computing Computing is any goal-oriented activity requiring, benefiting from, or creating computer, computing machinery. It includes the study and experimentation of algorithmic processes, and the development of both computer hardware, hardware and softw ...

, an extent is a contiguous area of storage reserved for a file in a file system, represented as a range of block numbers, or tracks on

count key data Count key data (CKD) is a direct-access storage device (DASD) data recording format introduced in 1964, by IBM with its IBM System/360 and still being emulated on IBM mainframes. It is a self-defining format with each data record represented by a ...

devices. A file can consist of zero or more extents; one file fragment requires one extent. The direct benefit is in storing each range compactly as two numbers, instead of canonically storing every block number in the range. Also, extent allocation results in less

file fragmentation In computing, file system fragmentation, sometimes called file system aging, is the tendency of a file system to lay out the contents of files non-continuously to allow in-place modification of their contents. It is a special case of data fragme ...

. Extent-based file systems can also eliminate most of the

metadata Metadata (or metainformation) is "data that provides information about other data", but not the content of the data itself, such as the text of a message or the image itself. There are many distinct types of metadata, including: * Descriptive ...

overhead of large files that would traditionally be taken up by the block-allocation tree. But because the savings are small compared to the amount of stored data (for all file sizes in general) but make up a large portion of the metadata (for large files), the overall benefits in storage efficiency and performance are slight. In order to resist fragmentation, several extent-based file systems do

allocate-on-flush Allocate-on-flush (also called delayed allocation) is a file system feature implemented in HFS+, XFS, Reiser4, ZFS, Btrfs, and ext4. The feature also closely resembles an older technique that Berkeley's UFS called "block reallocation". When bloc ...

. Many modern

fault-tolerant Fault tolerance is the ability of a system to maintain proper operation despite failures or faults in one or more of its components. This capability is essential for high-availability, mission-critical, or even life-critical systems. Fault to ...

file systems also do

copy-on-write Copy-on-write (COW), also called implicit sharing or shadowing, is a resource-management technique used in programming to manage shared data efficiently. Instead of copying data right away when multiple programs use it, the same data is shared ...

, although that increases fragmentation. As a similar design, the CP/M file system uses extents as well, but those do not correspond to the definition given above.

CP/M CP/M, originally standing for Control Program/Monitor and later Control Program for Microcomputers, is a mass-market operating system created in 1974 for Intel 8080/Intel 8085, 85-based microcomputers by Gary Kildall of Digital Research, Dig ...

's extents appear contiguously as a single block in the combined directory/allocation table, and they do not necessarily correspond to a contiguous data area on disk. IBM

OS/360 and successors OS/360, officially known as IBM System/360 Operating System, is a discontinued batch processing operating system developed by IBM for their then-new System/360 mainframe computer, announced in 1964; it was influenced by the earlier IBSYS/IBJOB a ...

allocate files in multiples of disk tracks or

cylinders A cylinder () has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infinite ...

. Files could originally have up to 16 extents, but this restriction has since been lifted. The initial allocation size, and the size of additional extents to be allocated if required, are specified by the user via

Job Control Language Job Control Language (JCL) is a scripting language used on IBM mainframe operating systems to instruct the system on how to run a batch processing, batch job or start a subsystem. The purpose of JCL is to say which programs to run, using which fi ...

. The system attempts to allocate the initial size as a contiguous area, although this may be split if contiguous space is not available.

Adoption

The systems supporting file system extents include the following: *

APFS Apple File System (APFS) is a proprietary file system developed and deployed by Apple Inc. for macOS Sierra (10.12.4) and later, iOS 10.3, tvOS 10.2, watchOS 3.2, and all versions of iPadOS. It aims to fix core problems of HFS+ (also ca ...

Apple File System * ASM Automatic Storage Management Oracle's database-oriented file system * BFS

BeOS BeOS is a discontinued operating system for personal computers that was developed by Be Inc. It was conceived for the company's BeBox personal computer which was released in 1995. BeOS was designed for multitasking, multithreading, and a graph ...

Zeta Zeta (, ; uppercase Ζ, lowercase ζ; , , classical or ''zē̂ta''; ''zíta'') is the sixth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 7. It was derived from the Phoenician alphabet, Phoenician letter zay ...

and

Haiku is a type of short form poetry that originated in Japan. Traditional Japanese haiku consist of three phrases composed of 17 Mora (linguistics), morae (called ''On (Japanese prosody), on'' in Japanese) in a 5, 7, 5 pattern; that include a ''kire ...

operating systems *

Btrfs Btrfs (pronounced as "better F S", "butter F S", "b-tree F S", or "B.T.R.F.S.") is a computer storage format that combines a file system based on the copy-on-write (COW) principle with a logical volume manager (distinct from Linux's LVM), d ...

Extent-based

(COW) file system for Linux * EFS Extent File System SGI's first-generation file system for

IRIX IRIX (, ) is a discontinued operating system developed by Silicon Graphics (SGI) to run on the company's proprietary MIPS architecture, MIPS workstations and servers. It is based on UNIX System V with Berkeley Software Distribution, BSD extensio ...

ext4 ext4 (fourth extended filesystem) is a journaling file system for Linux, developed as the successor to ext3. ext4 was initially a series of backward-compatible extensions to ext3, many of them originally developed by Cluster File Systems for ...

Linux Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...

file system (when the configuration enables extents the default in Linux since version 2.6.23) *

Files-11 Files-11 is the file system used in the RSX-11 and OpenVMS operating systems from Digital Equipment Corporation. It supports record-oriented I/O, remote network access, and file versioning. The original ODS-1 layer is a flat file system; th ...

