File system

See Filing system for this term as it is used in libraries and offices

In computing, a file system is a method for storing and organizing computer files and the data they contain to make it easy to find and access them. File systems may use a storage device such as a hard disk or CD-ROM and involve maintaining the physical location of the files, or they may be virtual and exist only as an access method for virtual data or for data over a network (e.g. NFS).

More formally, a file system is a set of abstract data types that are implemented for the storage, hierarchical organization, manipulation, navigation, access, and retrieval of data.


Aspects of file systems

The most familiar file systems make use of an underlying data storage device that offers access to an array of fixed-size blocks, sometimes called sectors, generally 512 bytes each. The file system software is responsible for organizing these sectors into files and directories, and keeping track of which sectors belong to which file and which are not being used.

However, file systems need not make use of a storage device at all. A file system can be used to organize and represent access to any data, whether it be stored or dynamically generated (eg, from a network connection).

Whether the file system has an underlying storage device or not, file systems typically have directories which associate file names with files, usually by connecting the file name to an index into a file allocation table of some sort, such as the FAT in an MS-DOS file system, or an inode in a UNIX-like filesystem. Directory structures may be flat, or allow hierarchies where directories may contain subdirectories. In some file systems, file names are structured, with special syntax for filename extensions and version numbers. In others, file names are simple strings, and per-file metadata is stored elsewhere.

The hierarchical filesystem was an early research interest of Dennis Ritchie of Unix fame; previous implementations were restricted to only a few levels, notably the IBM implementations, even of their early databases like IMS. After the success of Unix, Ritchie extended the filesystem concept to every object in his later operating system developments, such as Plan 9.

Traditional filesystems offer facilities to create, move and delete both files and directories. They lack facilities to create additional links to a directory (hard links in Unix), rename parent links (".." in Unix-like OS), and create bidirectional links to files.

Traditional filesystems also offer facilities to truncate, append to, create, move, delete and in-place modify files. They do not offer facilities to prepend to or truncate from the beginning of a file, let alone arbitrary insertion into or deletion from a file. The operations provided are highly asymmetric and lack the generality to be useful in unexpected contexts. For example, interprocess pipes in Unix have to be implemented outside of the filesystem because it does not offer truncation from the beginning of files.

Secure access to basic file system operations can be based on a scheme of access control lists or capabilities. Research has shown access control lists to be difficult to secure properly, which is why research operating systems tend to use capabilities. Commercial file systems still use access control lists. see: secure computing

Types of file systems

File system types can be classified into disk file systems, network file systems and special purpose file systems.

Disk file systems

A disk file system is a file system designed for the storage of files on a data storage device, most commonly a disk drive, which might be directly or indirectly connected to the computer. Examples of disk file systems include FAT, NTFS, ext2, ISO 9660, ODS-5, and UDF.

Some disk file systems are also journaling file systems or versioning file systems.

Network file systems

A network file system (also known as a distributed file system) is a file system where the files are accessed over a network, potentially simultaneously by several computers. Ideally, access to network filesystems is user transparent. Examples include NFS, CIFS, Lustre, and Global File System.

Database file systems

Missing image

New concepts for file management are database-based file systems. Instead of hierarchical structured management, files are identified by their characteristics, like type of file, topic, author, or similar metadata. Therefore a file search can be formulated in SQL or in natural speech. The example on the right side shows a query for "Movies that were directed by Spielberg". Examples include BFS, GNOME Storage, and WinFS.

Special purpose file systems

A special purpose file system is basically any file system that is not a disk file system or network file system. This includes systems where the files are arranged dynamically by software, intended for such purposes as communication between computer processes or temporary file space.

Special purpose file systems are most commonly used by file-centric operating systems such as Unix. Examples include the '/proc' filesystem used by some Unix variants, which grants access to information about processes and other operating system features.

Deep space science exploration craft, like Voyager I & II used digital tape based special file systems. Most modern space exploration craft like Cassini-Huygens used RTOS file systems or RTOS influenced file systems. The mars rovers are one such example of RTOS file systems, important in this case because they are implemented in flash memory.

File systems and operating systems

Most operating systems provide a file system, as a file system is an integral part of any modern operating system. Early microcomputer operating systems' only real task was file management - a fact reflected in their names (see DOS and QDOS). Some early operating systems had a separate component for handling file systems which was called a disk operating system. On some microcomputers, the disk operating system was loaded separately from the rest of the operating system.

Because of this, there needs to be an interface provided by the operating system software between the user and the file system. This interface can be textual (such as provided by a command line interface, such as the Unix shell, or OpenVMS DCL) or graphical (such as provided by a graphical user interface, such as file browsers). If graphical, the metaphor of the folder, containing documents, other files, and nested folders is often used (see also: directory and folder).

File systems under Unix

Unix and Unix-like operating systems assign a device name to each device, but this is not how the files on that device are accessed. Unix creates a virtual file system, which makes all the files on all the devices appear to exist under the one hierarchy. This means, in Unix, there is one root directory, and every file existing on the system is located under it somewhere. Furthermore, the Unix root directory does not have to be in any physical place. It might not be on your first hard drive - it might not even be on your computer. Unix can use a network shared resource as its root directory.

