[stephengillie]

In the full-OS VM (KVM, VMWare, etc) this is known as disk snapshotting. Another way to look at it is putting an RDBM in full-recovery mode, so the database itself remains the same and replaying logs is required to get the data's true state.

You shut down a VM and instruct the hypervisor system to take a "snapshot" which locks the original VHD file and creates a new one. When writes happen, they're performed on the new VHD, and reads have to use both the main and the snapshot VHD. And you can create a chain of snapshots, each pointing to the previous snapshot, for versioning. Or you can have several VM snapshots use the same master VHD, like for CI or data deduplication.

To roll back, it's usually as simple as shutting down the VM and removing the snapshot file.