File system

This article is about how a computer organizes and accesses computer files. For library and office filing systems, seeLibrary classification.

Incomputing,afile systemorfilesystem(often abbreviated toFSorfs) governsfileorganization and access. Alocalfile system is a capability of anoperating systemthat services the applications running on the samecomputer.Adistributed file systemis aprotocolthat provides file access betweennetworkedcomputers.

A file system provides adata storageservicethat allowsapplicationsto sharemass storage.Without a file system, applications could access the storage inincompatibleways that lead toresource contention,data corruptionanddata loss.

There are many file systemdesignsandimplementations– with various structure and features and various resulting characteristics such as speed, flexibility, security, size and more.

Files systems have been developed for many types ofstorage devices,includinghard disk drives(HDDs),solid-state drives(SSDs),magnetic tapesandoptical discs.

A portion of the computermain memorycan be set up as aRAM diskthat serves as a storage device for a file system. File systems such astmpfscan store files invirtual memory.

Avirtualfile system provides access to files that are either computed on request, calledvirtual files(seeprocfsandsysfs), or are mapping into another, backing storage.

Etymology

Fromc. 1900and before the advent of computers the termsfile system,filing systemandsystem for filingwere used to describe methods of organizing, storing and retrieving paper documents.By 1961, the termfile systemwas being applied to computerized filing alongside the original meaning.By 1964, it was in general use.

Architecture

A local file system'sarchitecturecan be described aslayers of abstractioneven though a particular file system design may not actually separate the concepts.

Thelogical file systemlayer provides relatively high-level access via anapplication programming interface(API) for file operations including open, close, read and write – delegating operations to lower layers. This layer manages open file table entries and per-process file descriptors.It provides file access, directory operations, security and protection.

Thevirtual file system,an optional layer, supports multiple concurrent instances of physical file systems, each of which called a file system implementation.

Thephysical file systemlayer provides relatively low-level access to a storage device (e.g. disk). It reads and writesdata blocks,providesbufferingand othermemory managementand controls placement of blocks in specific locations on the storage medium. This layer usesdevice driversorchannel I/Oto drive the storage device.

Attributes

File names

Main article:Filename

Afile name,orfilename,identifies a file to consuming applications and in some cases users.

A file name is unique so that an application can refer to exactly one file for a particular name. If the file system supports directories, then generally file name uniqueness is enforced within the context of each directory. In other words, a storage can contain multiple files with the same name, but not in the same directory.

Most file systems restrict the length of a file name.

Some file systems match file names ascase sensitiveand others as case insensitive. For example, the namesMYFILEandmyfilematch the same file for case insensitive, but different files for case sensitive.

Most modern file systems allow a file name to contain a wide range of characters from theUnicodecharacter set. Some restrict characters such as those used to indicate special attributes such as a device, device type, directory prefix, file path separator, or file type.

Directories

Main article:Directory (computing)

File systems typically support organizing files intodirectories,also calledfolders,which segregate files into groups.

This may be implemented by associating the file name with an index in atable of contentsor aninodein aUnix-likefile system.

Directory structures may be flat (i.e. linear), or allow hierarchies by allowing a directory to contain directories, called subdirectories.

The first file system to support arbitrary hierarchies of directories was used in theMulticsoperating system.The native file systems of Unix-like systems also support arbitrary directory hierarchies, as do,Apple'sHierarchical File Systemand its successorHFS+inclassic Mac OS,theFATfile system inMS-DOS2.0 and later versions of MS-DOS and inMicrosoft Windows,theNTFSfile system in theWindows NTfamily of operating systems, and the ODS-2 (On-Disk Structure-2) and higher levels of theFiles-11file system inOpenVMS.

Metadata

In addition to data, the file content, a file system also manages associatedmetadatawhich may include but is not limited to:

• name
• sizewhich may be stored as the number of blocks allocated or as abytecount
• whencreated, last accessed, last backed-up
• owneruserandgroup
• access permissions
• file attributessuch as whether the file is read-only,executable,etc.
• device type(e.g.block,character,socket,subdirectory,etc.)

A file system stores associated metadata separate from the content of the file.

Most file systems store the names of all the files in one directory in one place—the directory table for that directory—which is often stored like any other file. Many file systems put only some of the metadata for a file in the directory table, and the rest of the metadata for that file in a completely separate structure, such as theinode.

Most file systems also store metadata not associated with any one particular file. Such metadata includes information about unused regions—free space bitmap,block availability map—and information aboutbad sectors. Often such information about anallocation groupis stored inside the allocation group itself.

