This isn't necessarily a xen question, but it's related. I run a number of 
servers on Xen 2 (netbsd-3) and one of the exciting possibilities would seem 
to be that I could easily get a domU up and running on an alternative server 
if the primary was down because of hardware failure. I'm using file backed 
xbds and if I could easily snapshot the filesystem onto a central store, 
this would make it much easier to get the domU up and running elsewhere. The 
obvious solution to this is fss(4), the use of which is hampered by the poor 
documentation and it being declared experimental. I want to use this in 
production.

For example, I could add a new vnd block device from the dom0 and use fss to 
snapshot xbd0a onto xbd1b before unconfiguring it and using rsync to copy 
the snapshot vnd elsewhere. Is there prior art for this sort of thing? Does 
Xen-3 offer anything over Xen-2 in this respect?

der Mouse's live backup stuff looks interesting too: 
ftp://ftp.rodents.montreal.qc.ca/mouse/livebackup

-- 
Stephen