A file system (often also written as filesystem) is a method of storing and organizing Organizing is the act of rearranging elements following one or more rules computer files A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished. Computer files can be considered as the modern and their data The term data refers to groups of information that represent the qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which information and. Essentially, it organizes these files into a database A database consists of an organized collection of data for one or more uses, typically in digital form. One way of classifying databases involves the type of their contents, for example: bibliographic, document-text, statistical. Digital databases are managed using database management systems, which store database contents, allowing data creation for the storage, organization, manipulation, and retrieval by the computer's operating system An operating system is the software on a computer that manages the way different programs use its hardware, and regulates the ways that a user controls the computer. Operating systems are found on almost any device that contains a computer with multiple programs—from cellular phones and video game consoles to supercomputers and web servers. Some.

File systems are used on data storage devices A data storage device is a device for recording information (data). Recording can be done using virtually any form of energy, spanning from manual muscle power in handwriting, to acoustic vibrations in phonographic recording, to electromagnetic energy modulating magnetic tape and optical discs such as a hard disks A hard disk drive is a non-volatile storage device for digital data. It features one or more rotating rigid platters on a motor-driven spindle within a metal case. Data is encoded magnetically by read/write heads that float on a cushion of air above the platters, with modern storage capacity measured in gigabytes and terabytes or CD-ROMs CD-ROM is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback, the 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data to maintain the physical location of the files. Beyond this, they might provide access to data on a file server by acting as clients for a network protocol In computing, a protocol is a set of rules which is used by computers to communicate with each other across a network. A protocol is a convention or standard that controls or enables the connection, communication, and data transfer between computing endpoints. In its simplest form, a protocol can be defined as the rules governing the syntax, (e.g., NFS Network File System is a network file system protocol originally developed by Sun Microsystems in 1984, allowing a user on a client computer to access files over a network in a manner similar to how local storage is accessed. NFS, like many other protocols, builds on the Open Network Computing Remote Procedure Call (ONC RPC) system. The Network, SMB In computer networking, Server Message Block operates as an application-layer network protocol mainly used to provide shared access to files, printers, serial ports, and miscellaneous communications between nodes on a network. It also provides an authenticated inter-process communication mechanism. Most usage of SMB involves computers running, or 9P 9P is a network protocol developed for the Plan 9 from Bell Labs distributed operating system as the means of connecting the components of a Plan 9 system. Files are key objects in Plan 9. They represent windows, network connections, processes, and almost anything else available in the operating system. Unlike NFS, 9P encourages caching and also clients), or they may be virtual and exist only as an access method for virtual data (e.g., procfs On many Unix-like computer systems, procfs, short for process file system, consists of a pseudo file system used to access process information from the kernel. The file system is often mounted at the /proc directory and this is its location in the Filesystem Hierarchy Standard. Because /proc is dynamically generated rather than stored, it consumes). It is distinguished from a directory service A directory service is simply the software system that stores, organizes and provides access to information in a directory. In software engineering, a directory is a map of the differences between names and values. It allows the lookup of values given a name, similar to a dictionary. As a word in a dictionary may have multiple definitions, in a and registry The Windows Registry is a hierarchical database that stores configuration settings and options on Microsoft Windows operating systems. It contains settings for low-level operating system components as well as the applications running on the platform: the kernel, device drivers, services, SAM, user interface and third party applications all make.

