Minimum disk-space requirements before block caching

Currently it is possible that a FlexApp that downloads all cache blocks into DiskShadowData will immediately (or over time) fill the entire disk space, which could lead to operating system problems. Can we have a configurable threshold option that says that, for example, at least 5GB must be free before it downloads or stops downloading when it reaches that number, and write out an event if it fails for that reason.