The "ballast file" idea doesn't really change that spill-to-disk crash, as far as I can tell. You have to delete it manually; it already crashed by the time you realize it.

Seems like the sort of thing that only makes sense in a "I know my cheapskate boss won't have larger drives ready to go (or be willing to pay to expand it in a cloud scenario), and he insists that the alarm not go off until 95%, but it'll be my fault if we have a bad incident we can't recover quickly from, so I'm gonna give myself some headroom by padding things a bit" extra-paranoid scenario.