Disk Defragmenter is a computer program included in Microsoft Windows designed to increase access speed by rearranging files stored on a disk to occupy contiguous storage locations.It also attempts to create larger regions of free space using compaction to impede the return of fragmentation.Disk Defragmenter is also used to improve system startup times.

Hard disks are slow in computer terms. Compared to the speed of the processor and its memory, the time it takes for the arm to move and for a sector to spin into place is an eon ,so we want minimize arm movement as much as possible, and make data stored in sequential segments on the disk .

What happens is that the disk fills up. Then you erase files to reclaim space. These files that you delete are scattered all over the surface of the disk. When you load a new application or a large file onto the disk, it ends up being stored in hundreds or thousands of these scattered pockets of space. Now when the computer tries to load the scattered pieces, the disk’s arm has to move all over the surface and it takes forever.

The idea behind the disk defragmenter is to move all the files around so that every file is stored on sequential sectors on sequential rings of the disk.

Disk Defragmenter  limitations:

• It does not defragment files residing in the Recycle Bin or files that are in use.
• Only one volume can be analyzed or defragmented at a time and only one instance can run.
• Only local volumes can be defragmented, network volumes are not supported.
• Defragmenting NTFS volumes with cluster sizes larger than 4 kilobytes (KB) is not possible.
• EFS encrypted files are not defragmented.
• NTFS metadata, such as the Master File Table (MFT), or metadata that describes a directory’s contents is not defragmented.