Availability is one of the most important properties of a storage system. However,it is very difficult to guarantee availability in P2P storage system because of peer churn. This paper argues that it is unfeasible to provide the same availability level to all peers,so it presents a novel P2P storage architecture which builds on the basis of hierarchical management and differentiated service. This architecture has two important characters: First it uses a hierarchical organization according to peers’ character instead of organization as a whole; second,it provides different availability according to peer’s contribution instead of an unbiased service. This not only simplifies the organization of large-scale peers,but also provides a good incentive mechanism. This paper firstly presents a more precise peer behavior model,and then proposes three peer organization strategies and examines their efficiency on different hierarchy in order to study their applicable scopes. Finally,it summarizes the strategies of keeping availability in different hierarchies.