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Web proxy caching servers play a key role in today's Web infrastructure. Previous studies have shown that disk I/O is one of the major performance bottlenecks of proxy servers. Most conventional file systems do not work well for proxy server workloads and have high overheads. This paper presents a novel, User-space, Customized File System, called UCFS, that can drastically improve the I/O performance of proxy servers. UCFS is a user-level software component of a proxy server which manages data on a raw disk or disk partition. Since the entire system runs in the user space, it is easy and inexpensive to implement. It also has good portability and maintainability. UCFS uses efficient in-memory meta-data tables to eliminate almost all I/O overhead of meta-data searches and updates. It also includes a novel file system called Cluster-structured File System (CFS). Similarly to the Log-structured File Systems (LFS), CFS uses large disk transfers to significantly improve disk write performance. However, CFS can also markedly improve file read operations and it does not generate garbage. Comprehensive simulation experiments using five representative real-world traces show that UCFS can significantly improve proxy server performance. For example, UCFS achieves 8-19 times better I/O performance than the state-of-the-art SQUID server running on a Unix Fast File System (FFS), 4-7.5 times better than SQUID on asynchronous FFS, and 3-9 times better than the improved SQUIDML.