Delete all snapshots Fails

Today I encountered an old misunderstood principle again. An admin had created several snapshots on a virtual machine. Several… well to be exact 15. All snapshots were larger than 20GB. When the VMFS volume, on which this VM was located, ran out of diskspace he decided to use the button “Delete All”, but within a couple of minutes the VMFS volume ran out of diskspace again. What happened?

Situation:
Snapshot 1 – 20GB
Snapshot 2 – 10GB
Snapshot 3 – 30GB

When you choose “delete all” the following will happen:

1. Snapshot 2 will grow to 40GB at most
2. Snapshot 1 will grow to 60GB at most
3. Snapshot 1 will be committed to the original VMDK
4. All snapshot files are deleted

In other words: Snapshot 3 is merged into Snapshot 2, Snapshot 2 is merged into Snapshot 1, Snapshot 1 is merged into the original flat.vmdk and afterwards all snapshot files are deleted. This means that if you want to delete all snapshots at once you will need around 130GB of free diskspace. So think twice when you press the “delete all” button.