Aspects of file systems

Most file systems make use of an underlying data storage device A data storage device is a device for recording information (data). Recording can be done using virtually any form of energy, spanning from manual muscle power in handwriting, to acoustic vibrations in phonographic recording, to electromagnetic energy modulating magnetic tape and optical discs that offers access to an array of fixed-size physical sectors In the context of computer disk storage, a sector is a subdivision of a track on a magnetic disk or optical disc. Each sector stores a fixed amount of data. The typical formatting of these media provides space for 512 bytes (for magnetic disks) or 2048 bytes (for optical discs) of user-accessible data per sector, generally a power of 2 In mathematics, a power of two is any of the integer powers of the number two; in other words, two multiplied by itself a certain number of times. Note that one is a power of two. Written in binary, a power of two always has the binary form 100…0, just like a power of ten in the decimal system in size (512 bytes The byte is a unit of digital information in computing and telecommunications. It is an ordered collection of bits, in which each bit denotes the binary value of 1 or 0. Historically, a byte was the number of bits (typically 5, 6, 7, 8, 9, or 16) used to encode a single character of text in a computer and it is for this reason the basic or 1, 2, or 4 KiB The kibibyte is a multiple of the unit byte for quantities of digital information. The binary prefix kibi means 1024, therefore 1 kibibyte is 1024bytes. The unit symbol for the kibibyte is KiB. The unit was established by the International Electrotechnical Commission in 2000 and has been accepted for use by all major standards organizations. It are most common). The file system is responsible for organizing these sectors into files A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished. Computer files can be considered as the modern and directories Folder, directory, catalog, or drawer, in computing, is a virtual container within a digital file system, in which groups of computer files and other folders can be kept and organized, and keeping track of which sectors belong to which file and which are not being used. Most file systems address In computing, a physical address, also real address, or binary address, is the memory address that is electronically presented on the computer address bus circuitry in order to enable the data bus to access a particular storage cell of main memory data in fixed-sized units called "clusters In computer file systems, a cluster is the unit of disk space allocation for files and directories. To reduce the overhead of managing on-disk data structures, the filesystem does not allocate individual disk sectors, but contiguous groups of sectors, called clusters" or "blocks In computing , a block is a sequence of bytes or bits, having a nominal length (a block size). Data thus structured are said to be blocked. The process of putting data into blocks is called blocking. Blocking is used to facilitate the handling of the data-stream by the computer program receiving the data. Blocked data are normally read a whole" which contain a certain number of disk sectors In the context of computer disk storage, a sector is a subdivision of a track on a magnetic disk or optical disc. Each sector stores a fixed amount of data. The typical formatting of these media provides space for 512 bytes (for magnetic disks) or 2048 bytes (for optical discs) of user-accessible data per sector (usually 1-64). This is the smallest amount of disk space that can be allocated to hold a file. 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 is stored or dynamically generated (e.g., procfs On many Unix-like computer systems, procfs, short for process file system, consists of a pseudo file system used to access process information from the kernel. The file system is often mounted at the /proc directory and this is its location in the Filesystem Hierarchy Standard. Because /proc is dynamically generated rather than stored, it consumes).

File names

A file name is a name assigned to a file in order to secure storage location in the computer memory Computer memory refers to devices that are used to store data or programs on a temporary or permanent basis for use in an electronic digital computer. Computers represent information in binary code, written as sequences of 0s and 1s. Each binary digit (or "bit") may be stored by any physical system that can be in either of two stable. By this file name a file can be further accessed. Whether the file system has an underlying storage device or not, file systems typically have directories which associate file names The filename is metadata about a file; a special kind of string used to uniquely identify a file stored on the file system of a computer. Some operating systems also identify directories in the same way. Different operating systems impose different restrictions on length and allowed characters on filenames. A filename includes one or more of these with files, usually by connecting the file name to an index in a file allocation table File Allocation Table or FAT is a computer file system architecture now widely used on many computer systems and most memory cards, such as those used with digital cameras. FAT file systems are commonly found on floppy disks, flash memory cards, digital cameras, and many other portable devices because of their relative simplicity. Performance of of some sort, such as the FAT File Allocation Table or FAT is a computer file system architecture now widely used on many computer systems and most memory cards, such as those used with digital cameras. FAT file systems are commonly found on floppy disks, flash memory cards, digital cameras, and many other portable devices because of their relative simplicity. Performance of in a DOS DOS, short for "Disk Operating System", is a shorthand term for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition file system, or an inode In computing, an inode is a data structure on a traditional Unix-style file system such as UFS. An inode stores basic information about a regular file, directory, or other file system object in a Unix-like A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification file system. 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 A filename extension is a suffix to the name of a computer file applied to indicate the encoding convention of its contents and version numbers. In others, file names are simple strings, and per-file metadata Metadata is "data about data", of any sort in any media. Metadata is text, voice, or image that describes what the audience wants or needs to see or experience. The audience could be a person, group, or software program. Metadata is important because it aids in clarifying and finding the actual data. An item of metadata may describe an is stored elsewhere..

