Virtual file system

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A virtual file system (VFS) or virtual filesystem switch is an abstraction layer on top of a more concrete file system. The purpose of a VFS is to allow for client applications to access different types of concrete file systems in a uniform way. A VFS can for example be used to access local and network storage devices transparently without the client application noticing the difference. It can be used to bridge the differences in Windows, Mac OS and Unix filesystems, so that applications could access files on local file systems of those types without having to know what type of file system they're accessing.

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.

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[edit] Implementations

One of the first virtual file system mechanisms in Unix-like systems was introduced by Sun Microsystems in SunOS 2.0 in 1985. It allowed UNIX system calls to access local UFS file systems and remote NFS file systems transparently. For this reason, UNIX vendors who licensed the NFS code from Sun often copied the design of Sun's VFS. Other file systems could be plugged into it as well: there was an implementation of the MS-DOS 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.

The Filesystem in Userspace (FUSE) mechanism allows userland code to plug into the virtual file system mechanism in Linux, FreeBSD, OpenSolaris, and Mac OS X.

In Microsoft Windows, virtual filesystems can also be implemented through userland Shell namespace extensions; however, they do not support the lowest-level file system access application programming interfaces in Windows, so not all applications will be able to access file systems that implemented as namespace extensions.. KIO and the GNOME VFS provide similar mechanisms in the KDE and GNOME desktop environments, with similar limitations, although they can be made to use FUSE techniques and therefore integrate smoothly into the system.

[edit] Single-file virtual file systems

Some virtual file systems are implemented in such a way that the illusion of a file system can be created by using access to a single file on the underlying file system. The primary benefit to this type of implementation is that it is centralized and easy to remove. A single-file virtual file system may include all the basic features expected of any file system (virtual or otherwise), but access to the internal structure of these file systems is often limited to programs specifically written to make use of the single-file virtual file system (instead of implementation through a driver allowing universal access). Another major drawback is that performance is relatively low when compared to other virtual file systems. Low performance is mostly due to the cost of shuffling virtual files when data is written or deleted from the virtual file system.

[edit] References

[edit] See also

  • 9P - Distributed file system protocol that maps directly to the Plan 9 from Bell Labs VFS layer making all file system access network transparent.
  • Gnome VFS
  • FUSE (Linux)
  • Installable File System, a filesystem API for IBM OS/2 and Microsoft Windows NT.
  • Toronto Virtual File System, a VFS for OS/2 developed by IBM Toronto that allows mounting diverse filesystems under a common structure.

[edit] External links

Retrieved from "http://en.wikipedia.org../../../v/i/r/Virtual_file_system.html"