hywei, did you get this worked out? Not clear from your original post whether it would be a problem with how you are specifying the details to rsync, or whether perhaps you are dealing with disk images for virtual systems. If the latter, and you are using rsync from outside those virtual systems, then rsync will see the disk image as a single file, and that could be a problem. If you run rsync from within the virtual system, then you can be more appropriate and conservative in what you copy. It is sort of like the idea of backing up your data rather than your whole system on the assumption that you will reinstall the OS and recover the data.
However, there may be some tricks you can apply to get virtual system disk images copied efficiently, if that's what you are doing and what you want to do. See, for example, http://www.finalcog.com/rsync-vm-spa...ace-kvm-vmware