Metadata

Other bookkeeping information is typically associated with each file within a file system. The length File size measures the size of a computer file. Typically it is measured in bytes with a prefix. The actual amount of disk space consumed by the file depends on the file system of the data contained in a file may be stored as the number of blocks allocated for the file or as an exact byte The byte is a unit of digital information in computing and telecommunications. It is an ordered collection of bits, in which each bit denotes the binary value of 1 or 0. Historically, a byte was the number of bits (typically 5, 6, 7, 8, 9, or 16) used to encode a single character of text in a computer and it is for this reason the basic count. The time In computer science and computer programming, system time represents a computer system's notion of the passing of time. In this sense, time also includes the passing of days on the calendar that the file was last modified may be stored as the file's timestamp. Some file systems also store the file creation time, the time it was last accessed, and the time that the file's meta-data Metadata is “data about data.” Metadata is an emerging practice with close ties to librarianship, information science, information technology and GIS. It can be applied to a vast array of objects including both physical and electronic items such as raw data, books, CDs, DVDs, images, maps, database tables, and web pages. Since the emergence of was changed. (Note that many early PC A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end user, with no intervening computer operator. This is in contrast to the batch processing or time-sharing models which allowed large expensive mainframe operating systems did not keep track of file times.) Other information can include the file's device type In Unix-like operating systems, a device file or special file is an interface for a device driver that appears in a file system as if it were an ordinary file. There are also special device files in MS-DOS and Microsoft Windows. They allow software to interact with a device driver using standard input/output system calls, which simplifies many (e.g., block In Unix-like operating systems, a device file or special file is an interface for a device driver that appears in a file system as if it were an ordinary file. There are also special device files in MS-DOS and Microsoft Windows. They allow software to interact with a device driver using standard input/output system calls, which simplifies many, character In Unix-like operating systems, a device file or special file is an interface for a device driver that appears in a file system as if it were an ordinary file. There are also special device files in MS-DOS and Microsoft Windows. They allow software to interact with a device driver using standard input/output system calls, which simplifies many, socket In computer networking, an Internet socket or network socket is the endpoint of a bidirectional inter-process communication flow across an Internet Protocol-based computer network, such as the Internet. Internet sockets are an application programming interface (API) application program and the TCP/IP stack, usually provided by the operating system, subdirectory Folder, directory, catalog, or drawer, in computing, is a virtual container within a digital file system, in which groups of computer files and other folders can be kept and organized, etc.), its owner user-ID In computing, a user is a person who uses a computer or Internet service. A user may have a user account that identifies the user by a username , screen name (also screenname), or "handle", which is derived from the identical Citizen's Band radio term. To log in to an account, a user is typically required to authenticate himself/herself/ and group-ID In computing, the term group generally refers to a grouping of users. In principle, users may belong to none, one, or many groups The primary purpose of user groups is to simplify access control to computer systems, and its access permission Most current file systems have methods of administering permissions or access rights to specific users and groups of users. These systems control the ability of the users affected to view or make changes to the contents of the file system settings (e.g., whether the file is read-only, executable In computing, an executable file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a data file that must be parsed by a program to be meaningful. These instructions are traditionally machine code instructions for a physical CPU. However, in a more general sense, a file containing instructions, etc.).

Arbitrary attributes can be associated on advanced file systems, such as NTFS, XFS, ext2/ext3, some versions of UFS, and HFS+, using extended file attributes. This feature is implemented in the kernels of Linux, FreeBSD and Mac OS X operating systems, and allows metadata to be associated with the file at the file system level. This, for example, could be the author of a document, the character encoding of a plain-text document, or a checksum.

Hierarchical file systems

The hierarchical file system (not to be confused with Apple's HFS) 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 file system concept to every object in his later operating system developments, such as Plan 9 and Inferno.

Facilities

Traditional file systems 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 file systems 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 file system because the pipes concept does not offer truncation from the beginning of files.

Secure access

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.^{[citation needed]} Commercial file systems still use access control lists.

