On-disk image much bigger than VM shows with df.

Hey, all. I've got a VM -- this is in Juno -- which du shows as using 152 GB on the compute node's disk. In the VM, itself, it says it's using 39 GB of an 80 GB volume. So, clearly, that sparse file has gotten crazy huge with un-reclaimed stuff. I seem to recall doing snapshots and then defragging them, "back in the day," but I have no idea if there's a way to do that to an instance *without* snapshotting. So: * If I snapshot it, can I then just defrag the snapshot and then make use of it to bring my system back? * Even better: is there a way to defrag the on-disk VM files without snapshotting? Thanks... -Ken