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A VFS specifies an Interface (or a contract) between the kernel and a concrete file system. Therefore, it is easy to add new file systems to the kernel simply by fulfilling the contract. The terms of the contract might change incompatibly from release to release, which would require that concrete file systems be recompiled, and possibly modified before recompilation, to allow it to work with a new release of the operating system, or the supplier of the operating system might make only backward-compatible changes to the contract, so that a concrete file system built for a given release of the operating system would work with future versions of the operating system. IMPLEMENTATIONS One of the first virtual file system mechanisms in FAT file system developed at Sun that plugged into the SunOS VFS, although it wasn't shipped as a product until SunOS 4.1. The SunOS implementation was the basis of the VFS mechanism in System V Release 4 . John Heidemann developed a ''stacking'' VFS under SunOS 4.0 for the experimental Ficus File System . This design provided for Code Reuse among file system types with differing but similar semantics (''e.g.'', an encrypting file system could reuse all of the naming and storage-management code of a non-encrypting file system). Heidemann adapted this work for use in 4.4BSD as a part of his Thesis research; descendants of this code underpin the file system implementations in modern BSD derivatives including Mac OS X . Other virtual file system mechanisms in UNIX-like systems include the File System Switch in System V Release 3 , the Generic File System in Ultrix , and the VFS in Linux . In OS/2 and Microsoft Windows , the virtual file system mechanism is called the Installable File System . REFERENCES
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