There are two classes of snapshots for QEMU guests. Internal snapshots are contained completely within a qcow2 file, and fully supported by libvirt, allowing for creating, deleting, and reverting of snapshots. This is the default setting used by libvirt when creating a snapshot, especially when no option is specified. Although this file type takes a bit longer than others in creating the snapshot, it is required by libvirt to use qcow2 disks. Another drawback to this file type is that qcow2 disks are not subject to receive improvements from QEMU.
External snapshots, on the other hand work with any type of original disk image, can be taken with no guest downtime, and are able to receive active improvements from QEMU. In libvirt, they are created when using the
--disk-only option to
snapshot-create-as (or when specifying an explicit XML file to
snapshot-create that does the same). At the moment external snapshots are a one-way operation as libvirt can create them but cannot do anything further with them.