OpenVMS OpenVMS, often referred to as just VMS, is a multi-user, multiprocessing and virtual memory-based operating system. It is designed to support time-sharing, batch processing, transaction processing and workstation applications. Customers using Op ...

file system *

HFS HFS may refer to: Businesses and organisations * Croatian Film Association () * Hellenic Fire Service, Greece * Hospitality Franchise Systems, US Computing * Hierarchical file system, a system for organizing directories and files * Hierarchica ...

and

HFS Plus HFS Plus or HFS+ (also known as Mac OS Extended or HFS Extended) is a journaling file system developed by Apple Inc. It replaced the Hierarchical File System (HFS) as the primary file system of Apple computers with the 1998 release of Mac OS 8. ...

Hierarchical File System

Apple An apple is a round, edible fruit produced by an apple tree (''Malus'' spp.). Fruit trees of the orchard or domestic apple (''Malus domestica''), the most widely grown in the genus, are agriculture, cultivated worldwide. The tree originated ...

Macintosh Mac is a brand of personal computers designed and marketed by Apple Inc., Apple since 1984. The name is short for Macintosh (its official name until 1999), a reference to the McIntosh (apple), McIntosh apple. The current product lineup inclu ...

file systems *

High Performance File System HPFS (High Performance File System) is a file system created specifically for the OS/2 operating system to improve upon the limitations of the FAT file system. It was written by Gordon Letwin and others at Microsoft and added to OS/2 version ...

(HPFS) on

OS/2 OS/2 is a Proprietary software, proprietary computer operating system for x86 and PowerPC based personal computers. It was created and initially developed jointly by IBM and Microsoft, under the leadership of IBM software designer Ed Iacobucci, ...

eComStation eComStation or eCS is an operating system based on OS/2 Warp for the 32-bit x86 architecture. It was originally developed by Serenity Systems and Mensys BV under license from IBM. It includes additional applications, and support for new hardwa ...

and

ArcaOS ArcaOS is a Proprietary software, proprietary operating system based on OS/2, developed and marketed by Arca Noae, LLC under license from IBM. It was first released in 2017 and builds on OS/2 Warp 4.52 by adding support for new hardware, fixing ...

* IceFS IceFileSystem optional file system for

MorphOS MorphOS is an AmigaOS-like operating system designed for Power and PowerPC based computers. The core, based on the Quark microkernel, is proprietary, although several libraries and other parts are open source, such as the Ambient desktop. The p ...

* JFS Journaled File System used by

AIX Aix or AIX may refer to: Computing * AIX, a line of IBM computer operating systems *Alternate index, for an IBM Virtual Storage Access Method key-sequenced data set * Athens Internet Exchange, a European Internet exchange point Places Belg ...

/eComStation/ArcaOS and

operating systems *

ISO 9660 ISO 9660 (also known as ECMA-119) is a file system for optical disc media. The file system is an international standard available from the International Organization for Standardization (ISO). Since the specification is publicly available, im ...

Extent-based file system for optical disc media * MPE File System the file system of the Multi-Programming Executive operating system. *

NTFS NT File System (NTFS) (commonly called ''New Technology File System'') is a proprietary journaling file system developed by Microsoft in the 1990s. It was developed to overcome scalability, security and other limitations with File Allocation Tabl ...

used by Windows *

OCFS2 The Oracle Cluster File System (OCFS, in its second version OCFS2) is a shared disk file system developed by Oracle Corporation and released under the GNU General Public License. The first version of OCFS was developed with the main focus to accom ...

Oracle Cluster File System a shared-disk file system for Linux *

Reiser4 Reiser4 is a computer file system, successor to the ReiserFS file system, developed from scratch by Namesys and sponsored by DARPA as well as Linspire. Reiser4 was named after its former lead developer Hans Reiser. , the Reiser4 patch set is s ...

Linux file system (in "extents" mode) * SINTRAN III file system used by early computer company

Norsk Data Norsk Data was a minicomputer manufacturer located in Oslo, Norway. Existing from 1967 to 1998, it had its most active period from the early 1970s to the late 1980s. At the company's peak in 1987, it was the second largest company in Norway and em ...

* UDF Universal Disk Format standard for optical media *

VERITAS File System The VERITAS File System (or VxFS; called JFS and OnlineJFS in HP-UX) is an extent-based file system. It was originally developed by VERITAS Software. Through an OEM agreement, VxFS is used as the primary filesystem of the HP-UX operating s ...

enabled via the pre-allocation API and CLI *

XFS XFS is a high-performance 64-bit journaling file system created by Silicon Graphics, Inc (SGI) in 1993. It was the default file system in SGI's IRIX operating system starting with its version 5.3. XFS was ported to the Linux kernel in 2001; a ...

SGI's second-generation file system for

and

Adoption outside of file systems include the following: *

Microsoft SQL Server Microsoft SQL Server is a proprietary relational database management system developed by Microsoft using Structured Query Language (SQL, often pronounced "sequel"). As a database server, it is a software product with the primary function of ...

versions support 64 KB extents consisting of eight 8 KB pages. *

Oracle Database Oracle Database (commonly referred to as Oracle DBMS, Oracle Autonomous Database, or simply as Oracle) is a proprietary multi-model database management system produced and marketed by Oracle Corporation. It is a database commonly used for ru ...

groups blocks into extents and extents into segments.

References

External links

Getting to know the Solaris filesystem, Part 1
Allocation and storage strategy a comparison of block-based and extent-based allocation {{File systems, state=collapsed Computer file systems

Adoption

See also

References

External links