In Stu's example snapshot C will report being "0MB" even after deleting snapshot B.
Hmmm... so why don't we always take two snapshots and immediately delete the first one thus saving us loads of disk space?
Well, it's really a technical illusion. So just to clarify for those who might think they can get "something for nothing" out of this...
The illusion is caused because the space taken by snapshot B is not reclaimed until the snapshots are defragged. Technically speaking, snapshot C isn't taking any room because the details of snapshot B still exist even though it's been deleted. But in reality, snapshot C is taking up 1000MB !
How can I prove this? It becomes apparent when you defrag the snapshots and watch how long it takes to defrag snapshot C. It's taking up 0MB remember, so you would expect defragging it to be really quick, right? Wrong. Once defragging is complete the size of Snapshot C will change from 0MB to correctly report as being 1000MB.
So if you need accurate stats, always defrag your snapshots after deleting!
(Nick10 - I carried out a small test to verify this, but please correct me if I'm wrong about any of the above)