To gain access to files on another device, you must first inform the operating system where in the directory tree you would like those files to appear. This process is called mounting a file system. For example, to access the files on a CD-ROM, informally, one must tell the operating system "Take the file system from this CD-ROM and make it appear under the directory /mnt". The directory given to the operating system is called the mount point - in this case it is /mnt. The /mnt directory exists on all Unix systems (it is specified in the Filesystem Hierarchy Standard) and it is intended specifically for use as a mount point for temporary media like floppy disks or CDs. It may be empty, or it may contain subdirectories for mounting individual devices. Generally, only the administrator (i.e. root user) may authorize the mounting of file systems.

Unix-like operating systems often include software and tools that assist in the mounting process and provide it new functionality. Some of these strategies have been coined "auto-mounting" as a reflection of their purpose.

  1. In many situations, filesystems other than the root need to be available as soon as the operating system has booted. All Unix-like systems therefore provide a facility for mounting filesystems at boot time. System administrators define these filesystems in the configuration file fstab, which also indicates options and mount points.
  2. In some situations, there is no need to mount certain filesystems at boot time, although their use may be desired thereafter. There are some utilities for Unix-like systems that allow the mounting of predefined filesystems upon demand.
  3. Removable media have become very common with microcomputer platforms. They allow programs and data to be transferred between machines without a physical connection. Two common examples include CD-ROMs and DVDs. Utilities have therefore been developed to detect the presence and availability of a medium and then mount that medium without any user intervention.
  4. Progressive Unix-like systems have also introduced a concept called supermounting. For example, a floppy disk that has been supermounted can be physically removed from the system. Under normal circumstances, the disk should have been synchronised and then unmounted before its removal. Provided synchronisation has occurred, a different disk can be inserted into the drive. The system automatically notices that the disk has changed and updates the mount point contents to reflect the new medium.

File systems under Mac OS X

Mac OS X uses a file system that it inherited from Mac OS called HFS Plus. HFS Plus is a metadata-rich and case preserving but case insensitive file system. Due to the Unix roots of Mac OS X, Unix permissions were added to HFS Plus. Later versions of HFS Plus added a journal to prevent corruption of the file system structure and introduced a number of optimisations to the allocation algorithms in an attempt to defragment files automatically without requiring an external defragmenter.

Filenames can be up to 255 characters. HFS Plus uses Unicode to store filenames. On Mac OS X, the filetype can come from the Type code stored in file's metadata or the filename.

HFS Plus has three kinds of links: Hard links, Symbolic links and Aliases. Aliases are designed to maintain a link to their original file even if they are moved or renamed.

File systems under Plan 9

Plan 9 was originally designed to extend some of Unix's good points, and to introduce some new ideas of its own. With respect to file systems, the Unix system of treating things as files was continued, but in Plan 9, everything is treated as a file, and accessed as a file would be. Secondly, the underlying 9P protocol was used to ensure that the difference between a file existing on a remote system and a file existing on a local system was basically nil (apart from a possible difference in latency). This had the advantage that a device or devices, represented by files, on a remote computer, could be used as though it were the local computer's own device(s). This means that under Plan 9, multiple file servers provide access to devices, classing them as special file systems.

Everything on a Plan 9 system has, then, an abstraction as a file. For example, FTP connections are not handled by a dedicated program, but instead the ftpfs server mounts the remote hierarchy as part of the local filesystem hierarchy, and is accessed as if the remote files were local. Another example, the mail system uses file servers that synthesize virtual files and directories to represent your mailbox as /mail/fs/mbox.

File systems under Microsoft Windows

Microsoft Windows developed out of an earlier operating system (MS-DOS which in turn was based on CP/M-80, which took many ideas from still earlier operating systems, notably several from DEC), and has added both file system and user interface ideas from several other sources since its first release (Unix, OS/2, etc). As such, Windows makes use of the File Allocation Table and NTFS filesystems. Older versions of the FAT file system had file name length limits, plus had restrictions on the maximum size of FAT-formatted disks or partitions.

NTFS, introduced with the Windows NT operating system, allowed ACL-based permission control. Hard links, multiple file streams, attribute indexing, quota tracking, compression and mount-points for other file systems (called "junctions") are also supported, though not all well-documented.

Unlike many other operating systems, Windows uses a drive letter abstraction at the user level to distinguish one disk or partition from another. For example, the path C:\WINDOWS\ represents a directory WINDOWS on the partition represented by the letter C. The C drive is most commonly used for the primary hard disk partition, on which Windows is installed and from which it boots. This "tradition" has become so firmly ingrained that bugs came about in older versions of Windows which made assumptions that the drive that the operating system was installed on was C. The tradition of using "C" for the drive letter can be traced to MS-DOS, where the letters A and B were reserved for up to two floppy disk drives. Network drives may also be mapped to drive letters.

Since Windows interacts with the user via a graphical user interface, its documentation refers to directories as a folder which contains files, and is represented graphically with a folder icon.

File systems under OpenVMS

This topic is discussed here: Files-11

See also

External links

es:Sistema de archivos fi:Tiedostojrjestelm fr:Systme de fichiers hr:Datotečni sustav it:File system ja:ファイルシステム nl:Bestandssysteem pl:System plikw sk:Sborov systm ru:Файловая система uk:Файлова система


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