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Used in the maintenance of file systems, defragmenters are software programs designed to reduce the degree of fragmentation of data on a disk by physically organizing its contents into the smallest number of contiguous regions, while creating larger regions of free space using compaction, thus slowing the return of fragmentation.

Defragmentation utilities may also keep smaller files within a single directory together, since they are often accessed in sequence.

Fragmentation occurs because of the tendency of a file system to layout the contents of files non-contiguously. Because data is not placed in the order in which it is used, disk head movement is increased, along with the amount of time that it takes to run a program. Resulting problems increase as more and more of the hard drive becomes filled with disorganized data. Defragmentation is designed to reorganize file data and free space back into contiguous areas on the disk.

Defragmentation is not as critical in computers with SSD disks, as they don't rotate and are not actually discs in the conventional sense. As there is no movement of heads or discs with SSD drives, fragmentation doesn't pose as much of a performance problem.

Otherwise, fragmentation occurs when a file system does not (or cannot) allocate enough contiguous space to store a complete file as a unit, but instead places parts of it in gaps between existing data, which are usually there because they previously held a file that the system has deleted, or because the system had allocated excess space for the file in the first place.

Defragmentation moves the physical allocation blocks for an application so that they eventually merge, preferably into one. This usually requires at least two copy operations, the first moving the blocks into a temporary hold space on the disk so that further movement can occur, and the other to move the blocks to their intended place. As no data is actually removed from the disk, a power loss during the defragmentation process would not result in a problem.

Defragmentation software only moves files around within the available free space, and it cannot occur without free space.

As a temporary degradation of system performance may occur during the defragmentation process, it's best to leave the computer alone until defragmentation is completed.

Successful defragmentation can increase the speed at which programs load, as well as the efficiency of their operation, and the amount of time that it takes to open files

Defragmentation utilities are often included as modules in a disk or file management suite, but they may also be available as standalone programs. The focus of this category is on software designed to defragment a drive.



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