This counter reports the amount of free space, in megabytes, available for the currently active snapshot on this resource volume, with 4,294,967,295 returned if the value cannot be determined. An admin monitors this in real time to catch a shrinking active snapshot before it runs out of room, since an active copy-on-write snapshot that exhausts its free space can fail or be invalidated. For example, watching this value trend toward zero during a period of heavy write activity would prompt expanding the resource volume before the active snapshot is lost.