Editing File system (section)

== Implementations ==

An [[operating system]] (OS) typically supports one or more file systems. Sometimes an OS and its file system are so tightly interwoven that it is difficult to describe them independently.

An OS typically provides file system access to the user. Often an OS provides [[command line interface]], such as [[Unix shell]], Windows [[Command Prompt]] and [[PowerShell]], and [[DIGITAL Command Language|OpenVMS DCL]]. An OS often also provides [[graphical user interface]] [[file browser]]s such as MacOS [[Finder (software)|Finder]] and Windows [[File Explorer]].

===Unix and 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 [[mount (computing)|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]]''&nbsp;– it might, for example, be {{mono|/media}}. The {{mono|/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 [[System administrator|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.
* In many situations, file systems other than the root need to be available as soon as the operating system has [[booting|booted]]. All Unix-like systems therefore provide a facility for mounting file systems at boot time. [[System administrator]]s define these file systems in the configuration file [[fstab]] (''vfstab'' in [[Solaris (operating system)|Solaris]]), which also indicates options and mount points.
* In some situations, there is no need to mount certain file systems at [[booting|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.
* Removable media allow programs and data to be transferred between machines without a physical connection. Common examples include [[USB flash drive]]s, [[CD-ROM]]s, and [[DVD]]s. Utilities have therefore been developed to detect the presence and availability of a medium and then mount that medium without any user intervention.
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* Progressive Unix-like systems have also introduced a concept called '''supermounting'''; see, for example, [http://sourceforge.net/projects/supermount-ng 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.
* An [[automounter]] will automatically mount a file system when a reference is made to the directory atop which it should be mounted. This is usually used for file systems on network servers, rather than relying on events such as the insertion of media, as would be appropriate for removable media.

===={{Anchor|LINUX}}Linux====
[[Linux]] supports numerous file systems, but common choices for the system disk on a block device include the ext* family ([[ext2]], [[ext3]] and [[ext4]]), [[XFS]], [[JFS (file system)|JFS]], and [[btrfs]]. For raw flash without a [[flash translation layer]] (FTL) or [[Memory Technology Device]] (MTD), there are [[UBIFS]], [[JFFS2]] and [[YAFFS]], among others. [[SquashFS]] is a common compressed read-only file system.

====Solaris====
[[Solaris (operating system)|Solaris]] in earlier releases defaulted to (non-journaled or non-logging) [[Unix File System|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 file system|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]].

====macOS====
[[macOS|macOS (formerly Mac OS X)]] uses the [[Apple File System]] (APFS), which in 2017 replaced a file system inherited from [[classic Mac OS]] called [[HFS Plus]] (HFS+). Apple also uses the term "Mac OS Extended" for HFS+.<ref>{{cite web|title=Mac OS X: About file system journaling|url=http://support.apple.com/kb/ht2355|publisher=Apple|access-date=8 February 2014}}</ref> HFS Plus is a [[metadata (computing)|metadata]]-rich and [[case preservation|case-preserving]] but (usually) [[case sensitivity|case-insensitive]] file system. Due to the Unix roots of macOS, 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.

File names can be up to 255 characters. HFS Plus uses [[Unicode]] to store file names. On macOS, the [[file format|filetype]] can come from the [[type code]], stored in file's metadata, or the [[filename extension]].

HFS Plus has three kinds of links: Unix-style [[hard link]]s, Unix-style [[symbolic link]]s, and [[alias (Mac OS)|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 (computing)|userland]].

macOS 10.13 High Sierra, which was announced on June 5, 2017, at Apple's WWDC event, uses the [[Apple File System]] on [[solid-state drive]]s.

macOS also supported the [[Unix File System|UFS]] file system, derived from the [[BSD]] Unix Fast File System via [[NeXTSTEP]]. However, as of [[Mac OS X Leopard]], macOS could no longer be installed on a UFS volume, nor can a pre-Leopard system installed on a UFS volume be upgraded to Leopard.<ref>{{cite web|url=http://docs.info.apple.com/article.html?artnum=306516|title=Mac OS X 10.5 Leopard: Installing on a UFS-formatted volume|work=apple.com|date=19 October 2007|access-date=29 April 2016|archive-url=https://web.archive.org/web/20080316033439/http://docs.info.apple.com/article.html?artnum=306516|archive-date=16 March 2008|url-status=dead}}</ref> As of [[Mac OS X Lion]] UFS support was completely dropped.

