Snapshot is a special way to catch the virtual machine state. This feature is very useful when you need to fix the state prior to any change in the system, or vice versa after reaching a certain state. For example, this may be the software installation or a large-scale configuration update that affects a large number of system packages and files. The reasons may be very different. Creating a snapshot implies a checkpoint where the state of the filesystem is fixed. In addition to creating images for block devices, you can create a snapshot of memory.
In fact, a snapshot is a single file or block device. For storage with file access (such as NFS or Gluster), snapshot is a separate file. For storages with block access (such as iSCSI or FC) snapshot is a separate LVM volume, which is located at the LVM physical volume (oVirt virtualization hosts use the attached storage as LVM). However, snapshots cause decrease in the VM's performance. When a snapshot is created, the original disk...