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A heavily fragmented hard drive makes reading and writing data slower; 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).
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 used formerly to hold a file that the operating system has subsequently deleted).
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 filesystem 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).
The presence of immovable system files (or of files that the defragmenter will not move in order to simplfy its task), especially a swap file, can impede defragmentation. ntfsresize can safely move these files in order to resize an NTFS partition.
Certain file systems exhibit a greater susceptibility to fragmentation than others, for example, a FAT file system becomes fragmented more quickly than NTFS or ext3 file systems. Most Unix-like systems do not have the ability to explicitly defragment at all (though they may have the ability to perform other maintence tasks, eg repacking). These systems attempt to keep fragmentation below a certain point so defragmenting is not necessary.
Defragmentation programs often come bundled with an operating system (although Windows NTMicrosoft Windows NT is an operating system produced by Microsoft Corporation. It is the ancestor of their current flagship Windows XP. Development When development started in 1988, Windows NT was to be known as OS/2 3. 0, the third version of the operati 4 notably did not include one).
A defragmentation utility moves the scattered clusterSee: In astrophysics and astrometry: star cluster In music: tone cluster In statistics: cluster sampling or cluster analysis metall cluster In economics: Porter's cluster Techno cluster In space sciences: An ESA mission to study the magnetosphere: Clusters of each file into consecutive clusters. A hard disk is fragmented when its files are not written into their consecutive clusters.