Replication to multiple nodes is an effective way to improve the availability in the P2P environment. It is difficult to determine how many replicas can fulfill the availability of user request, because of the unreliable peers, networks and uncertainty of user access mode.

We emphasize the availability QoS in the way that replica number should be adaptive to both user demands on service capability and system environment. Aiming to supply service to users with availability QoS at a low replica cost, we introduce a replica cost model to predict the optimal replica number and dynamically control the replicas number and the cost.

We compare the cost and availability in our system to those in others under different cost strategies. The result shows that our system provides better availability QoS at an optimal cost. The cost strategies in our model can be flexibly reconfigured according to the replica service providers.