[$] Changed-block tracking and differential backups in QEMU

The block layer of QEMU, the open-sourcemachine emulator and virtualizer, forms the backbone of many storagevirtualization features: the QEMU Copy-On-Write (QCOW2) disk-image file format,disk image chains, point-in-time snapshots, backups, and more. At therecently concluded 2020 KVM Forumvirtual event, Eric Blake gave a talkon the current work in QEMU and libvirtto make differential backups more powerful. As the name implies,"differential backups" address the efficiency problems of full diskbackups: space usage and speed of backup creation.