Additional attributes can be associated on file systems, such asNTFS,XFS,ext2,ext3,some versions ofUFS,andHFS+,usingextended file attributes.Some file systems provide for user defined attributes such as the author of the document, the character encoding of a document or the size of an image.

Some file systems allow for different data collections to be associated with one file name. These separate collections may be referred to asstreamsorforks.Apple has long used a forked file system on the Macintosh, and Microsoft supports streams in NTFS. Some file systems maintain multiple past revisions of a file under a single file name; the file name by itself retrieves the most recent version, while prior saved version can be accessed using a special naming convention such as "filename;4" or "filename(-4)" to access the version four saves ago.

Seecomparison of file systems#Metadatafor details on which file systems support which kinds of metadata.

Storage space organization

A local file system tracks which areas of storage belong to which file and which are not being used.

When a file system creates a file, it allocates space for data. Some file systems permit or require specifying an initial space allocation and subsequent incremental allocations as the file grows.

To delete a file, the file system records that the file's space is free; available to use for another file.

An example of slack space, demonstrated with 4,096-byteNTFS clusters: 100,000 files, each five bytes per file, which equal to 500,000 bytes of actual data but require 409,600,000 bytes of disk space to store

A local file system manages storage space to provide a level of reliability and efficiency. Generally, it allocates storage device space in a granular manner, usually multiple physical units (i.e.bytes). For example, inApple DOSof the early 1980s, 256-byte sectors on 140 kilobyte floppy disk used atrack/sector map.

The granular nature results in unused space, sometimes calledslack space,for each file except for those that have the rare size that is a multiple of the granular allocation.For a 512-byte allocation, the average unused space is 256 bytes. For 64 KB clusters, the average unused space is 32 KB.

Generally, the allocation unit size is set when the storage is configured. Choosing a relatively small size compared to the files stored, results in excessive access overhead. Choosing a relatively large size results in excessive unused space. Choosing an allocation size based on the average size of files expected to be in the storage tends to minimize unusable space.

Fragmentation

File systems may becomefragmented

As a file system creates, modifies and deletes files, the underlying storage representation may becomefragmented.Files and the unused space between files will occupy allocation blocks that are not contiguous.

A file becomes fragmented if space needed to store its content cannot be allocated in contiguous blocks. Free space becomes fragmented when files are deleted.

This is invisible to the end user and the system still works correctly. However this can degrade performance on some storage hardware that work better with contiguous blocks such ashard disk drives.Other hardware such assolid-state drivesare not affected by fragmentation.

Access control

See also:Computer security,Password cracking,Filesystem-level encryption,andEncrypting File System

A file system often supports access control of data that it manages.

The intent of access control is often to prevent certain users from reading or modifying certain files.

Access control can also restrict access by program in order to ensure that data is modified in a controlled way. Examples include passwords stored in the metadata of the file or elsewhere andfile permissionsin the form of permission bits,access control lists,orcapabilities.The need for file system utilities to be able to access the data at the media level to reorganize the structures and provide efficient backup usually means that these are only effective for polite users but are not effective against intruders.

Methods for encrypting file data are sometimes included in the file system. This is very effective since there is no need for file system utilities to know the encryption seed to effectively manage the data. The risks of relying on encryption include the fact that an attacker can copy the data and use brute force to decrypt the data. Additionally, losing the seed means losing the data.

Storage quota

Some operating systems allow a system administrator to enabledisk quotasto limit a user's use of storage space.

Data integrity

A file system typically ensures that stored data remains consistent in both normal operations as well as exceptional situations like:

• accessing program neglects to inform the file system that it has completed file access (to close a file)
• accessing program terminates abnormally (crashes)
• media failure
• loss of connection to remote systems
• operating system failure
• system reset (soft reboot)
• power failure (hard reboot)

Recovery from exceptional situations may include updating metadata, directory entries and handling data that was buffered but not written to storage media.

Recording

A file system might record events to allow analysis of issues such as:

• file or systemic problems and performance
• nefarious access

Data access

Byte stream access

Many file systems access data as a stream ofbytes.Typically, to read file data, a program provides amemory bufferand the file system retrieves data from the medium and then writes the data to the buffer. A write involves the program providing a buffer of bytes that the file system reads and then stores to the medium.

Record access

Some file systems, or layers on top of a file system, allow a program to define arecordso that a program can read and write data as a structure; not an unorganized sequence of bytes.

If afixed lengthrecord definition is used, then locating the n^threcord can be calculated mathematically, which is relatively fast compared to parsing the data for record separators.

An identification for each record, also known as a key, allows a program to read, write and update records without regard to their location in storage. Such storage requires managing blocks of media, usually separating key blocks and data blocks. Efficient algorithms can be developed with pyramid structures for locating records.

