Extent (file systems)

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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.[1]

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,[2] whereas the reduction in metadata is significant and reduces exposure to file system corruption[citation needed] 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[edit]

The following systems support extents:

See also[edit]

References[edit]

  1. ^ "Understanding Ext4 (part1): Extents". 2010-12-20. Retrieved 2015-02-02. What's really a departure for EXT4 however, is the use of extents rather than the old, inefficient indirect block mechanism used by earlier Unix file systems (e.g. EXT2/EXT3) for tracking file content. Extents are similar to cluster runs in the NTFS file system — essentially they specify an initial block address and the number of blocks that make up the extent. A file that is fragmented will have multiple extents, but EXT4 tries very hard to keep files contiguous. 
  2. ^ "Ext4 Disk Layout". 2015-01-26. Retrieved 2015-02-02. If flex_bg is enabled, it is possible to allocate very large files with a single extent, at a considerable reduction in metadata block use, and some improvement in disk efficiency. 
  3. ^ "Understanding Pages and Extents". msdn.microsoft.com. Retrieved 2014-08-14. 

External links[edit]