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In the context of administering computer systems, defragmentation (or defragging) is a process that eliminates fragmentation in file systems. It does this by physically reorganizing the contents of the disk in order to store the pieces of each file close together and in order (contiguously). It also attempts to create large regions of free space using compaction, to impede the return of fragmentation.

Aims of defragmentation

Reading and writing data on a heavily fragmented hard drive is slowed down as the time for the heads to move between fragments on the disk surface can be substantial. The disk operates at speeds millions of times slower than the CPU; thus the desire to process more efficiently encourages defragmentation. Operating system manufacturers often recommend periodic defragmentation in order to keep hard drive access as fast as possible.

Fragmented data also spreads over more disk than it needs. Thus one may defragment in order to compact data storage before splitting a single partition into two or more partitions (for example, with FIPS, or PartitionMagic).

Defragmentation: Causes and Cures

Fragmentation occurs when the operating system cannot or will not allocate enough contiguous space to store a complete file as a unit, but instead puts parts of it in gaps between other files (usually those gaps exist because they formerly held a file that the operating system has subsequently deleted or because the operating system allocated excess space for the file in the first place). As advances in technology bring us larger disk drives, the performance loss due to fragmentation squares with each doubling of the size of the drive. Larger files and greater numbers of files also contribute to fragmentation and consequent performance loss. Defragmentation restores a drive to its original speed.

A defragmentation program must move files around within the free space available in order to undo fragmentation. This is a memory intensive operation and cannot be performed on a file system with no free space. The reorganization involved in defragmentation does not change logical location of the files (defined as their location within the directory structure).

Microsoft Windows Disk Defragmenter

Defragmentation programs often come bundled with modern operating systems, beginning with Windows XP and is accessible through the Properties dialogue menu of a local disk.

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