P2P存储系统中基于用户体验的可用性模型与应用

User-experience-based availability analysis model and its application in P2P storage systems

保证数据的可用性是Peer-to-Peer(P2P)存储系统最重要的属性之一.可用性分析模型和数据的放置是P2P存储系统设计的两个关键问题.由于用户在P2P存储系统中同时作为服务节点和访问节点,决定了可用性分析必须以用户为中心,从而提高服务质量,并降低系统开销.目前广泛使用的可用性分析模型以及随机放置方法存在以下缺点:1.忽视了节点在线时间的模式,会在不同时间段高估或低估节点的可用性;2.忽视了节点对数据的访问规律,不能准确评估用户体验到的可用性;3.忽视了节点可用性差异,缺乏激励机制.本文提出了一个新的基于用户体验的可用性模型,它能够从用户体验的角度,评价P2P存储系统的可用性,同时它也可以兼容传统的可用性分析模型.在新模型的基础上,本文针对两种典型的P2P存储应用:数据共享和个人备份,提出了相应的分布式数据分发算法.通过真实日志驱动实验证明,新的算法在数据共享应用中能大大降低可用性方差,减少低可用性数据;同时,在个人备份应用中能针对不同用户的贡献提供不同层次的服务,起到有效的激励作用.

Data availability is one of the most important properties of peer-to-peer（P2P） storage systems.Availability analysis model and data placement are two key design choices.Users in P2P storage system are both providers and customers.This characteristic determines that the availability analysis must be user-centric,and thereby enhance the quality of service and decrease the system cost.The popular approach in recent studies is simple random placement with steady-state model,which has the following drawbacks：1） It ignores the up/down patterns of nodes,whose availability is over-estimated or under-estimated at di？erent periods of time.2） It ignores the access patterns of users,so the availability perceived by users is hard to evaluate precisely.3） It ignores the huge di？erence of nodes’ availability,thus leading to the absence of incentive.This paper proposes a novel user-experience-based availability model,which evaluates the availability of P2P storage system in terms of user experience,which can degenerate to traditional availability analysis model.Based on the new model,this paper proposes decentralized data placement algorithms for two typical P2P storage applications：＂data sharing＂ and ＂personal backup＂.By the trace-driven simulation,we prove that our methods can enhance the availability perceived by users greatly,reduce the variance of the availability dramatically and eliminate the nodes with low availability in data-sharing applications;meanwhile,it can provide different-level service to encourage users according to their contributions.