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According to rpalethorpe (the author of the big QEMU rewrite, merged as #985) including snapshots inside the qcow2 image is only semi supported (libvirt does not use this feature so it is barely supported). Thus everybody uses overlay files instead. That's why he rolled our own snapshotting solution using backing files/overlays (otherwise doing basic things like reverting to a snapshot uses wierd code nobody supports).

The implementation saves overlay files in vm-snapshots directory (in pool directory) and qemu_state.json describes the snapshots. We need to allow user to download these files. I suggest to introduce new variable SAVE_SNAPSHOTS which would be by default off.

• document how to use these overlay files Some more reading about overlays (external snapshots) and internal snapshots (qemu-img):
• https://www.linux-kvm.org/images/6/65/02x08B-Max_Reitz-Backups_with_QEMU.pdf
• https://kashyapc.fedorapeople.org/virt/LinuxCon-NA-2016/A-Practical-Look-at-QEMU-Block-Layer-Primitives-LC-NA-2016.pdf
• https://libvirt.org/kbase/incrementalbackupinternals.html