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 (FAT12, FAT16, FAT32, exFAT), NTFS, HFS and HFS+, HPFS, UFS, ext2, ext3, ext4, btrfs, ISO 9660, ODS-5, Veritas File System, ZFS, ReiserFS, Linux SWAP and UDF. Some disk file systems are journaling file systems or versioning file systems.

ISO 9660 and Universal Disk Format are the two most common formats that target Compact Discs and DVDs. Mount Rainier is a newer extension to UDF supported by Linux 2.6 series and Windows Vista that facilitates rewriting to DVDs in the same fashion as has been possible with floppy disks.

Flash file systems

A flash file system is a file system designed for storing files on flash memory devices. These are becoming more prevalent as the number of mobile devices is increasing, and the capacity of flash memories increase.

While a disk file system can be used on a flash device, this is suboptimal for several reasons:

• Erasing blocks: Flash memory blocks have to be explicitly erased before they can be rewritten. The time taken to erase blocks can be significant, thus it is beneficial to erase unused blocks while the device is idle.
• Random access: Disk file systems are optimized to avoid disk seeks whenever possible, due to the high cost of seeking. Flash memory devices impose no seek latency.
• Wear levelling: Flash memory devices tend to wear out when a single block is repeatedly overwritten; flash file systems are designed to spread out writes evenly.

Log-structured file systems have many of the desirable properties for a flash file system. Such file systems include JFFS2 and YAFFS.

Tape file systems

A tape file system is a file system and tape format designed to store files on tape in a self-describing form. Magnetic tapes are sequential storage media, posing challenges to the creation and efficient management of a general-purpose file system. IBM has recently announced a new file system for tape called the Linear Tape File System. The IBM implementation of this file system has been released as the open-source IBM Long Term File System product.

Database file systems

A new concept for file management is the concept of a database-based file system. Instead of, or in addition to, hierarchical structured management, files are identified by their characteristics, like type of file, topic, author, or similar metadata.

Transactional file systems

Some programs need to update multiple files "all at once." For example, a software installation may write program binaries, libraries, and configuration files. If the software installation fails, the program may be unusable. If the installation is upgrading a key system utility, such as the command shell, the entire system may be left in an unusable state.

Transaction processing introduces the isolation guarantee, which states that operations within a transaction are hidden from other threads on the system until the transaction commits, and that interfering operations on the system will be properly serialized with the transaction. Transactions also provide the atomicity guarantee, that operations inside of a transaction are either all committed, or the transaction can be aborted and the system discards all of its partial results. This means that if there is a crash or power failure, after recovery, the stored state will be consistent. Either the software will be completely installed or the failed installation will be completely rolled back, but an unusable partial install will not be left on the system.

Windows, beginning with Vista, added transaction support to NTFS, abbreviated TxF. TxF is the only commercial implementation of a transactional file system, as transactional file systems are difficult to implement correctly in practice. There are a number of research prototypes of transactional file systems for UNIX systems, including the Valor file system^[1], Amino^[2], LFS ^[3], and a transactional ext3 file system on the TxOS kernel^[4], as well as transactional file systems targeting embedded systems, such as TFFS ^[5].

Ensuring consistency across multiple file system operations is difficult, if not impossible, without file system transactions. File locking can be used as a concurrency control mechanism for individual files, but it typically does not protect the directory structure or file metadata. For instance, file locking cannot prevent TOCTTOU race conditions on symbolic links. File locking also cannot automatically roll back a failed operation, such as a software upgrade; this requires atomicity.

Journaling file systems are one technique used to introduce transaction-level consistency to file system structures. Journal transactions are not exposed to programs as part of the OS API; they are only used internally to ensure consistency at the granularity of a single system call.

Network file systems

A network file system is a file system that acts as a client for a remote file access protocol, providing access to files on a server. Examples of network file systems include clients for the NFS, AFS, SMB protocols, and file-system-like clients for FTP and WebDAV.

Shared disk file systems

A shared disk file system is one in which a number of machines (usually servers) all have access to the same external disk subsystem (usually a SAN). The file system arbitrates access to that subsystem, preventing write collisions. Examples include GFS from Red Hat, GPFS from IBM, and SFS from DataPlow.