Utilities

Typically, a file system can be managed by the user via various utility programs.

Some utilities allow the user to create, configure and remove an instance of a file system. It may allow extending or truncating the space allocated to the file system.

Directory utilities may be used to create, rename and deletedirectory entries,which are also known asdentries(singular:dentry),and to alter metadata associated with a directory. Directory utilities may also include capabilities to create additional links to a directory (hard linksinUnix), to rename parent links ( ".." inUnix-likeoperating systems),and to create bidirectional links to files.

File utilities create, list, copy, move and delete files, and alter metadata. They may be able to truncate data, truncate or extend space allocation, append to, move, and modify files in-place. Depending on the underlying structure of the file system, they may provide a mechanism to prepend to or truncate from the beginning of a file, insert entries into the middle of a file, or delete entries from a file. Utilities to free space for deleted files, if the file system provides an undelete function, also belong to this category.

Some file systems defer operations such as reorganization of free space, secure erasing of free space, and rebuilding of hierarchical structures by providing utilities to perform these functions at times of minimal activity. An example is the file systemdefragmentationutilities.

Some of the most important features of file system utilities are supervisory activities which may involve bypassing ownership or direct access to the underlying device. These include high-performance backup and recovery, data replication, and reorganization of various data structures and allocation tables within the file system.

File system API

Utilities, libraries and programs usefile system APIsto make requests of the file system. These include data transfer, positioning, updating metadata, managing directories, managing access specifications, and removal.

Multiple file systems within a single system

Frequently, retail systems are configured with a single file system occupying the entirestorage device.

Another approach is topartitionthe disk so that several file systems with different attributes can be used. One file system, for use as browser cache or email storage, might be configured with a small allocation size. This keeps the activity of creating and deleting files typical of browser activity in a narrow area of the disk where it will not interfere with other file allocations. Another partition might be created for the storage of audio or video files with a relatively large block size. Yet another may normally be setread-onlyand only periodically be set writable. Some file systems, such asZFSandAPFS,support multiple file systems sharing a common pool of free blocks, supporting several file systems with different attributes without having to reserved a fixed amount of space for each file system.

A third approach, which is mostly used in cloud systems, is to use "disk images"to house additional file systems, with the same attributes or not, within another (host) file system as a file. A common example is virtualization: one user can run an experimental Linux distribution (using theext4file system) in a virtual machine under his/her production Windows environment (usingNTFS). The ext4 file system resides in a disk image, which is treated as a file (or multiple files, depending on thehypervisorand settings) in the NTFS host file system.

Having multiple file systems on a single system has the additional benefit that in the event of a corruption of a single file system, the remaining file systems will frequently still be intact. This includes virus destruction of thesystemfile system or even a system that will not boot. File system utilities which require dedicated access can be effectively completed piecemeal. In addition,defragmentationmay be more effective. Several system maintenance utilities, such as virus scans and backups, can also be processed in segments. For example, it is not necessary to backup the file system containing videos along with all the other files if none have been added since the last backup. As for the image files, one can easily "spin off" differential images which contain only "new" data written to the master (original) image. Differential images can be used for both safety concerns (as a "disposable" system - can be quickly restored if destroyed or contaminated by a virus, as the old image can be removed and a new image can be created in matter of seconds, even without automated procedures) and quick virtual machine deployment (since the differential images can be quickly spawned using a script in batches).

Types

Disk file systems

Adisk file systemtakes advantages of the ability of disk storage media to randomly address data in a short amount of time. Additional considerations include the speed of accessing data following that initially requested and the anticipation that the following data may also be requested. This permits multiple users (or processes) access to various data on the disk without regard to the sequential location of the data. Examples includeFAT(FAT12,FAT16,FAT32),exFAT,NTFS,ReFS,HFSandHFS+,HPFS,APFS,UFS,ext2,ext3,ext4,XFS,btrfs,Files-11,Veritas File System,VMFS,ZFS,ReiserFS,NSSand ScoutFS. Some disk file systems arejournaling file systemsorversioning file systems.

Optical discs

ISO 9660andUniversal Disk Format(UDF) are two common formats that targetCompact Discs,DVDsandBlu-raydiscs.Mount Rainieris an extension to UDF supported since 2.6 series of the Linux kernel and since Windows Vista that facilitates rewriting to DVDs.

Flash file systems

Main article:Flash file system

Aflash file systemconsiders the special abilities, performance and restrictions offlash memorydevices. Frequently a disk file system can use a flash memory device as the underlying storage media, but it is much better to use a file system specifically designed for a flash device.

