This article presents the design, implementation, and evaluation of IO -Lite, a unified I/O buffering and caching system for general-purpose operating systems. IO-Lite unifies all buffering and caching in the system, to the extent permitted by the hardware. In particular, it allows applications, the interprocess communication system, the file system, the file cache, and the network subsystem to safely and concurrently share a single physical copy of the data. Protection and security are maintained through a combination of access control and read-only sharing. IO-Lite eliminates all copying and multiple buffering of I/O data, and enables various cross-subsystem optimizations. Experiments with a Web server show performance improvements between 40 and 80% on real workloads as a result of IO-Lite.