Newer versions of macOS are capable of reading and writing to the legacy [[File Allocation Table|FAT]] file systems (16 and 32) common on Windows. They are also capable of ''reading'' the newer [[NTFS]] file systems for Windows. In order to ''write'' to NTFS file systems on macOS versions prior to [[Mac OS X Snow Leopard]] third-party software is necessary. Mac OS X 10.6 (Snow Leopard) and later allow writing to NTFS file systems, but only after a non-trivial system setting change (third-party software exists that automates this).<ref>{{cite web|last=OSXDaily|title=How to Enable NTFS Write Support in Mac OS X|url=http://osxdaily.com/2013/10/02/enable-ntfs-write-support-mac-os-x/|access-date=6 February 2014|date=2013-10-02}}</ref>

Finally, macOS supports reading and writing of the [[exFAT]] file system since Mac OS X Snow Leopard, starting from version 10.6.5.<ref name="encase-book">{{cite book|url=https://books.google.com/books?id=c1mezk6uHfIC&q=os+x+exfat+10.6.5&pg=PA79 |title=EnCase Computer Forensics - The Official EnCE: EnCase Certified Examiner |author=Steve Bunting |date=2012-08-14 |publisher=Wiley |access-date=2014-02-07|isbn=9781118219409 }}</ref>

===OS/2===
[[OS/2]] 1.2 introduced the [[High Performance File System]] (HPFS). HPFS supports mixed case file names in different [[code page]]s, long file names (255 characters), more efficient use of disk space, an architecture that keeps related items close to each other on the disk volume, less fragmentation of data, [[Extent (file systems)|extent-based]] space allocation, a [[B+ tree]] structure for directories, and the root directory located at the midpoint of the disk, for faster average access. A [[JFS (file system)|journaled filesystem]] (JFS) was shipped in 1999.

===PC-BSD===
[[PC-BSD]] is a desktop version of FreeBSD, which inherits [[FreeBSD]]'s [[ZFS]] support, similarly to [[FreeNAS]]. The new graphical installer of [[PC-BSD]] can handle ''/ ([[Root directory|root]]) on ZFS'' and [[RAID-Z]] pool installs and [[disk encryption]] using [[Geli (software)|Geli]] right from the start in an easy convenient ([[GUI]]) way. The current PC-BSD 9.0+ 'Isotope Edition' has ZFS filesystem version 5 and ZFS storage pool version 28.

===Plan 9===
[[Plan&nbsp;9 from Bell Labs]] treats everything as a file and accesses all objects as a file would be accessed (i.e., there is no [[ioctl]] or [[mmap]]): networking, graphics, debugging, authentication, capabilities, encryption, and other services are accessed via I/O operations on [[file descriptor]]s. The [[9P (protocol)|9P]] protocol removes the difference between local and remote files. File systems in Plan&nbsp;9 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)|Inferno]] operating system shares these concepts with Plan&nbsp;9.

===Microsoft Windows===
[[File:Dir command in Windows Command Prompt.png|thumb|right|300px|Directory listing in a [[Microsoft Windows|Windows]] command shell]]
Windows makes use of the [[File Allocation Table|FAT]], [[NTFS]], [[exFAT]], [[Live File System]] and [[ReFS]] file systems (the last of these is only supported and usable in [[Windows Server 2012]], [[Windows Server 2016]], [[Windows 8]], [[Windows 8.1]], and [[Windows 10]]; Windows cannot boot from it).

