Virtual File Systems

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How does an OS allow multiple types of file systems to be integrated into a directory structure?

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An obvious but suboptimal method of implementing multiple types of file systems is to write directory and file routines for each type.

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Instead, most OSs, including UNIX, use object-oriented techniques to simplify, organize, and modularize the implementation.

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The use of these methods allows very dissimilar file-system types to be implemented within the same structure, including network file systems, such as NFS.

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The file-system implementation consists of three major layers, as depicted schematically in Fig. 5.

Figure 5: Schematic view of a virtual file system.

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The first layer is the file-system interface, based on the $open ()$, $read ()$, $write ()$, and $close ()$ calls and on file descriptors.

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The second layer is called the virtual file system (VFS) layer; it serves two important functions:

• It separates file-system-generic operations from their implementation by defining a clean VFS interface. Several implementations for the VFS interface may coexist on the same machine, allowing transparent access to different types of file systems mounted local1y.
• The VFS provides a mechanism for uniquely representing a file throughout a network. The VFS is based on a file-representation structure, called a $vnode$, that contains a numerical designator for a network-wide unique file.

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The VFS distinguishes local files from remote ones, and local files are further distinguished according to their file-system types.