I’ve been asked a few times if it would be possible to use virtnbdbackup as some kind of “replication” utility, to keep cold standby virtual machines on other libvirt hosts.

Usually i would tell to use underlying filesystem features (such as zfs send/recv, with incremental snapshots) to keep cold, standby copies on other hosts.

As for qcow based virtual machines, using the dirty bitmaps is not only a valid feature to create backups, but to (incrementally) replicate virtual machines, too.

I’ve released vmsync. A small golang utility that implements a simple replication tool using the NBD protocol to sync virtual machines to other hosts.