And how does restic behave if a file changes during its backup? I have a KVM with a qcow2 disk image, can I back it up with restic without turning off the VM?
If a file is change while restic reads it then the backup contains a mix of old and new data. To get a consistent snapshot you’ll have to use LVM snapshots or those provided by the individual file systems.
The same behavior applies by default for Windows. Although there’s the --use-fs-snapshot option to let restic create a snapshot before backup.