Re: Hacking parted to work with BSD disklabels ...

[Andrew Clausen]

Timshel Knoll wrote:
> 
> On Wed, Oct 18, 2000 at 05:14:44AM +1100, Andrew Clausen wrote:
> > Hi Timshel!
> >
> > I haven't seen you for AGES... should meet in the city some time.
> > Lunch on Mondays and Tuesdays are good :-)  Dinner on Thursday is
> > also good.
> >
> 
> OK, I'm kind of busy this week ... but I'll let you know for next week.
> Exams coming up ... FUN ;-P

hehe...  my first exam (applied maths, yuck) is in 2 weeks... all
my exams should be rather easy...

> Both :). The problem is that a BSD disklabel can be used in two ways.
> The first (and most common on ix86 boxen) is to have the BSD partition
> housed within a dos primary partition, ie. a type of `extended' partition.
> The second is to have the disklabel used as the main partition table
> for the disk, much like a `traditional' partition table (this is usually
> called 'dedicated' style. The problem is that I'll have to do one of a
> number of things:
>  - only use support for 1 use of the disklabel at this stage (not good)
>    (I've coded quite a bit of this - for only the 'dedicated' disklabel)
>  - lots of code duplication to get it working for both (also not good)
>  - somehow create a nasty hack that works around both solutions
>    (can't think of a way to do this)
>  - Other ideas that I haven't come up with yet :-)

You do realise that 1.4.x allows disk-label specific metadata (both
per-disk and per-partition)?  You could have a BSD field inside the
MSDOS specific metadata for this.

For RAID and LVM, I've used the partition flag system, rather than
file system types.  I guess it would make sense to do that for
BSD as well (i.e. make a PED_PARTITION_BSD flag)

Maybe.  But, it seems you can have data partitions inside BSD
partitions (sort of like extended partitions), etc.

How do you envisage Parted (the front end) interfacing this kind
of stuff?  I'm scared!  Having more than one extended partition
would require a fair bit of code to change... and the general
case gets much more difficult...

(/me has nightmares about alignment and metadata allocation)

One (yucky) option might be to allow users to create a BSD
partition (say, /dev/hda3), and then do:

        (parted) select /dev/hda3

To edit it, as if it is it's own block device.  This will probably
break, though, if *anything* is expressed in terms of the entire
/dev/hda (not just /dev/hda3).

> > BTW: I don't know much about BSD slices... are there any good docs,
> > or readable code I can look at?  It's hard for me to comment, until I
> > know what I'm talking about ;-)
> 
> OK, check include/linux/genhd.h for some basic info on the internals ...
> struct bsd_disklabel is the actual disklabel, and is found at sector
> 1 in the partition that the disklabel is found in.

I'm sure I said "readable"... :-P
Yoshi's link is pretty good though...

Andrew Clausen