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Date: Fri, 4 Jan 2013 23:22:31 +0100
From: Christian Weisgerber <naddy@mips.inka.de>
To: Warren Block <wblock@wonkity.com>
Subject: Re: bsdinstall misaligns partitions
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Warren Block:

> > * Funnily enough, the ancient BSD "dangerously dedicated" scheme
> >  is the only one that out of the box does not misalign partitions.
> 
> The filesystems don't begin at the start of the slice anyway.  There is 
> a bsdlabel there.

Yes and no.

If you look at the bsdlabel(8) output, the size of 'c' is exactly
the same as the sum of the sizes of the other partitions, as well
as exactly the size of the fdisk slice.  There is no additional
reserved space for the label.

So where does the disklabel hide?  FFS1 (FFS2) leaves 8 kB (64 kB)
of space at the start of _every_ filesystem.  The first 8 kB of the
slice--overlapping with the start of 'c' and the start of 'a'--hold
boot1, the disklabel, and boot2.  If you hexdump /boot/boot2, you'll
notice that the first 0x114 bytes are zeroed out; those 276 bytes
are exactly where the disklabel is located on disk.

See <sys/disklabel.h> and <ufs/ffs/fs.h>.

-- 
Christian "naddy" Weisgerber                          naddy@mips.inka.de