We present UBackup-II, an anonymous storage overlay network based on personal multi-cloud storages, with flexible QoS awareness. We reform the original Tor protocol by extending the command set and adding a tail part to the Tor cell, which makes it possible for coordination among proxy servers and still keeps the anonymity. Thus, users can upload and download files secretly under the cover of several proxy servers.
Moreover, users can develop a personalized QoS policy leading different hidden access patterns according to their own QoS requirement. We presented the design of UBackup-II in detail, analyzed the security policy and showed how different QoS policies works by conducting a simulating experiment.