You never can and that is sadly the truth. You will always run in situation in which you have in sum enough memory available, but it is so fragmented, that you cannot start your VM or you will receive OOM actions. The chances of that are increasing with uptime and memory operations (balloonig, starting/stopping processes etc.). You are right in that sense, that using ZFS will also increase the chances for such a situation, so you need to constraint your ARC in order to predict your memory usage and optimize for that.
