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VOD (video-on-demand) system must satisfy the user's demands of real-time VCR (video cassette recording) operations; this will bring an unbearable load to server if there are a large number of concurrent users. P2P (peer to peer) technology is the key solution to the problem. With the technology, multimedia data will be cached in peers and transmitted between peers, idle bandwidth between nodes will be used, and then server load will be reduced. In the design of P2P VOD system, cache allocation mechanism determines data scheduling mechanism, thus determines the performance of the system. According to the study of available cache allocation mechanisms, this article addressed a new cache allocation mechanism based on rarity of data, it make peers cache the scarce data as possible to prove usability of system under the VCR operations. Experimental results show that the performance of VOD system using the mechanism can meet the application requirements.