Extent (file systems)

Extent (file systems)

An extent is a contiguous area of storage in a computer file system, reserved for a file. When starting to write to a file, a whole extent is allocated. When writing to the file again, possibly after doing other write operations, the data continues where the previous write left off. This reduces or eliminates file fragmentation.

Extents are supported in:
* Macintosh Hierarchical File System and HFS Plus
* SGI XFS (Designed to be an extent based file system)
* Reiser4 (in "extents" mode)
* Universal Disk Format (UDF)
* VERITAS File System (via the preallocation API and CLI).
* Linux ext4 (when extents are enabled, the default since kernel 2.6.23)
* OS/2 and eComStation HPFS
* HP Multi-Programming Executive file system
* JFS for AIX, OS/2/eComStation and Linux
* BFS for BeOS, Zeta and Haiku
* The SINTRAN III file system
* Oracle Automatic Storage Management

The CP/M file system uses extents as well, but those don't correspond to the definition given above. CP/M's extents are contiguous only in the sense that they appear as a single block in the combined directory/allocation table; they are not necessarily contiguous on the data area of the disk.

Note that a file system can be extent-based (i.e., addressing via extents rather than in single blocks) without requiring that each file be limited to a single, contiguous extent.

ee also

* Comparison of file systems

External links

* [http://www.solarisinternals.com/si/reading/sunworldonline/swol-05-1999/swol-05-filesystem.html Getting to know the Solaris filesystem, Part 1: Allocation and storage strategy] - comparison of block-based and extent-based allocation


Wikimedia Foundation. 2010.

Игры ⚽ Нужна курсовая?

Look at other dictionaries:

  • Extent (file systems) — …   Википедия

  • List of file systems — The following lists identify, characterize and link to more thorough information on computer file systems.Many older operating systems support only their one native file system, which does not bear any name apart from the name of the operating… …   Wikipedia

  • Extent — has several meanings: *Extent (file systems), a contiguous piece of a file on a computer storage medium *Wingspan, the extent between the tips of the wings of a bird, bat, or other flying animal *Reach (physical measurement), the extent between… …   Wikipedia

  • File system fragmentation — In computing, file system fragmentation, sometimes called file system aging, is the inability of a file system to lay out related data sequentially (contiguously), an inherent phenomenon in storage backed file systems that allow in place… …   Wikipedia

  • File manager — For the Windows 3.x file manager, see File Manager (Windows). A file manager or file browser is a computer program that provides a user interface to work with file systems. The most common operations performed on files or groups of files are:… …   Wikipedia

  • File format — A file format is a particular way that information is encoded for storage in a computer file. Since a disk drive, or indeed any computer storage, can store only bits, the computer must have some way of converting information to 0s and 1s and vice …   Wikipedia

  • JFS (file system) — infobox filesystem name = JFS full name = IBM Journaled File System developer = IBM et al. introduction os = JFS1 in AIX 3.1 and JFS in OS/2 4.5 introduction date = 1990 and 1999 partition id = 0x35 (MBR) file struct = Bitmap/extents bad blocks… …   Wikipedia

  • Hierarchical File System — Infobox filesystem name = HFS full name = Hierarchical File System developer = Apple Computer introduction os = System 2.1 introduction date = September 17 1985 partition id = Apple HFS (Apple Partition Map) 0xAF (MBR) directory struct = B* tree… …   Wikipedia

  • Virtual file system — A virtual file system (VFS) or virtual filesystem switch is an abstraction layer on top of a more concrete file system. The purpose of a VFS is to allow client applications to access different types of concrete file systems in a uniform way. A… …   Wikipedia

  • Veritas File System — For other uses, see Veritas (disambiguation). VERITAS File System Full name VERITAS File System Introduced 1991 Structures Directory contents extensible hash Limits Max file size 8 EB ( …   Wikipedia

Share the article and excerpts

Direct link
… Do a right-click on the link above
and select “Copy Link”