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* The decrease of [[locality of reference]] between separate, but related files.
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===File fragmentation===
Individual file fragmentation occurs when a single file has been broken into multiple pieces (called [[extent]]s on extent-based file systems). While disk file systems attempt to keep individual files contiguous, this is not often possible without significant performance penalties. File fragmentation
===Free space fragmentation===
Free (unallocated) space fragmentation occurs when there are several unused areas of the file system where new files or metadata can be written to. Unwanted free space fragmentation is generally caused by deletion or truncation of files, but file systems may also intentionally insert fragments (sometimes called "bubbles") of free space in order to facilitate extending nearby files (see [[#Proactive techniques|proactive techniques]] below).
==Techniques for mitigating fragmentation==
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====Retroactive techniques====
Retroactive techniques attempt to reduce fragmentation after it has occurred. Many file systems provide [[defragmentation]] tools, which attempt to reorder fragments of files, and often also increase [[locality of reference]] by keeping smaller files in [[directory (file systems)|directories]], or directory trees, close to each another on the disk. Some file systems, such as [[HFS Plus]], exploit idle time to defragment data on the disk in the background.
==See also==
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