Handbook mirroring section

[Anton Shterenlikht]

On Thu, May 17, 2012 at 03:17:29PM -0600, Warren Block wrote:
> 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.

I haven't read your update thoroughly.
This is just to mention that you might
want to look at 
http://www.freebsd.org/cgi/query-pr.cgi?pr=docs/136712

It might be completely out of date by now, in which
case feel free to close this PR.

-- 
Anton Shterenlikht
Room 2.6, Queen's Building
Mech Eng Dept
Bristol University
University Walk, Bristol BS8 1TR, UK
Tel: +44 (0)117 331 5944
Fax: +44 (0)117 929 4423