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 procfs (/proc) file system 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 and II used digital tape-based special file systems. Most modern space exploration craft like Cassini-Huygens used Real-time operating system file systems or RTOS influenced file systems. The Mars Rovers are one such example of an RTOS file system, 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). 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. On early operating systems, there was usually support for only one, native, unnamed file system; for example, CP/M supports only its own file system, which might be called "CP/M file system" if needed, but which didn't bear any official name at all.

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).

Flat file systems

In a flat file system, there are no subdirectories—everything is stored at the same (root) level on the media, be it a hard disk, floppy disk, etc. While simple, this system rapidly becomes inefficient as the number of files grows, and makes it difficult for users to organize data into related groups.

Like many small systems before it, the original Apple Macintosh featured a flat file system, called Macintosh File System. Its version of Mac OS was unusual in that the file management software (Macintosh Finder) created the illusion of a partially hierarchical filing system on top of EMFS. This structure meant that every file on a disk had to have a unique name, even if it appeared to be in a separate folder. MFS was quickly replaced with Hierarchical File System, which supported real directories.

A recent addition to the flat file system family is Amazon's S3, a remote storage service, which is intentionally simplistic to allow users the ability to customize how their data is stored. The only constructs are buckets (imagine a disk drive of unlimited size) and objects (similar, but not identical to the standard concept of a file). Advanced file management is allowed by being able to use nearly any character (including '/') in the object's name, and the ability to select subsets of the bucket's content based on identical prefixes.

File systems under Unix-like operating systems

Unix-like operating systems create a virtual file system, which makes all the files on all the devices appear to exist in a single hierarchy. This means, in those systems, there is one root directory, and every file existing on the system is located under it somewhere. Unix-like systems can use a RAM disk or network shared resource as its root directory.

Unix-like systems assign a device name to each device, but this is not how the files on that device are accessed. Instead, to gain access to files on another device, the operating system must first be informed where in the directory tree those files should appear. This process is called mounting a file system. For example, to access the files on a CD-ROM, one must tell the operating system "Take the file system from this CD-ROM and make it appear under such-and-such directory". The directory given to the operating system is called the mount point – it might, for example, be /media. The /media directory exists on many Unix systems (as specified in the Filesystem Hierarchy Standard) and is intended specifically for use as a mount point for removable media such as CDs, DVDs, USB drives or floppy disks. 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, file systems 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 file systems at boot time. System administrators define these file systems in the configuration file fstab or vfstab in Solaris Operating Environment, which also indicates options and mount points.
2. In some situations, there is no need to mount certain file systems at boot time, although their use may be desired thereafter. There are some utilities for Unix-like systems that allow the mounting of predefined file systems 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. Common examples include USB flash drives, 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; see, for example, the Linux supermount-ng project. For example, a floppy disk that has been supermounted can be physically removed from the system. Under normal circumstances, the disk should have been synchronized and then unmounted before its removal. Provided synchronization 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. Similar functionality is found on Windows machines.
5. A similar innovation preferred by some users is the use of autofs, a system that, like supermounting, eliminates the need for manual mounting commands. The difference from supermount, other than compatibility in an apparent greater range of applications such as access to file systems on network servers, is that devices are mounted transparently when requests to their file systems are made, as would be appropriate for file systems on network servers, rather than relying on events such as the insertion of media, as would be appropriate for removable media.

File systems under Linux

Linux supports many different file systems, but common choices for the system disk include the ext* family (such as ext2, ext3 and ext4), XFS, JFS, ReiserFS and btrfs.

File systems under Solaris

The Sun Microsystems Solaris operating system in earlier releases defaulted to (non-journaled or non-logging) UFS for bootable and supplementary file systems. Solaris defaulted to, supported, and extended UFS.

Support for other file systems and significant enhancements were added over time, including Veritas Software Corp. (Journaling) VxFS, Sun Microsystems (Clustering) QFS, Sun Microsystems (Journaling) UFS, and Sun Microsystems (open source, poolable, 128 bit compressible, and error-correcting) ZFS.

