Extent (file systems)

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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 and possibly file scattering too.

Extents are supported in:

• Microsoft NTFS (Called Runs)
• 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
• Digital Equipment Corporation (now Hewlett-Packard) OpenVMS Files-11
• 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
• The Btrfs file system
• Oracle Automatic Storage Management
• SQL Server 2000-2008 supports extents of 64KB in size, as discussed on the MSDN page [1]

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.

[edit] See also

• Comparison of file systems

[edit] External links

Look up extent in Wiktionary, the free dictionary.

• Getting to know the Solaris filesystem, Part 1: Allocation and storage strategy - comparison of block-based and extent-based allocation