Re: BSD disklabel partition letters in NetBSD

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

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

>So, the distinction between systems that only reserve partition "c" and
>those who also reserve partition "d" is the necessity to use MBR.
>I wonder if in those systems one can install BSD disklabel to replace MBR
>(without caring about incompatibilities with other OSs): and, if yes,
>what would be the use of "c" and "d" when MBR doesn't exist at all.

The system always uses the disklabel, only if none is written to disk,
a fictious label is generated from other data like an MBR.

The raw partitition is always special. The extra 'c' partition on MBR
systems (where the raw partition is 'd') is just helpful but not
strictly needed. But it's still a good idea to not use it for anything
else.


>In other words: with amd64, if it is possible to use the whole disk and
>to install BSD disklabel as the only partition table, what would happen?

The disklabel would be used and 'd' would still be the raw partition.
The disklabel would also be placed on sector 1.


>> All other partition letters are free for arbitrary use. Very common (but not
>> mandatory) is "a" as root partition, and "b" as swap.

>Ok!

It's better to use 'a' only as root and 'b' only as swap.

While it is possible to move root and swap and sometimes it comes handy
(e.g. to have an alternate root), the default and fallback is to use
'a' and 'b'.


>Like for `rawdisk', is there any other sysctl parameter with the number of
>available partitions in the current system?

There is a build parameter MAXPARTITIONS, depending on platform it's 8,12 or 16.
The maximum value for MAXPARTITIONS is MAXMAXPARTITIONS which is 22.

This limit is caused by the size of the disklabel structure that has to
fit into a standard block of 512 bytes.

If you don't use disklabel but the wedge system, you get a device per
partitition, so there is no inherent limit.

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