Re: BSD disklabel partition letters in NetBSD

["Rocky Hotas" <rockyhotas%post.com@localhost>]

> Sent: Friday, September 28, 2018 at 9:04 PM
> From: "Don NetBSD" <netbsd-embedded%gmx.com@localhost>
> To: netbsd-users%netbsd.org@localhost
> Subject: Re: BSD disklabel partition letters in NetBSD

[...]

Hi and sorry for the delay!

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

Yes, of course.

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

This is exactly the degenerate example I wanted to refer to. Let's consider
a BSD disklabel in the first sector of a hard disk (so, without MBR) with
the following partitions defined:

a: /, root partition of system A
b: unused
c: unused (in some cases, it represents the whole disk)
d: unused
e: /homeA, home partition of system A
f: swap partition of system A
g: /, root partition of system B
h: /homeB, home partition of system B
i: swap partition of system B

Let both system A and B be NetBSD, for example 8.0 e 7.1, so we are sure
that they are both fully-compatible with the BSD disklabel layout.

Now, boot system A from partition `a'. First: is it possible to do so? Then,
how would partitions `g', `h' and `i' be detected? In other words, what
would the output of `disklabel wd0' be from system A?

Ideally, at least partitions `g' and `h' should be mounted, fully readable
and writeable. But as regards mountpoints, there is some confusion, given
that partition `g' should be mounted in the same place as the already
mounted partition `a'. Or does this only depend on how fstab(5) has been set
up?

Then, boot system B from partition `g'. Same questions as above.

> 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!

This would be extreme :).

> 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).

Don't worry, I know this distinction, it's maybe the easier thing here!
Thank you :),

Rocky