> An "rsync copy" of a file system is not a point-in-time backup (unless the system is offline), because the system changes constantly. A point-in-time backup is a backup in which each block/file/.. maps to the same exact timestamp.

You can do this with some extra steps in between. Specifically you need a snapshotting file system like zfs. You run the rsync on the snapshot to get an atomic view of the file system.

Of course if you’re using zfs, you might just want to export the actual snapshot at that point.