I use vac -a to back up several unix systems to my main Plan 9 file server. Currently I'm doing two nightly via cron and two sporadically (laptops); there have been more of each in the past. In addition to storing the scores locally, I wrote a little rc script that lives in /rc/bin/service.auth and accepts scores and stores them on my CPU server. Combined with a simple 9fs addition, I can always "9fs foo.vac" and get foo's history of archives from all my plan9 systems. I've also done restores from p9p, but have not exercised that well. I considered using vbackup instead of vac on the unix side, but it didn't look practical to get at the results from plan9. Also, I really needed to be selective in what I saad backing up (there's a few hundred Gb of post-processed video I don't need to store on one of these systems, and growing daily).
This has worked very well for me overall; my only problem has been vtcache exhaustion on some backups, which I haven't been able to track down. Thankfully the error is on backup, not restore, so you know something's gone wrong when it happens. My message about vtcache is at http://9fans.net/archive/2010/02/307. I'm not sure I pushed the absolute latest to sources, but some version of my unix backup script is at /n/sources/contrib/anothy/bin/rc/vacbak.
