mirroring FreeBSD-9 disks with GPT

I recently tried to mirror my hard drives in a new machine. The Handbook instructions, and those in my own Absolute FreeBSD, didn't work well. (The Handbook now warns about this in a big, friendly, hard-to-miss red box.) So how can I mirror my disk? By using per-partition mirroring rather than full-disk mirroring.

I should note up front that this article is the result of my researches and testing. I am not a filesystem developer. I'm not even a FreeBSD committer any more. You should check the ...