Tape file systems

Atape file systemis a file system and tape format designed to store files on tape.Magnetic tapesare sequential storage media with significantly longer random data access times than disks, posing challenges to the creation and efficient management of a general-purpose file system.

In a disk file system there is typically a master file directory, and a map of used and free data regions. Any file additions, changes, or removals require updating the directory and the used/free maps. Random access to data regions is measured in milliseconds so this system works well for disks.

Tape requires linear motion to wind and unwind potentially very long reels of media. This tape motion may take several seconds to several minutes to move the read/write head from one end of the tape to the other.

Consequently, a master file directory and usage map can be extremely slow and inefficient with tape. Writing typically involves reading the block usage map to find free blocks for writing, updating the usage map and directory to add the data, and then advancing the tape to write the data in the correct spot. Each additional file write requires updating the map and directory and writing the data, which may take several seconds to occur for each file.

Tape file systems instead typically allow for the file directory to be spread across the tape intermixed with the data, referred to asstreaming,so that time-consuming and repeated tape motions are not required to write new data.

However, a side effect of this design is that reading the file directory of a tape usually requires scanning the entire tape to read all the scattered directory entries. Most data archiving software that works with tape storage will store a local copy of the tape catalog on a disk file system, so that adding files to a tape can be done quickly without having to rescan the tape media. The local tape catalog copy is usually discarded if not used for a specified period of time, at which point the tape must be re-scanned if it is to be used in the future.

IBM has developed a file system for tape called theLinear Tape File System.The IBM implementation of this file system has been released as the open-sourceIBM Linear Tape File System — Single Drive Edition (LTFS-SDE)product. The Linear Tape File System uses a separate partition on the tape to record the index meta-data, thereby avoiding the problems associated with scattering directory entries across the entire tape.

Tape formatting

Writing data to a tape, erasing, or formatting a tape is often a significantly time-consuming process and can take several hours on large tapes.With many data tape technologies it is not necessary to format the tape before over-writing new data to the tape. This is due to the inherently destructive nature of overwriting data on sequential media.

Because of the time it can take to format a tape, typically tapes are pre-formatted so that the tape user does not need to spend time preparing each new tape for use. All that is usually necessary is to write an identifying media label to the tape before use, and even this can be automatically written by software when a new tape is used for the first time.

Database file systems

Another concept for file management is the idea 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 similarrich metadata.

IBM DB2 for i(formerly known as DB2/400 and DB2 for i5/OS) is a database file system as part of the object basedIBM ioperating system (formerly known as OS/400 and i5/OS), incorporating asingle level storeand running on IBM Power Systems (formerly known as AS/400 and iSeries), designed by Frank G. Soltis IBM's former chief scientist for IBM i. Around 1978 to 1988 Frank G. Soltis and his team at IBM Rochester had successfully designed and applied technologies like the database file system where others like Microsoft later failed to accomplish.These technologies are informally known as 'Fortress Rochester'and were in few basic aspects extended from early Mainframe technologies but in many ways more advanced from a technological perspective.

Some other projects that are not "pure" database file systems but that use some aspects of a database file system:

• ManyWeb content management systemsuse arelational DBMSto store and retrieve files. For example,XHTMLfiles are stored asXMLor text fields, while image files are stored as blob fields;SQLSELECT (with optionalXPath) statements retrieve the files, and allow the use of a sophisticated logic and more rich information associations than "usual file systems." Many CMSs also have the option of storing onlymetadatawithin the database, with the standard filesystem used to store the content of files.
• Very large file systems, embodied by applications likeApache HadoopandGoogle File System,use somedatabase file systemconcepts.

Transactional file systems

Some programs need to either make multiple file system changes, or, if one or more of the changes fail for any reason, make none of the changes. For example, a program which is installing or updating software may write executables, libraries, and/or configuration files. If some of the writing fails and the software is left partially installed or updated, the software may be broken or unusable. An incomplete update of a key system utility, such as the commandshell,may leave the entire system in an unusable state.

Transaction processingintroduces theatomicityguarantee, ensuring 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. Transactions also provide theisolationguarantee,meaning 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 properlyserializedwith the transaction.

Windows, beginning with Vista, added transaction support toNTFS,in a feature calledTransactional NTFS,but its use is now discouraged.There are a number of research prototypes of transactional file systems for UNIX systems, including the Valor file system,Amino,LFS,and a transactionalext3file system on the TxOS kernel,as well as transactional file systems targeting embedded systems, such as TFFS.

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

Journaling file systemsis 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.

