syncing bhyve instances

[Paul Vixie]

i've used zfs for this, but i have to shut the guest down to do it. i'm 
using zvols for my guest system disks, so it's

shut the guest down (maybe just to single-user mode)
make a zfs checkpoint
start the guest back up
zfs send the checkpoint

this also assumes that the sync-destination is a cold spare, sitting in 
its shutdown state until needed.

for live sync you'll have to run software inside the guest that knows 
how to properly freeze state. for example if there's a live database of 
any kind you'll want it to be in its quiet state before you sync from 
it. in those situations, i do use rsync.

bhyve could conceivably offer a feature to export the guest RAM, and 
with a little page-stealing, this could be made into an incremental sync 
feed. but i predict it would be enormous in size for any non-trivial guest.