The spacer solution guarantees you will always have one instance of "disk got full", even if the fill rate is slow. With an alarm there is a chance you end up s

The spacer solution guarantees you will always have one instance of "disk got full", even if the fill rate is slow. With an alarm there is a chance you end up solving the problem with zero instances of "disk got full" if the fill rate is not that big. That is the main difference.