Data backup systems typically do not provide support for direct backup of data stored in a transactional manner, which makes the recovery of reliable and consistent data sets difficult. Most backup software simply notes what files have changed since a certain time, regardless of the transactional state shared across multiple files in the overall dataset. As a workaround, some database systems simply produce an archived state file containing all data up to that point, and the backup software only backs that up and does not interact directly with the active transactional databases at all. Recovery requires separate recreation of the database from the state file after the file has been restored by the backup software.

Network file systems

Main article:Distributed file system

Anetwork file systemis a file system that acts as a client for a remote file access protocol, providing access to files on a server. Programs using local interfaces can transparently create, manage and access hierarchical directories and files in remote network-connected computers. Examples of network file systems include clients for theNFS,AFS,SMBprotocols, and file-system-like clients forFTPandWebDAV.

Shared disk file systems

Main article:Shared disk file system

Ashared disk file systemis one in which a number of machines (usually servers) all have access to the same external disk subsystem (usually astorage area network). The file system arbitrates access to that subsystem, preventing write collisions.Examples includeGFS2fromRed Hat,GPFS,now known as Spectrum Scale, from IBM,SFSfrom DataPlow,CXFSfromSGI,StorNextfromQuantum Corporationand ScoutFS from Versity.

Special file systems

Some file systems expose elements of the operating system as files so they can be acted on via thefile system API.This is common inUnix-likeoperating systems, and to a lesser extent in other operating systems. Examples include:

• devfs,udev,TOPS-10expose I/O devices or pseudo-devices as special files
• configfsandsysfsexpose special files that can be used to query and configureLinuxkernel information
• procfsexposes process information as special files

Minimal file system / audio-cassette storage

In the 1970s disk and digital tape devices were too expensive for some earlymicrocomputerusers. An inexpensive basic data storage system was devised that used commonaudio cassettetape.

When the system needed to write data, the user was notified to press "RECORD" on the cassette recorder, then press "RETURN" on the keyboard to notify the system that the cassette recorder was recording. The system wrote a sound to provide time synchronization, thenmodulated soundsthat encoded a prefix, the data, achecksumand a suffix. When the system needed to read data, the user was instructed to press "PLAY" on the cassette recorder. The system wouldlistento the sounds on the tape waiting until a burst of sound could be recognized as the synchronization. The system would then interpret subsequent sounds as data. When the data read was complete, the system would notify the user to press "STOP" on the cassette recorder. It was primitive, but it (mostly) worked. Data was stored sequentially, usually in an unnamed format, although some systems (such as theCommodore PETseries of computers) did allow the files to be named. Multiple sets of data could be written and located by fast-forwarding the tape and observing at the tape counter to find the approximate start of the next data region on the tape. The user might have to listen to the sounds to find the right spot to begin playing the next data region. Some implementations even included audible sounds interspersed with the data.

Flat file systems

Not to be confused withFlat file database.

In a flat file system, there are nosubdirectories;directory entries for all files are stored in a single directory.

Whenfloppy diskmedia was first available this type of file system was adequate due to the relatively small amount of data space available.CP/Mmachines featured a flat file system, where files could be assigned to one of 16user areasand generic file operations narrowed to work on one instead of defaulting to work on all of them. These user areas were no more than special attributes associated with the files; that is, it was not necessary to define specificquotafor each of these areas and files could be added to groups for as long as there was still free storage space on the disk. The earlyApple Macintoshalso featured a flat file system, theMacintosh File System.It was unusual in that the file management program (Macintosh Finder) created the illusion of a partially hierarchical filing system on top of EMFS. This structure required every file to have a unique name, even if it appeared to be in a separate folder.IBMDOS/360andOS/360store entries for all files on a disk pack (volume) in a directory on the pack called aVolume Table of Contents(VTOC).

While simple, flat file systems become awkward as the number of files grows and makes it difficult to organize data into related groups of files.

A recent addition to the flat file system family isAmazon'sS3,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.

Implementations

Anoperating 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 providescommand line interface,such asUnix shell,WindowsCommand PromptandPowerShell,andOpenVMS DCL.An OS often also providesgraphical user interfacefile browserssuch as MacOSFinderand WindowsFile Explorer.

Unix and Unix-like operating systems

Unix-likeoperating 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 oneroot directory,and every file existing on the system is located under it somewhere. Unix-like systems can use aRAM diskor 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 calledmountinga file system. For example, to access the files on aCD-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 themount point– it might, for example, be/media.The/mediadirectory exists on many Unix systems (as specified in theFilesystem 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 theadministrator(i.e.root user) may authorize the mounting of file systems.

