A + B would be best. Warn at 200, file to reserve the last 100 (or 50 or whatever). That way if the fill is too fast to react to in time, you still have a quick way to temporarily gain disk space, if needed to solve the problem.

I like that idea. Belt and suspenders.

Alerting on an unexpectedly high rate-of-change, as some others have suggested, also seems good for some workloads.