Kernel extensions were added to Solaris to allow for bootable Veritas VxFS operation. Logging or Journaling was added to UFS in Sun's Solaris 7. Releases of Solaris 10, Solaris Express, OpenSolaris, and other open source variants of the Solaris operating system later supported bootable ZFS.

Logical Volume Management allows for spanning a file system across multiple devices for the purpose of adding redundancy, capacity, and/or throughput. Legacy environments in Solaris may use Solaris Volume Manager (formerly known as Solstice DiskSuite.) Multiple operating systems (including Solaris) may use Veritas Volume Manager. Modern Solaris based operating systems eclipse the need for Volume Management through leveraging virtual storage pools in ZFS.

File systems under Mac OS X

Mac OS X uses a file system that it inherited from classic Mac OS called HFS Plus, sometimes called Mac OS Extended. HFS Plus is a metadata-rich and case preserving file system. Due to the Unix roots of Mac OS X, Unix permissions were added to HFS Plus. Later versions of HFS Plus added journaling to prevent corruption of the file system structure and introduced a number of optimizations 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: Unix-style hard links, Unix-style symbolic links and aliases. Aliases are designed to maintain a link to their original file even if they are moved or renamed; they are not interpreted by the file system itself, but by the File Manager code in userland.

Mac OS X also supports the UFS file system, derived from the BSD Unix Fast File System via NeXTSTEP. However, as of Mac OS X 10.5 (Leopard), Mac OS X can no longer be installed on a UFS volume, nor can a pre-Leopard system installed on a UFS volume be upgraded to Leopard.^[6]

File systems under Plan 9 from Bell Labs

Plan 9 from Bell Labs was originally designed to extend some of Unix's good points, and to introduce some new ideas of its own while fixing the shortcomings of Unix.

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 (i.e., no ioctl or mmap). Perhaps surprisingly, while the file interface is made universal it is also simplified considerably: symlinks, hard links and suid are made obsolete, and an atomic create/open operation is introduced. More importantly the set of file operations becomes well defined and subversions of this like ioctl are eliminated.

Secondly, the underlying 9P protocol was used to remove the difference between local and remote files (except for a possible difference in latency or in throughput). This has 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 file systems. Servers for "synthetic" file systems can also run in user space bringing many of the advantages of micro kernel systems while maintaining the simplicity of the system.

Everything on a Plan 9 system has an abstraction as a file; networking, graphics, debugging, authentication, capabilities, encryption, and other services are accessed via I-O operations on file descriptors. For example, this allows the use of the IP stack of a gateway machine without need of NAT, or provides a network-transparent window system without the need of any extra code.

Another example: a Plan-9 application receives FTP service by opening an FTP site. The ftpfs server handles the open by essentially mounting the remote FTP site as part of the local file system. With ftpfs as an intermediary, the application can now use the usual file-system operations to access the FTP site as if it were part of the local file system. A further example is the mail system which uses file servers that synthesize virtual files and directories to represent a user mailbox as /mail/fs/mbox. The wikifs provides a file system interface to a wiki.

These file systems are organized with the help of private, per-process namespaces, allowing each process to have a different view of the many file systems that provide resources in a distributed system.

The Inferno operating system shares these concepts with Plan 9.

File systems under Microsoft Windows

Windows makes use of the FAT and NTFS file systems.

FAT

The File Allocation Table (FAT) filing system, supported by all versions of Microsoft Windows, was an evolution of that used in Microsoft's earlier operating system (MS-DOS which in turn was based on 86-DOS). FAT ultimately traces its roots back to the short-lived M-DOS project and Standalone disk BASIC before it. Over the years various features have been added to it, inspired by similar features found on file systems used by operating systems such as Unix.

Older versions of the FAT file system (FAT12 and FAT16) had file name length limits, a limit on the number of entries in the root directory of the file system and had restrictions on the maximum size of FAT-formatted disks or partitions. Specifically, FAT12 and FAT16 had a limit of 8 characters for the file name, and 3 characters for the extension (such as .exe). This is commonly referred to as the 8.3 filename limit. VFAT, which was an extension to FAT12 and FAT16 introduced in Windows NT 3.5 and subsequently included in Windows 95, allowed long file names (LFN).

