Well, I always try to add a 2nd disk, mirror everything, and for upgrades, drop the mirror and use it to install the new version into - sort of a manual equivalent of a live upgrade, but actually a fresh install. Which means I have to bring over all changes to volatile files myself, but I can know that I'm starting clean.
In a perfect world, I'd have _3_ disks for just the OS (so I'd never have to drop a mirror, and would thus always have redundancy); but some boxes don't have room for 3 disks internally. But then I'd also have a fast NVRAM disk for highly volatile files (one of the worst is /etc/rmtab on a busy NFS server with lots of client automounts). This message posted from opensolaris.org
