This paper presents the design, implementation, and evaluation ofIO-Lite, a unified I/O buffering and caching system. IO-Lite unifies all buffering and caching in the system, to the extent permitted by the hardware. In particular, it allows applications, interprocess communication, the file system, the file cache, and the network subsystem to share a single physical copy of the data safely and concurrently. Protection and security are maintained through a combination of access control and read-only sharing. The various subsystems use (mutable) buffer aggregates to access the data according to their needs. IO-Lite eliminates all copying and multiple buffering of I/Odata, and enables various cross-subsystem optimizations. Performance measurements show significant performance improvements on Web servers and other I/O intensive applications.