Peer-to-Peer (P2P) has been one of the most important architectures for Internet applications for its inherent scalability, fault tolerance, and high performance. The research of P2P storage systems is one of the hottest issues, and P2P storage system is regarded as one of the most promising P2P applications. However, to provide durable data storage is not trivial work and sets great barrier to real deployed systems. This paper surveys the P2P storage systems and techniques for durable storage. This paper first introduces the basic components of a durable P2P storage system and the advantages by using P2P architecture, and then introduces a series of techniques for durable storage including data redundancy, data placement, failure detection and data maintenance. Following the research framework, some typical P2P storage systems and their techniques are introduced. By a detailed comparison, the pros and cons of the techniques are discussed. In the end, the problems in current research and some future research issues are outlined.