Unix-likeoperating 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 hasbooted.All Unix-like systems therefore provide a facility for mounting file systems at boot time.System administratorsdefine these file systems in the configuration filefstab(vfstabinSolaris), which also indicates options and mount points.
• In some situations, there is no need to mount certain file systems atboot 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 includeUSB flash drives,CD-ROMs,andDVDs.Utilities have therefore been developed to detect the presence and availability of a medium and then mount that medium without any user intervention.
• Progressive Unix-like systems have also introduced a concept calledsupermounting;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.
• Anautomounterwill 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.

Linux

Linuxsupports numerous file systems, but common choices for the system disk on a block device include the ext* family (ext2,ext3andext4),XFS,JFS,andbtrfs.For raw flash without aflash translation layer(FTL) orMemory Technology Device(MTD), there areUBIFS,JFFS2andYAFFS,among others.SquashFSis a common compressed read-only file system.

Solaris

Solarisin earlier releases defaulted to (non-journaled or non-logging)UFSfor bootable and supplementary file systems. Solaris defaulted to, supported, and extended UFS.

Support for other file systems and significant enhancements were added over time, includingVeritas SoftwareCorp. (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 VeritasVxFSoperation. Logging orjournalingwas added to UFS in Sun'sSolaris 7.Releases ofSolaris 10,Solaris Express,OpenSolaris,and other open source variants of the Solaris operating system later supported bootableZFS.

Logical Volume Managementallows for spanning a file system across multiple devices for the purpose of adding redundancy, capacity, and/or throughput. Legacy environments in Solaris may useSolaris Volume Manager(formerly known asSolstice DiskSuite). Multiple operating systems (including Solaris) may useVeritas Volume Manager.Modern Solaris based operating systems eclipse the need for volume management through leveraging virtual storage pools inZFS.

macOS

macOS (formerly Mac OS X)uses theApple File System(APFS), which in 2017 replaced a file system inherited fromclassic Mac OScalledHFS Plus(HFS+). Apple also uses the term "Mac OS Extended" for HFS+.HFS Plus is ametadata-rich andcase-preservingbut (usually)case-insensitivefile 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 usesUnicodeto store file names. On macOS, thefiletypecan come from thetype code,stored in file's metadata, or thefilename extension.

HFS Plus has three kinds of links: Unix-stylehard links,Unix-stylesymbolic links,andaliases.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 inuserland.

macOS 10.13 High Sierra, which was announced on June 5, 2017, at Apple's WWDC event, uses theApple File Systemonsolid-state drives.

macOS also supported theUFSfile system, derived from theBSDUnix Fast File System viaNeXTSTEP.However, as ofMac 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.As ofMac OS X LionUFS support was completely dropped.

Newer versions of macOS are capable of reading and writing to the legacyFATfile systems (16 and 32) common on Windows. They are also capable ofreadingthe newerNTFSfile systems for Windows. In order towriteto NTFS file systems on macOS versions prior toMac OS X Snow Leopardthird-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).

Finally, macOS supports reading and writing of theexFATfile system since Mac OS X Snow Leopard, starting from version 10.6.5.

OS/2

OS/21.2 introduced theHigh Performance File System(HPFS). HPFS supports mixed case file names in differentcode pages,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-basedspace allocation, aB+ treestructure for directories, and the root directory located at the midpoint of the disk, for faster average access. Ajournaled filesystem(JFS) was shipped in 1999.

PC-BSD

PC-BSDis a desktop version of FreeBSD, which inheritsFreeBSD'sZFSsupport, similarly toFreeNAS.The new graphical installer ofPC-BSDcan handle/ (root) on ZFSandRAID-Zpool installs anddisk encryptionusingGeliright 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 9 from Bell Labstreats everything as a file and accesses all objects as a file would be accessed (i.e., there is noioctlormmap): networking, graphics, debugging, authentication, capabilities, encryption, and other services are accessed via I/O operations onfile descriptors.The9Pprotocol removes the difference between local and remote files. File systems in Plan 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.

TheInfernooperating system shares these concepts with Plan 9.

Microsoft Windows

Directory listing in aWindowscommand shell

Windows makes use of theFAT,NTFS,exFAT,Live File SystemandReFSfile systems (the last of these is only supported and usable inWindows Server 2012,Windows Server 2016,Windows 8,Windows 8.1,andWindows 10;Windows cannot boot from it).

Windows uses adrive letterabstraction at the user level to distinguish one disk or partition from another. For example, thepathC:\WINDOWSrepresents a directoryWINDOWSon the partition represented by the letter C. Drive C: is most commonly used for the primaryhard disk drivepartition, 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 toMS-DOS,where the letters A and B were reserved for up to two floppy disk drives. This in turn derived fromCP/Min the 1970s, and ultimately from IBM'sCP/CMSof 1967.

