Re: BSD disklabel partition letters in NetBSD

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

> Sent: Tuesday, September 25, 2018 at 7:52 PM
> From: "Michael van Elst" <mlelstv%serpens.de@localhost>
> To: "Rocky Hotas" <rockyhotas%post.com@localhost>
> Cc: netbsd-users%netbsd.org@localhost
> Subject: Re: BSD disklabel partition letters in NetBSD
>

[...]

> disklabel is a data structure. If there is none on the disk, it is
> generated from other information.

Do you mean that who generates disklabel collects information about the
disk, cylinders, etc. and creates a brand new data structure? I can't
understand what you are referring to as "other information".

> With MBR, the disklabel is usually placed on relative sector 1 of
> the MBR partition tagged as 'NetBSD' (type 169).
> 
> Without MBR, it is placed on absolute sector 1 of the medium.
> 
> N.B. the sector number can also be platform dependent, most use sector 1
> but some use other sectors.

Ok! Very clear.
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? 
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.
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).

> wedges are an abstraction that can be used for arbitrary partitioning
> schemes, GPT is the most popular. But I use it for everything, including
> disks that uses the BSD disklabel (you need a custom kernel for that).

Ok!
Again, thank you so much!

Rocky

> 
> 
> Greetings,
> -- 
>                                 Michael van Elst
> Internet: mlelstv%serpens.de@localhost
>                                 "A potential Snark may lurk in every tree."
>