Abstracttechnique [1,26,27]. In this method, client calls are directed to a single primary server, which communicates This paper describes the design and implementation of the with other backup servers and waits for them to respond Harp file system. Harp is a replicated Unix file system before replying to the client. The system masks failures by accessible via the VFS interface. It provides highly availperforming a failover algorithm in which an inaccessible able and reliable storage for files and guarantees that file server is removed from service. When a primary performs operations are executed atomically in spite of concurrency an operation, it must inform enough backups to guarantee and failures. It uses a novel variation of the primary copy that the effects of that operation will survive all subsequent replication technique that provides good performance befailovers. cause it allows us to trade disk accesses for network comHarp is one of the first implementations of a primary munication. Harp is intended to be used within a file sercopy scheme that runs on conventional hardware. It has vice in a distributed network; in our current implemensome novel features that allow it to perform well. The key tation, it is accessed via NFS. Preliminary performance performance issues are how to provide quick response for results indicate that Harp provides equal or better response user operations and how to provide good system capacity time and system capacity than an unreplicated implemen-(roughly, the number of operations the system can handle tation of NFS that uses Unix files directly.in some time period while still providing good response time). Harp achieves good performance by recording the