Implementing and maintaining file systems is painful. OS functionality is notoriously difficult to develop and debug, and file systems are more so than most because of their size and interactions with other OS components. In-kernel file systems must adhere to a large number of internal OS interfaces. Though difficult during initial file system development, these dependencies particularly complicate porting a file system to different OSs or even across OS versions. This dissertation describes an architecture that addresses the file system portability problem. Virtual machines are used to decouple the OS on which a file system runs from the OS on which user applications run. The file system is distributed as a file system virtual appliance (FSVA), a virtual machine running the file system developers ’ preferred OS (version). Users runs their applications in a separate virtual machine, using their preferred OS (version). An FSVA design and implementation is described that maintains file