At the end of a class, each student has changed many aspects of virtual machines (VM). In order to prepare them for the next class, it is important to be able to return them to its original state as soon as possible. The Logical Volume Manager snapshots are useful in these cases.
The Logical Volume Manager (LVM) is a method that has been used successfully on Linux for many years and managing storage in a flexible way. Instead of working with fixed storage allocation units, LVM work with versatile volumes that can be extended or reduced very easily. These volumes offer also other benefits, like the snapshots.
After creating a LVM snapshot, there will be two volumes that will reference the same data. When the changes are written in the original volume after the snapshot was taken, the original data blocks are copied to the snapshot volume.
When accessing the data blocks, the snapshot metadata point to two different locations. The data blocks that have not changed since the snapshot was created are still in the original volume, but the original version of the data blocks they have changed is saved in the snapshot. This allows to use the snapshot to access the status of the volume as it was at the time that the snapshot was created.
If you installed one VM KVM using all values by default, the back end of the VM disk storage will be in a file. If we create a LVM logical before starting the installation of the VM, this logical volume can be used as the storage back end of the VM.
If you want to know how an LVM disk is referred to from the KVM XML code in the configuration file read the full article in English.