FAT32 also addressed many of the limits in FAT12 and FAT16, but remains limited compared to NTFS.

exFAT (also known as FAT64) is the newest iteration of FAT, with certain advantages over NTFS with regards to file system overhead. exFAT is only compatible with newer Windows systems, such as Windows 2003, Windows Vista, Windows 2008, Windows 7 and more recently, support has been added for WinXP^[7].

NTFS

NTFS, introduced with the Windows NT operating system, allowed ACL-based permission control. Hard links, multiple file streams, attribute indexing, quota tracking, sparse files, encryption, compression, reparse points (directories working as mount-points for other file systems, symlinks, junctions, remote storage links) are also supported, though not all these features are 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 usually installed and from which it boots. This "tradition" has become so firmly ingrained that bugs came about in older applications 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. This in turn derived from CP/M in the 1970s, which however used A: and B: for hard drives, and C: for floppy disks, and ultimately from IBM's CP/CMS of 1967.

Network drives may also be mapped to drive letters.

File systems under OpenVMS

File systems under MVS [IBM Mainframe]

Other file systems

• The Prospero File System is a file system based on the Virtual System Model.^{[clarification needed]} The system was created by Dr. B. Clifford Neuman of the Information Sciences Institute at the University of Southern California.^[8]
• RSRE FLEX file system - written in ALGOL 68

See also

• Comparison of file systems
• Directory structure
• Disk sharing
• Distributed file system
• Filename extension
• File manager
• File system fragmentation
• Filesystem API
• Physical and logical storage
• List of file systems
• List of Unix programs
• Virtual file system
• Storage efficiency

References

Cited references

1. ^ Spillane, Richard; Gaikwad, Sachin; Chinni, Manjunath; Zadok, Erez and Wright, Charles P.; 2009; "Enabling transactional file access via lightweight kernel extensions"; Seventh USENIX Conference on File and Storage Technologies (FAST 2009)
2. ^ Wright, Charles P.; Spillane, Richard; Sivathanu, Gopalan; Zadok, Erez; 2007; "Extending ACID Semantics to the File System; ACM Transactions on Storage
3. ^ Selzter, Margo I.; 1993; "Transaction Support in a Log-Structured File System"; Proceedings of the Ninth International Conference on Data Engineering
4. ^ Porter, Donald E.; Hofmann, Owen S.; Rossbach, Christopher J.; Benn, Alexander and Witchel, Emmett; 2009; "Operating System Transactions"; In the Proceedings of the 22nd ACM Symposium on Operating Systems Principles (SOSP '09), Big Sky, MT, October 2009.
5. ^ Gal, Eran; Toledo, Sivan; "A Transactional Flash File System for Microcontrollers"
6. ^ [http://docs.info.apple.com/article.html?artnum=306516 Mac OS X 10.5 Leopard: Installing on a UFS-formatted volume Newer versions Mac OS X are capable of reading and writing to the legacy FAT file systems(16 & 32). They are capable of reading, but not writing to the NTFS file system. Third party software is still necessary to write to the NTFS file system under Snow Leopard 10.6.2. ]
7. ^ Microsoft WinXP exFat patch http://www.microsoft.com/downloads/details.aspx?FamilyID=1cbe3906-ddd1-4ca2-b727-c2dff5e30f61&displaylang=en
8. ^ http://www.cs.ucsb.edu/~ravenben/papers/fsml/prospero-gfsvsm.ps.gz

General references

• Jonathan de Boyne Pollard (1996). "Disc and volume size limits". Frequently Given Answers. http://homepage.ntlworld.com./jonathan.deboynepollard/FGA/os2-disc-and-volume-size-limits.html. Retrieved February 9, 2005.
• IBM. "OS/2 corrective service fix JR09427". ftp://service.boulder.ibm.com/ps/products/os2/fixes/v4warp/english-us/jr09427/JR09427.TXT. Retrieved February 9, 2005.
• "Attribute - $EA_INFORMATION (0xD0)". NTFS Information, Linux-NTFS Project. http://linux-ntfs.sourceforge.net/ntfs/attributes/ea_information.html. Retrieved February 9, 2005.
• "Attribute - $EA (0xE0)". NTFS Information, Linux-NTFS Project. http://linux-ntfs.sourceforge.net/ntfs/attributes/ea.html. Retrieved February 9, 2005.
• "Attribute - $STANDARD_INFORMATION (0x10)". NTFS Information, Linux-NTFS Project. http://linux-ntfs.sourceforge.net/ntfs/attributes/standard_information.html. Retrieved February 21, 2005.
• Apple Computer Inc. "Technical Note TN1150: HFS Plus Volume Format". Detailed HFS Plus and HFSX description. http://developer.apple.com/technotes/tn/tn1150.html. Retrieved May 2, 2006.
• File System Forensic Analysis, Brian Carrier, Addison Wesley, 2005.