FAT

Main article:File Allocation Table

The family ofFATfile systems is supported by almost all operating systems for personal computers, including all versions ofWindowsandMS-DOS/PC DOS,OS/2,andDR-DOS.(PC DOS is an OEM version of MS-DOS, MS-DOS was originally based onSCP's86-DOS.DR-DOS was based onDigital Research'sConcurrent DOS,a successor ofCP/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 inStandalone Disk BASICand the short-livedMDOS/MIDASproject.

Over the years, the file system has been expanded fromFAT12toFAT16andFAT32.Various features have been added to the file system includingsubdirectories,codepagesupport,extended attributes,andlong 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 theroot directoryof the file system and had restrictions on the maximum size of FAT-formatted disks orpartitions.

FAT32 addresses the limitations in FAT12 and FAT16, except for the file size limit of close to 4 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 the8.3 filenamelimit.VFAT,an optional extension to FAT12, FAT16 and FAT32, introduced inWindows 95andWindows NT 3.5,allowed long file names (LFN) to be stored in the FAT file system in a backwards compatible fashion.

NTFS

Main article:NTFS

NTFS,introduced with theWindows NToperating system in 1993, allowedACL-based permission control. Other features also supported byNTFSinclude hard links, multiple file streams, attribute inde xing, 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 article:exFAT

exFAThas certain advantages over NTFS with regard tofile system overhead.

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).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 GB.

OpenVMS

Main article:Files-11

MVS

Main article:MVS § MVS filesystem

Prior to the introduction ofVSAM,OS/360systems implemented a hybrid file system. The system was designed to easily supportremovable disk packs,so the information relating to all files on one disk (volumein IBM terminology) is stored on that disk in aflat system filecalled theVolume Table of Contents(VTOC). The VTOC stores all metadata for the file. Later a hierarchical directory structure was imposed with the introduction of theSystem 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 article:CMS file system

The IBMConversational Monitor System(CMS) component ofVM/370uses a separate flat file system for eachvirtual disk(minidisk). File data and control information are scattered and intermixed. The anchor is a record called theMaster 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 ofindirection,where the file's directory entry (called aFile 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 asingle-level store.Many types ofobjectsare 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 integratedrelational database.

Other file systems

• The Prospero File System is a file system based on the Virtual System Model.The system was created by B. Clifford Neuman of the Information Sciences Institute at the University of Southern California.
• RSRE FLEX file system- written inALGOL 68
• The file system of theMichigan 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.
• TempleOSuses RedSea, a file system made by Terry A. Davis.

Limitations

Design limitations

File systems limitstorable data capacity– generally driven by the typical size of storage devices at the time the file system is designed and anticipated into the foreseeable future.

Since storage sizes have increased at nearexponentialrate (seeMoore's law), newer storage devices often exceed existing file system limits within only a few years after introduction. This requires new file systems with ever increasing capacity.

With higher capacity, the need for capabilities and therefore complexity increases as well. File system complexity typically varies proportionally with available storage capacity. Capacity issues aside, the file systems of early 1980shome computerswith 50 KB to 512 KB of storage would not be a reasonable choice for modern storage systems with hundreds of gigabytes of capacity. Likewise, modern file systems would not be a reasonable choice for these early systems, since the complexity of modern file system structures would quickly consume the limited capacity of early storage systems.

Converting the type of a file system

It may be advantageous or necessary to have files in a different file system than they currently exist. Reasons include the need for an increase in the space requirements beyond the limits of the current file system. The depth of path may need to be increased beyond the restrictions of the file system. There may be performance or reliability considerations. Providing access to another operating system which does not support the existing file system is another reason.

In-place conversion

In some cases conversion can be done in-place, although migrating the file system is more conservative, as it involves a creating a copy of the data and is recommended.On Windows, FAT and FAT32 file systems can be converted to NTFS via the convert.exe utility, but not the reverse.On Linux, ext2 can be converted to ext3 (and converted back), and ext3 can be converted to ext4 (but not back),and both ext3 and ext4 can be converted tobtrfs,and converted back until the undo information is deleted.These conversions are possible due to using the same format for the file data itself, and relocating the metadata into empty space, in some cases usingsparse filesupport.

Migrating to a different file system

Migration has the disadvantage of requiring additional space although it may be faster. The best case is if there is unused space on media which will contain the final file system.

For example, to migrate a FAT32 file system to an ext2 file system, a new ext2 file system is created. Then the data from the FAT32 file system is copied to the ext2 one, and the old file system is deleted.

An alternative, when there is not sufficient space to retain the original file system until the new one is created, is to use a work area (such as a removable media). This takes longer but has the benefit of producing a backup.

