Re: BSD disklabel partition letters in NetBSD

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

On 9/25/2018 10:52 AM, Michael van Elst wrote:
> On Tue, Sep 25, 2018 at 05:05:44PM +0200, Rocky Hotas wrote:
>
> > > only if none is written to disk,
> > > a fictious label is generated from other data like an MBR.
> >
> > Sorry, I can't understand this. Maybe it's related to the following
> > description:
>
> disklabel is a data structure. If there is none on the disk, it is
> generated from other information.

The problem comes from the fact that the term is severely overloaded.
There is a "data structure", a "portion of the medium" and a "software
program" that all share the name "disklabel".

The (fictitious) data structure needs to be explicitly written to the
medium -- by a sysadm's deliberate actions (most typically, by disklabel(8)!).
Otherwise, it is a purely ephemeral concept.

> > > The disklabel would be used and 'd' would still be the raw partition.
> > > The disklabel would also be placed on sector 1.
> >
> > So, two disklabels in total. But what would be the contents of the
> > disklabel in sector 1?
>
> 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.

But it isn't placed in any of these places unless/until the system is
DIRECTED to do so.  I.e., you can access a brand-new, never-before-powered-on
disk drive via the fictitious IN-KERNEL disklabel STRUCTURE.  But, need
never actually write it to the medium to allow such access (on a NetBSD box).

[I.e., write the entire medium -- except the label portion -- and mount it on
some foreign OS and they won't see ANY label in place!  But, the DATA that
you wrote will still be there!]