Understanding vinum (bootstrapped vinum)

[Andriko Tamas]

Hi,

I'd like to setup a brand new server  with 2 pieces of 120GB SATA HDD.
So i have decided  I'd like to mirror the entire space of these disks 
with vinum. (including root)

I've goggling documentations for accomplish this, and i have find some 
documents.
This is some link.

http://devel.reinikainen.net/docs/how-to/Vinum/. 
<http://org.netbase.org/vinum-mirrored.html>
Bootstrapping Vinum by Robert A. Van Valzah
<http://www.freebsd.org/doc/en_US.ISO8859-1/articles/vinum/index.html>An 
introduction to Vinum on FreeBSD by Rocky 
<http://bsdvault.net/sections.php?op=viewarticle&artid=86>
Chapter 13 of the FreeBSD handbook: The Vinum Volume Manager, originally 
written by Greg Lehey. 
<http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/vinum-vinum.html>
Vinum HOWTO for two mirrored disks, by mic at netbase dot org 
<http://org.netbase.org/vinum-mirrored.html>
Chapter 12 of O'Reilly's The Complete FreeBSD: The Vinum Volume Manager 
<http://www.vinumvm.org/cfbsd/vinum.txt>
Replacing a failed Vinum drive 
<http://www.vinumvm.org/vinum/replacing-drive.html>.
Man vinum[4|8]

These are very clear documentations, but one thing have left what i 
couldn't understand.
The documents recommend me to make a vinum bsd partition on the root 
spindle with a 16 blocks offset,
and then make vinum volumes as i like, but they are warned me to make a 
fake root "a" partition to remain bootable
my OS. (and don't forget the extra 265 block space for vinum header). 
That is ok.

The problem is why do i have to allocate the 16 block at the beginning 
of the slice? (because of vinum will cut into bootstrapping. That was 
the explaining)

Ok its understandable, but what about that situation when the disklabel 
on the first slice looks like this:

# /dev/ad0s1:

8 partitions:
#      size   offset  fstype   [fsize bsize bps/cpg]
a:  2097152  1048576  4.2BSD        0     0     0
b:  1048576        0    swap
c: 12715857        0  unused        0     0         # "raw" part, don't edit
d:  2097152  3145728  4.2BSD        0     0     0
e:  1048576  5242880  4.2BSD        0     0     0
f:  6424401  6291456  4.2BSD        0     0     0

As it shows the "b" swap partition is located at the 0 offset . So this 
partition doesn't cut into bootstrapping, and if not, why not?

Sorry for my english, and the long theoretical mail, but i'd like to 
collect as information as possible, because it will  worst the effort
when i  find myself a data loss situation.

Thanks for any idea.

Andriko Tamas