Defragging
In the context of administering computer systems. defragmentation (or defragging) refers to the process of physically reorganizing files on a computer hard drive in order to store files as contiguously as possible.
Aims of Defragmenation
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 may also spread over more disk than it needs. Thus one may defragment in order to optimise compact data storage before splitting a single partition into two or more partitions (for example, with FIPS, Partition Magic, or ntfsresize).
Causes and Cures
The reorganization involved in defragmentation has nothing to do with the logical location of the files (defined as their location within the directory structure). 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.
Defragmentation Issues
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.
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.
Utilities
Defragmentation programs often come bundled with an operating system (although Windows NT 4 notably did not include one).
Defragmentation utility refers to the re-writing of scattered clusters into their consecutive order, whereas a hard disk qualifies as fragmented when its files are not written into their consecutive clusters
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