QEMU Block Layer currently (as of QEMU 2.10) supports four major kinds of live block device jobs – stream, commit, mirror, and backup. These can be used to manipulate disk image chains to accomplish certain tasks, e.g.: live copy data from backing files into overlays; shorten long disk image chains by merging data from overlays into backing files; live synchronize data from a disk image chain (including current active disk) to another target image; and point-in-time (and incremental) backups of a block device.
To that end, recently I have written documentation (thanks to the QEMU Block Layer maintainers & developers for the reviews) of the usage of following commands:
block-streamblock-commitdrive-mirror(andblockdev-mirror)drive-backup(andblockdev-backup)
Each of the above block device jobs, their QMP (QEMU Machine Protocol) invocation examples are documented.
Here’s the source. And here’s the Sphinx-rendered HTML version.
This documentation can be handy in those (debugging) scenarios when it’s instructive to look at what is happening behind the scenes of QEMU. For example, live storage migration (without shared storage setup) is one of the most common use-cases that takes advantage of the QMP drive-mirror command and QEMU’s built-in Network Block Device (NBD) server. Here’s the QMP-level workflow for it — this is the flow libvirt internally implements (with some additional niceties).
Kashyap Chamarthy 