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On Object Maintenance in Peer-to-Peer Systems



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On Object Maintenance in Peer to Peer Systems Kiran Tati and Geoffrey M Voelker Department of Computer Science and Engineering University of California San Diego 1 INTRODUCTION Storage is often a fundamental service provided by peer topeer systems where the system stores data objects on behalf of higher level services applications and users A primary challenge in peer to peer storage systems is to efficiently maintain object availability and reliability in the face of node churn Nodes in peer to peer systems exhibit both temporary and permanent failures requiring the use of redundancy to mask and cope with such failures e g 1 4 10 16 21 The cost of redundancy however is additional storage and bandwidth for creating and repairing stored data Since bandwidth is typically a much more scarce resource than storage in peer to peer systems strategies for efficiently maintaining objects focus on reducing the bandwidth overhead of managing redundancy trading off storage as a result Typically these strategies create redundant versions of object data using either replication or erasure coding as redundancy mechanisms and either react to node failures immediately or lazily as a repair policy In this paper we revisit object maintenance in peer to peer systems focusing on how temporary and permanent churn impact the overheads associated with object maintenance We have a number of goals to highlight how different environments exhibit different degrees of temporary and permanent churn to provide further insight into how churn in different environments affects the tuning of object maintenance strategies and to examine how object maintenance and churn interact with other constraints such as storage capacity When possible we highlight behavior independent of particular object maintenance strategies When an issue depends on a particular strategy though we explore it in the context of a strategy in essence similar to TotalRecall 4 which uses erasure coding lazy repair of data blocks and



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