Further reading

Books

• Carrier, Brian (2005). File System Forensic Analysis. Addison-Wesley. ISBN 0321268172.
• Custer, Helen (1994). Inside the Windows NT File System. Microsoft Press. ISBN 155615660X.
• Giampaolo, Dominic (1999) (PDF). Practical File System Design with the Be File System. Morgan Kaufmann Publishers. ISBN 1558604979. http://www.nobius.org/~dbg/practical-file-system-design.pdf. Retrieved 2010-01-22.
• McCoy, Kirby (1990). VMS File System Internals. VAX - VMS Series. Digital Press. ISBN 1555580564.
• Mitchell, Stan (1997). Inside the Windows 95 File System. O'Reilly. ISBN 156592200X.
• Nagar, Rajeev (1997). Windows NT File System Internals : A Developer's Guide. O'Reilly. ISBN 9781565922495.
• Pate, Steve D. (2003). UNIX Filesystems: Evolution, Design, and Implementation. Wiley. ISBN 0471164836.
• Rosenblum, Mendel (1994). The Design and Implementation of a Log-Structured File System. The Springer International Series in Engineering and Computer Science. Springer. ISBN 0792395417.
• Russinovich, Mark; Solomon, David A.; Ionescu, Alex (2009). "File Systems". Windows Internals (5th ed.). Microsoft Press. ISBN 0735625301.
• Prabhakaran, Vijayan (2006). IRON File Systems. PhD disseration, University of Wisconsin-Madison.
• Silberschatz, Abraham; Galvin, Peter Baer; Gagne, Greg (2004). "Storage Management". Operating System Concepts (7th ed.). Wiley. ISBN 0471694665.
• Tanenbaum, Andrew S. (2007). "File Systems". Modern operating Systems (3rd ed.). Prentice Hall. ISBN 0136006639.
• Tanenbaum, Andrew S.; Woodhull, Albert S. (2006). "File Systems". Operating Systems: Design and Implementation (3rd ed.). Prentice Hall. ISBN 0131429388.

Online articles

• Benchmarking Filesystems (outdated) by Justin Piszcz, Linux Gazette 102, May 2004
• Benchmarking Filesystems Part II using kernel 2.6, by Justin Piszcz, Linux Gazette 122, January 2006
• Filesystems (ext3, ReiserFS, XFS, JFS) comparison on Debian Etch
• Interview With the People Behind JFS, ReiserFS & XFS
• Journal File System Performance (outdated): ReiserFS, JFS, and Ext3FS show their merits on a fast RAID appliance
• Journaled Filesystem Benchmarks (outdated): A comparison of ReiserFS, XFS, JFS, ext3 & ext2
• Large List of File System Summaries
• Linux File System Benchmarks v2.6 kernel with a stress on CPU usage
• Linux Filesystem Benchmarks
• Linux large file support (outdated)
• Local Filesystems for Windows
• Overview of some filesystems (outdated)
• Sparse files support (outdated)
• Jeremy Reimer (March 16, 2008). "From BFS to ZFS: past, present, and future of file systems". arstechnica.com. http://arstechnica.com/articles/paedia/past-present-future-file-systems.ars. Retrieved 2008-03-18.

External links

• Filesystem Specifications - Links & Whitepapers
• Interesting File System Projects

Categories: Computer file systems

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• Embedded System: Encyclopedia
• Operating System: Encyclopedia
• NLS (Computer System): Encyclopedia
• File Transfer Protocol: Encyclopedia

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