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etworking security input output v t e this article needs more citations please help improve this article by adding citations to reliable sources unsourced material may be challenged and removed find sources file system news newspapers books scholar jstor november 2025 learn how and when to remove this message in computing a file system or filesystem fs or fs governs file organization and access a local file system is a capability of an operating system that services the applications running on the same computer 1 2 a distributed file system is a protocol that provides file access between networked computers a file system provides a data storage service that allows applications to share mass storage without a file system applications could access the storage in incompatible ways that lead to resource contention data corruption and data loss there are many file system designs and implementations with various structures and features and various resulting characteristics such as speed flexibility security size and more file systems have been developed for many types of storage devices including hard disk drives hdds solid state drives ssds magnetic tapes and optical discs 3 a portion of the computer main memory can be set up as a ram disk that serves as a storage device for a file system file systems such as tmpfs can store files in virtual memory a virtual file system provides access to files that are either computed on request called virtual files for example those provided by procfs and sysfs or are mapping into another backing storage etymology edit from c 1900 and before the advent of computers the terms file system filing system and system for filing were used to describe methods of organizing storing and retrieving paper documents 4 by 1961 the term file system was being applied to computerized filing alongside the original meaning 5 by 1964 it was in general use 6 architecture edit a local file system s architecture can be described as layers of abstraction even though a particular file system design may not actually separate the concepts 7 the logical file system layer provides relatively high level access via an application 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 8 it provides file access directory operations security and protection 7 the virtual file system an optional layer supports multiple concurrent instances of physical file systems each of which is called a file system implementation 8 the physical file system layer provides relatively low level access to a storage device e g disk it reads and writes data blocks provides buffering and other memory management and controls placement of blocks in specific locations on the storage medium this layer uses device drivers or channel i o to drive the storage device 7 attributes edit file names edit main article filename a file name or filename 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 as case sensitive and others as case insensitive for example the names myfile and myfile match 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 the unicode character 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 edit main article directory computing file systems typically support organizing files into directories also called folders which segregate files into groups this may be implemented by associating the file name with an index in a table of contents or an inode in a unix like file 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 the multics operating system 9 the native file systems of unix like systems also support arbitrary directory hierarchies as do apple s hierarchical file system and its successor hfs in classic mac os the fat file system in ms dos 2 0 and later versions of ms dos and in microsoft windows the ntfs file system in the windows nt family of operating systems and the ods 2 on disk structure 2 and higher levels of the files 11 file system in openvms metadata edit this section does not cite any sources please help improve this section by adding citations to reliable sources unsourced material may be challenged and removed november 2025 learn how and when to remove this message in addition to data the file content a file system also manages associated metadata which may include but is not limited to name size which may be stored as the number of blocks allocated or as a byte count when created last accessed last modified owner user and group access permissions file attributes such 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 the inode 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 about bad sectors often such information about an allocation group is stored inside the allocation group itself additional attributes can be associated on file systems such as ntfs xfs ext2 ext3 some versions of ufs and hfs using extended 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 as streams or forks 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 versions can be accessed using a special naming convention such as filename 4 or filename 4 to access the version four saves ago see comparison of file systems metadata for details on which file systems support which kinds of metadata storage space organization edit 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 byte ntfs 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 in apple dos of the early 1980s 256 byte sectors on 140 kilobyte floppy disk used a track sector map 10 the granular nature results in unused space sometimes called slack space for each file except for those that have the rare size that is a multiple of the granular allocation 11 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 edit file systems may become fragmented as a file system creates modifies and deletes files the underlying storage representation may become fragmented 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 12 fragmentation is invisible to the end user and the system still works correctly however this can degrade performance on some storage hardware that works better with contiguous blocks such as hard disk drives other hardware such as solid state drives are not affected by fragmentation access control edit see also computer security password cracking filesystem level encryption and encrypting 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 to ensure that data is modified in a controlled way examples include passwords stored in the metadata of the file or elsewhere and file permissions in the form of permission bits access control lists or capabilities 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 edit example of qgroup quota group of a btrfs filesystem some operating systems allow a system administrator to enable disk quotas to limit a user s use of storage space data integrity edit 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 edit a file system might record events to allow analysis of issues such as file or systemic problems and performance nefarious access data access edit byte stream access edit many file systems access data as a stream of bytes typically to read file data a program provides a memory buffer and 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 edit some file systems or layers on top of a file system allow a program to define a record so that a program can read and write data as a structure not an unorganized sequence of bytes if a fixed length record definition is used then locating the n th record 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 13 utilities edit 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 delete directory entries which are also known as dentries singular dentry 14 and to alter metadata associated with a directory directory utilities may also include capabilities to create additional links to a directory hard links in unix to rename parent links in unix like operating systems clarification needed 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 system defragmentation utilities 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 edit utilities libraries and programs use file system apis to 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 edit frequently retail systems are configured with a single file system occupying the entire storage device another approach is to partition the disk so that several file systems with different attributes can be used one 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