Windows uses a ''[[drive letter]]'' abstraction at the user level to distinguish one disk or partition from another. For example, the [[path (computing)|path]] {{mono|C:\WINDOWS}} represents a directory {{mono|WINDOWS}} on the partition represented by the letter C. Drive C: is most commonly used for the primary [[hard disk drive]] partition, on which Windows is usually installed and from which it boots. This "tradition" has become so firmly ingrained that bugs exist in many applications which make assumptions that the drive that the operating system is installed on is C. The use of drive letters, and the tradition of using "C" as the drive letter for the primary hard disk drive partition, 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, and ultimately from IBM's [[CP/CMS]] of 1967.

====FAT====
{{Main|File Allocation Table}}

The family of [[File Allocation Table|FAT]] file systems is supported by almost all operating systems for personal computers, including all versions of [[Microsoft Windows|Windows]] and [[MS-DOS]]/[[PC&nbsp;DOS]], [[OS/2]], and [[DR-DOS]]. (PC&nbsp;DOS is an OEM version of MS-DOS, MS-DOS was originally based on [[Seattle Computer Products|SCP]]'s [[86-DOS]]. DR-DOS was based on [[Digital Research]]'s [[Concurrent DOS]], a successor of [[CP/M-86]].) The FAT file systems are therefore well-suited as a universal exchange format between computers and devices of most any type and age.

The FAT file system traces its roots back to an (incompatible) 8-bit FAT precursor in [[Standalone Disk BASIC]] and the short-lived [[MIDAS (operating system)|MDOS/MIDAS]] project.{{Citation needed|date=September 2012}}

Over the years, the file system has been expanded from [[FAT12]] to [[FAT16]] and [[FAT32]]. Various features have been added to the file system including [[subdirectory|subdirectories]], [[codepage]] support, [[extended attribute]]s, and [[long filenames]]. Third parties such as Digital Research have incorporated optional support for deletion tracking, and volume/directory/file-based multi-user security schemes to support file and directory passwords and permissions such as read/write/execute/delete access rights. Most of these extensions are not supported by Windows.

The FAT12 and FAT16 file systems had 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 [[partition (computing)|partitions]].

FAT32 addresses the limitations in FAT12 and FAT16, except for the file size limit of close to 4&nbsp;GB, but it remains limited compared to NTFS.

FAT12, FAT16 and FAT32 also have a limit of eight characters for the file name, and three characters for the extension (such as [[.exe]]). This is commonly referred to as the [[8.3 filename]] limit. [[VFAT]], an optional extension to FAT12, FAT16 and FAT32, introduced in [[Windows 95]] and [[Windows NT 3.5]], allowed long file names ([[Long File Name|LFN]]) to be stored in the FAT file system in a backwards compatible fashion.

====NTFS====
{{Main|NTFS}}

[[NTFS]], introduced with the [[Windows NT]] operating system in 1993, allowed [[Access control list|ACL]]-based permission control. Other features also supported by [[NTFS]] include hard links, multiple file streams, attribute indexing, quota tracking, sparse files, encryption, compression, and reparse points (directories working as mount-points for other file systems, symlinks, junctions, remote storage links).

====exFAT====
{{Main|exFAT}}

[[exFAT]] has certain advantages over NTFS with regard to [[file system overhead]].{{citation needed|date=March 2021}}

exFAT is not backward compatible with FAT file systems such as FAT12, FAT16 or FAT32. The file system is supported with newer Windows systems, such as Windows XP, Windows Server 2003, Windows Vista, Windows 2008, Windows 7, Windows 8, Windows 8.1, Windows 10 and Windows 11.

exFAT is supported in macOS starting with version 10.6.5 (Snow Leopard).<ref name="encase-book" /> Support in other operating systems is sparse since implementing support for exFAT requires a license. exFAT is the only file system that is fully supported on both macOS and Windows that can hold files larger than 4&nbsp;GB.<ref>{{cite web|url=https://support.apple.com/guide/disk-utility/dsku19ed921c/mac|title=File system formats available in Disk Utility on Mac|website=Apple Support}}</ref><ref>{{cite web|url=https://docs.microsoft.com/en-us/windows/win32/fileio/exfat-specification|title=exFAT file system specification|website=Microsoft Docs}}</ref>

