR to create 4
different MBR "slices" -- and put NetBSD labels *in* each of those defining
4/8/16 different "NetBSD partitions".  Then boot some particular NetBSD
partition from that set of 4/8/16 available!

Sorry to further complicate the discussion by drawing a distinction between
MBR /slices/ (which the rest of the world calls a /partition/) and NetBSD
/partitions/ within said slice(s).

[Graphics would really make a lot of this easier to explain!  :< ]