4k alignment of MBR slices / BSD partitions is not working correctly

[Warren Block]

On Wed, 12 Dec 2012, Miroslav Lachman wrote:

> Next test with gpart -a 4k
>
> # gpart create -s mbr mirror/gm0
> mirror/gm0 created
>
> # gpart add -a 4k -s 40g -t freebsd mirror/gm0
> mirror/gm0s1 added
>
> # gpart show -p mirror/gm0
> =>        63  1951475648    mirror/gm0  MBR  (930G)
>          63          63                - free -  (31k)
>         126    83886012  mirror/gm0s1  freebsd  (40G)
>    83886138  1867589573                - free -  (890G)
>
>
> Same wrong result as in previous test

My possibly incorrect understanding of this is that the slice will start 
on a block that is aligned to the old CHS values for backwards 
compatibility.

When gpart -a4k is used to create FreeBSD partitions inside that slice,
those partitions will be 4K aligned.