Re: BSD disklabel partition letters in NetBSD

[mlelstv%serpens.de@localhost (Michael van Elst)]

rockyhotas%post.com@localhost ("Rocky Hotas") writes:

>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".

The driver provides a 'default label' based on information like cylinders.

Then the disk is searched for an MBR (to find the NetBSD partition type 169).

If this is found, it searches for a disklabel in that partition, otherwise
it computes a disklabel based on MBR partitions and continues to look for
a disklabel at the beginning of the disk.

Some non-MBR-based platforms have additional heuristics to read other
kinds of partitioning information, e.g. an Amiga "Rigid Disk Block".



>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? 

I guess you can have NetBSD and FreeBSD using different partitions but
the same disklabel on a single disk. OpenBSD did change the disklabel
format to support larger disks, so that probably conflicts.

Wether you can have a bootloader on that disk that handles different
systems is another question. But for data disks that's not a problem.


>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. But it's possible to boot NetBSD using a partition different from 'a'.


>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.

You don't need an MBR to make the disk accessible.

On x86 hardware you do need an MBR just for being able to boot an OS,
and I'm not even talk about UEFI, just about plain old BIOS.

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