Long file paths and long file names

Inhierarchical file systems,files are accessed by means of apaththat is a branching list of directories containing the file. Different file systems have different limits on the depth of the path. File systems also have a limit on the length of an individual file name.

Copying files with long names or located in paths of significant depth from one file system to another may cause undesirable results. This depends on how the utility doing the copying handles the discrepancy.

See also

• Comparison of file systems
• Disk quota
• List of file systems
• List of Unix commands
• Directory structure
• Shared resource
• Distributed file system
• Distributed Data Management Architecture
• File manager
• File system fragmentation
• Filename extension
• Global file system
• Object storage
• Computer data storage
• Storage efficiency
• Virtual file system

References

Sources

• de Boyne Pollard, Jonathan (1996)."Disc and volume size limits".Frequently Given Answers.RetrievedFebruary 9,2005.
• "OS/2 corrective service fix JR09427".IBM.RetrievedFebruary 9,2005.
• "Attribute - $EA_INFORMATION (0xD0)".NTFS Information, Linux-NTFS Project.RetrievedFebruary 9,2005.
• "Attribute - $EA (0xE0)".NTFS Information, Linux-NTFS Project.RetrievedFebruary 9,2005.
• "Attribute - $STANDARD_INFORMATION (0x10)".NTFS Information, Linux-NTFS Project.RetrievedFebruary 21,2005.
• "Technical Note TN1150: HFS Plus Volume Format".Apple Inc.RetrievedSeptember 22,2015.
• Brian Carrier (2005).File System Forensic Analysis.Addison Wesley.

Further reading

Books

• Arpaci-Dusseau, Remzi H.; Arpaci-Dusseau, Andrea C. (2014).Operating Systems: Three Easy Pieces.Arpaci-Dusseau Books.
• Carrier, Brian (2005).File System Forensic Analysis.Addison-Wesley.ISBN0-321-26817-2.
• Custer, Helen (1994).Inside the Windows NT File System.Microsoft Press.ISBN1-55615-660-X.
• Giampaolo, Dominic (1999).Practical File System Design with the Be File System(PDF).Morgan Kaufmann Publishers.ISBN1-55860-497-9.Archived(PDF)from the original on 2018-09-03.Retrieved2019-12-15.
• McCoy, Kirby (1990).VMS File System Internals.VAX - VMS Series.Digital Press.ISBN1-55558-056-4.
• Mitchell, Stan (1997).Inside the Windows 95 File System.O'Reilly.ISBN1-56592-200-X.
• Nagar, Rajeev (1997).Windows NT File System Internals: A Developer's Guide.O'Reilly.ISBN978-1-56592-249-5.
• Pate, Steve D. (2003).UNIX Filesystems: Evolution, Design, and Implementation.Wiley.ISBN0-471-16483-6.
• Rosenblum, Mendel (1994).The Design and Implementation of a Log-Structured File System.The Springer International Series in Engineering and Computer Science. Springer.ISBN0-7923-9541-7.
• Russinovich, Mark; Solomon, David A.; Ionescu, Alex (2009). "File Systems".Windows Internals(5th ed.).Microsoft Press.ISBN978-0-7356-2530-3.
• Prabhakaran, Vijayan (2006).IRON File Systems.PhD dissertation, University of Wisconsin-Madison.
• Silberschatz, Abraham; Galvin, Peter Baer; Gagne, Greg (2004). "Storage Management".Operating System Concepts(7th ed.). Wiley.ISBN0-471-69466-5.
• Tanenbaum, Andrew S.(2007).Modern operating Systems(3rd ed.).Prentice Hall.ISBN978-0-13-600663-3.
• Tanenbaum, Andrew S.;Woodhull, Albert S. (2006).Operating Systems: Design and Implementation(3rd ed.).Prentice Hall.ISBN0-13-142938-8.

Online

• Benchmarking Filesystems (outdated)by Justin Piszcz, Linux Gazette 102, May 2004
• Benchmarking Filesystems Part IIusing kernel 2.6, by Justin Piszcz, Linux Gazette 122, January 2006
• Filesystems (ext3, ReiserFS, XFS, JFS) comparison on Debian EtchArchived2008-09-13 at theWayback Machine2006
• 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 (most recent update 2006-11-19)
• Linux File System Benchmarksv2.6 kernel with a stress on CPU usage
• "Linux 2.6 Filesystem Benchmarks (Older)".Archived from the original on 2016-04-15.Retrieved2019-12-16.{{cite web}}:CS1 maint: unfit URL (link)
• 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.Retrieved2008-03-18.

External links

• "Filesystem Specifications - Links & Whitepapers".Archived from the original on 2015-11-03.{{cite web}}:CS1 maint: unfit URL (link)
• Interesting File System Projects