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__NOTOC__ An extent is a contiguous area of storage reserved for a file in a file system, represented as a range. 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. Extent-based file systems can also eliminate most of the metadata overhead of large files that would traditionally be taken up by the block allocation tree. 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 benefits in storage efficiency and performance are slight, whereas the reduction in metadata is significant and reduces exposure to file system corruption as one bad sector in the block allocation tree causes much greater data loss than one bad sector in stored data. In order to resist fragmentation, several extent based file systems do allocate-on-flush. Many modern fault tolerant file systems also do copy-on-write, 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'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. == Adoption == The following systems support extents: * ASM Automatic Storage Management Oracle's database-oriented filesystem * BFS BeOS, Zeta and Haiku operating systems * Btrfs GPL'd extent based file storage for Linux * Ext4 Linux filesystem (when the configuration enables extents the default in Linux since version 2.6.23) * Files-11 Digital Equipment Corporation (subsequently Hewlett-Packard) OpenVMS filesystem. * HFS and HFS Plus Hierarchical File System Apple Macintosh filesystems * High Performance File System (HPFS) on OS/2 and eComStation * IceFS IceFileSystem Optional file system for MorphOS * JFS Journaled File System used by AIX, OS/2/eComStation and Linux operating systems * Microsoft SQL Server versions 2000-2008 supports extents of up to 64 KB * Multi-Programming Executive a filesystem by Hewlett-Packard * NTFS Microsoft's latest-generation file system * OCFS2 Oracle Cluster File System a shared disk file system for Linux * Reiser4 Linux filesystem (in "extents" mode) * SINTRAN III File system used by early computer company Norsk Data * UDF Universal Disk Format standard for optical media * VERITAS File System enabled via the pre-allocation API and CLI * XFS SGI's second generation file system 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Extent (file systems)」の詳細全文を読む スポンサード リンク
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