File system: Meaning (information, definition, explanation, facts)

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 a 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 (taken from the earlier CP/M), 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

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 disk drive, which might be directly or indirectly connected to the computer. Examples of disk file systems include FAT. NTFS, ISO 9660 and UDF.

Some disk file systems are also journaling filesystems.

Network file systems

A network file system is a file system where the files are accessed over a network, potentially simultaneously by several computers.

Examples of network file systems include:

• AFS (Andrew File System)
• AppleShare
• CIFS (sometimes also called SMB or Samba filesystems)
• Coda
• GFS
• InterMezzo
• Lustre
• NFS

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.

• acme (Plan 9) (text windows)
• archfs (archive)
• cdfs (reading and writing of CDs)
• cfs (caching)
• DEVFS
• ftpfs (ftp access)
• lnfs (long names)
• nntpfs (netnews)
• plumber (Plan 9) (interprocess communication - pipes)
• PROCFS
• ROMFS
• TMPFS
• wikifs (wiki wiki)

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).

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 graphical (such as provided by a graphical user interface, such as a file browsers). If graphically, 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 assigns 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 hierachy. 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, 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.

At least one and perhaps many file systems are automatically mounted (automounting) by Unix at boot time. The system administrator can control which file systems are mounted at boot time, and can pre-determine the mount points for specific file systems. The sysadmin can also designate some file systems that may be mounted by normal users, and can specify when mounted file systems are checked for errors and backed up. All this information is stored in the fstab file, which anyone can read to discover what file systems are available and mountable by users.

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 (possibly modulo a perceptible 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 hierachy as part of the local filesystem hierachy, and is accessed as if the remote files were local.

File systems under Windows

Microsoft Windows developed from 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 modified versions of the simple FAT and HPFS file systems, FAT32 and NTFS respectively. 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 allowed Unix-style user based permission control, and inherited HPFS's automatic fragmentation control, which was a problem for FAT-based file systems.

Both FAT32 and NTFS make use of drive letters in order to distinguish the physical location of files on a disk. For example, the path C:\WINNT\ represents a directory WINNT on the drive represented by the letter C. The C drive is most commonly used for the primary hard disk, on which Windows is installed and from which it boots. This "tradition" has become so 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.

Since Windows interacts with the user with a graphical user interface, its literature (help files, icon labels, ...) all refer to a "directory" as a folder which contains files. Network locations can be mapped to drive letters: for example, Z: could represent a location on a different server.

Newer file systems, such as WinFS, aim to move beyond the folder metaphor and into more general categories, with greater use of metadata