bsdlabel offset

[Jerry McAllister]

On Fri, Mar 14, 2008 at 11:40:55AM +0100, Tektonaut wrote:

> Hi,
> 
> following bsdllabel output caught my attention:
> 
> #        size    offset    fstype   [fsize bsize bps/cpg]
>  a:  2097152         0    4.2BSD     2048 16384 28552 
>  b:  4194304   2097152      swap                    
>  c: 312576642        0    unused        0     0         # "raw" part, don't 
>  edit
>  d: 33554432   6291456    4.2BSD     2048 16384 28552 
> ...
> 
> I created this disk with sade or sysinstall. What I'm not sure
> about is that partition 'a' has an offset of 0. With an 8k big
> /boot/boot I would guess offset should be 16block large.
> 
> But since the disk is booting, some boot1 loader ist located at
> sector 0 (from the beginning of this slice). How is it assured,
> that the first block will never be overwritten? Where is boot1
> located, where boot2?
> 
> Comparing the first sector with boot and boot1 differs already
> at the first char. (and there were no updates so far)

That sector 0 lies outside of the slice block 0.   What you are
seeing is not an absolute disk offset, but the offset in to the slice.  

It is possible to create it otherwise but isn't done that
way by default.    Nowdays, actually a whole track is held
out, instead of just sector 0 and that is where some of the
fancier MBRs such as GRUB get their extra space to work.
But, the standard FreeBSD MBR sticks to the official standard
of just one sector - which is why it is so plain vanilla.

////jerry

> 
> 
> Little bemused regards,
> Adam
> 
> -- 
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