Handbook mirroring section

[Warren Block]

Summary: Handbook mirror example broken, new rewrite here, feedback 
desired before commit.

http://www.wonkity.com/~wblock/mirror/book.html
http://www.wonkity.com/~wblock/mirror/chapter.diff


More detail:

The existing mirroring section in the GEOM chapter of the Handbook uses 
a workaround to add another drive and make a mirror with one existing 
drive without moving data.

http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/geom-mirror.html

That workaround creates mirrors that FreeBSD 9 and later versions of 8 
see as having invalid, overlapping areas.  The more strict tests prevent 
at least FreeBSD 9 from even booting from mirrors created that way:

http://www.freebsd.org/releases/9.0R/relnotes-detailed.html#AEN1277

Ideally, the procedure would be able to create a single-drive mirror but 
use only a limited amount of disk space on the new drive, making that 
space the same size as the existing drive.  (Here's a new 80G drive, but 
I only want to use 40G of space for the mirror.)  Of course this can be 
done with partitions, but that is more complex and there may be 
thrashing issues with multiple partitions.

It turns out to be fairly difficult to do this in a situation that will 
work reliably for the average user.

I've rewritten the mirroring section to just use two new drives.

A mirror *can* be created with two different-sized drives, provided they 
are both seen at the time of creation.  The smaller size is used; 
exposing that size with a command-line option would be very useful.