The increasing popularity of Cloud storage services is leading end-users to store their digital lives (including photos, videos, work documents, etc.) in the Cloud. However, many users are still reluctant to move their data to the Cloud due to the amount of control ceded to Cloud vendors. To let users retain the control over their data, Friend-to-Friend (F2F) storage systems have been presented in the literature as a promising alternative.However, as we show in this paper, pure F2F storage systems present a poor QoS, mainly due to availability correlations, which results in a loss of attractiveness by end users. To overcome this limitation, we propose a hybrid architecture that combines F2F storage systems and the availability of Cloud storage services to let users infer the right balance between user control and quality of service. This architecture, we called it F2BOX, is able to deliver such a balance thanks to the development of a new suite of data transfer scheduling strategies and a new redundancy calculation algorithm. The main feature of this algorithm is that allow users to adjust the amount of redundancy according to the availability patterns exhibited by friends. Our simulation and experimental results (in Amazon S3) demonstrate the high benefits experienced by end users as a result of the "cloudification" of F2F systems.

Abstract-Personal storage is a mainstream service used by millions of users. Among the existing alternatives, Friend-toFriend (F2F) systems are nowadays an interesting research topic aimed to leverage a secure and private off-site storage service.However, the specific characteristics of F2F storage systems (reduced node degree, correlated availabilities) represent a hard obstacle to their performance. Actually, it is extremely difficult for a F2F system to guarantee an acceptable storage service quality in terms of transference times and data availability to end-users. In this landscape, we propose to resort to the Cloud for improving the storage service of a F2F system.We present FriendBox: a hybrid F2F personal storage system. FriendBox is the first F2F system that efficiently combines resources of trusted friends with Cloud storage for improving the service quality achievable by pure F2F systems.We evaluated FriendBox through a real deployment in our university campus. We demonstrated that FriendBox achieves high transfer performance and flexible user-defined data availability guarantees. Furthermore, we analyzed the costs of FriendBox demonstrating its economic feasibility.