KVM Forum 2020

The premise of incremental backups is simple: if you can keep track of what changed, you can optimize a backup to visit only those portions of a disk image. But under the hood, there are a lot of moving parts that have been added and refined in the past few years to make incremental backups a reality when using qcow2 images. In this talk, Eric Blake will explore recent work in qemu to make bitmap tracking more powerful, enabling libvirt to finally add support for incremental backups even when a disk image is split across a backing chain involving multiple qcow2 files. Whether deciding which bitmaps should be active, or accessing the contents of those bitmaps over Network Block Device (NBD) for consumption by an arbitrary client, having an understanding of change block tracking and related technology can help you get the most performance from your incremental backups.