Actually the real issue is, "when the system is 65% full and you reboot, how long will it take for ZFS to mount it"? Perhaps he has split up ZFS into a number

Actually the real issue is, "when the system is 65% full and you reboot, how long will it take for ZFS to mount it"? Perhaps he has split up ZFS into a number of different pools and they can be mounted in parallel (depends on the init script and whether ZFS can do this). But I do recall that larger ZFS pools can take a bit of time to mount; maybe the updated ZFS for Linux is faster....