===OpenVMS===
{{Main|Files-11}}

===MVS===
{{Main|MVS#MVS filesystem}}
Prior to the introduction of [[VSAM]], [[OS/360]] systems implemented a hybrid file system. The system was designed to easily support [[Removable media|removable disk packs]], so the information relating to all files on one disk (''volume'' in IBM terminology) is stored on that disk in a [[Flat file database|flat system file]] called the ''[[Volume Table of Contents]]'' (VTOC). The VTOC stores all metadata for the file. Later a hierarchical directory structure was imposed with the introduction of the ''System Catalog'', which can optionally catalog files (datasets) on resident and removable volumes. The catalog only contains information to relate a dataset to a specific volume. If the user requests access to a dataset on an offline volume, and they have suitable privileges, the system will attempt to mount the required volume. Cataloged and non-cataloged datasets can still be accessed using information in the VTOC, bypassing the catalog, if the required volume id is provided to the OPEN request. Still later the VTOC was indexed to speed up access.

===Conversational Monitor System===
{{Main|CMS file system}}
The IBM [[Conversational Monitor System]] (CMS) component of [[VM/370]] uses a separate flat file system for each [[virtual disk]] (''minidisk''). File data and control information are scattered and intermixed. The anchor is a record called the ''Master File Directory'' (MFD), always located in the fourth block on the disk. Originally CMS used fixed-length 800-byte blocks, but later versions used larger size blocks up to 4K. Access to a data record requires two levels of [[indirection]], where the file's directory entry (called a ''File Status Table'' (FST) entry) points to blocks containing a list of addresses of the individual records.

===AS/400 file system===
Data on the AS/400 and its successors consists of system objects mapped into the system virtual address space in a [[single-level store]]. Many types of [[Object (IBM i)|object]]s are defined including the directories and files found in other file systems. File objects, along with other types of objects, form the basis of the AS/400's support for an integrated [[relational database]].

===Other file systems===
* The Prospero File System is a file system based on the Virtual System Model.<ref>{{cite book|url=http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.132.7982|title=The Prospero File System: A Global File System Based on the Virtual System Model|year=1992}}</ref> The system was created by B. Clifford Neuman of the Information Sciences Institute at the University of Southern California.
* [[Flex machine#RSRE FLEX Computer System|RSRE FLEX file system]] - written in [[ALGOL 68]]
* The file system of the [[Michigan Terminal System]] (MTS) is interesting because: (i) it provides "line files" where record lengths and line numbers are associated as metadata with each record in the file, lines can be added, replaced, updated with the same or different length records, and deleted anywhere in the file without the need to read and rewrite the entire file; (ii) using program keys files may be shared or permitted to commands and programs in addition to users and groups; and (iii) there is a comprehensive file locking mechanism that protects both the file's data and its metadata.<ref>{{cite journal|url=https://ieeexplore.ieee.org/document/1451786|title=A file system for a general-purpose time-sharing environment|first=G. C.|last=Pirkola|journal=[[Proceedings of the IEEE]]|date=June 1975|volume=63|issue=6|pages=918–924|doi=10.1109/PROC.1975.9856 |s2cid=12982770 |issn=0018-9219|url-access=subscription}}</ref><ref name=Protection1977>{{cite conference|url=https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxtaWNoaWdhbnRlcm1pbmFsc3lzdGVtfGd4Ojc5MTAxNzg1NTVmMjg5Mzk|title=The Protection of Information in a General Purpose Time-Sharing Environment|first1=Gary C.|last1=Pirkola|first2=John|last2=Sanguinetti|book-title=Proceedings of the IEEE Symposium on Trends and Applications 1977: Computer Security and Integrity|volume=10|issue=4|pages=106–114}}</ref>
* [[TempleOS]] uses RedSea, a file system made by Terry A. Davis.<ref name="00yK1">{{Cite web|last=Davis|first=Terry A.|date=n.d.|url=http://www.templeos.org/Wb/Doc/Features.html#l1|title=The Temple Operating System|website=www.templeos.org|access-date=March 30, 2017|url-status=dead|archive-url=https://web.archive.org/web/20170331120502/http://www.templeos.org/Wb/Doc/Features.html#l1|archive-date=March 31, 2017}}</ref>