What is Defragmentation?

Defragmentation is the process of locating the non-contiguous fragments of data into which a computer file may be divided as it is stored on a hard disk, and rearranging the fragments and restoring them into fewer fragments or into the whole file. Defragmentation reduces data access time and allows storage to be used more efficiently. Some operating systems automatically defragment storage periodically; others require that the user occasionally use a special utility for this purpose.

Windows 98 comes with a built-in defragmenter as a "system tool" that the user can run. Windows NT did not come with a defragmenter because its file system, NTFS, was designed to minimize fragmentation; however, NT users often find one necessary and several vendors provide defragmenters. Windows 2000 comes with a "light" version of the Diskeeper defragmenter; some users (especially corporate users) use Diskeeper or some other full-function defragmentation program to manage storage efficiency and performance. Windows XP comes with a utility called "Disk Defragmenter."

What causes files to become fragmented?

Fragmentation occurs when the file 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 existing files (usually those gaps exist because they formerly held a file that the operating system has subsequently deleted

Disk Defragmenter also shipped as part of Windows 95, Windows 98 and Windows Me. It could be scheduled using a Maintenance Wizard and supported command line switches. This version had the limitation that if the contents of the drive changed during defragmentation, it rescanned the drive and restarted the process from where it left off.

The Disk Defragmenter in Windows 2000 was a stripped-down licensed version of Diskeeper, licensed from Diskeeper Corporation. It uses the following techniques.

1. Moving all the index or directory information to one spot. Moving this spot into the centre of the data, i.e. one third of the way in, so that average head travel to data is halved compared to having directory information at the front.
2. Moving infrequently used files further from the directory area.
3. Obeying a user-provided table of file descriptions to emphasize or ignore.
4. Making files contiguous so that they can be read without unnecessary seeking.

In Windows 2000 and later operating systems, Disk Defragmenter has the following limitations:

1. It does not defragment files residing in the Recycle Bin or files that are in use. In particular, this includes the registry, page file and hibernation file.
2. Prior to the Windows Vista release, only one volume could be analysed or defragmented at a time and only one instance could run.
3. Only local volumes can be defragmented; network volumes are not supported
4. The GUI version prior to Windows Vista cannot be scheduled. However, the command line utility since Windows XP and later can be scheduled
5. Unlike previous versions, the GUI version in Windows Vista does not display a map of disk fragmentation, nor does it display progress during defragmentation.

In addition, the Windows 2000 version has the following limitations which were removed in Windows XP

1. Defragmenting NTFS volumes with cluster sizes larger than 4 kilobytes (KB) is not possible.
2. It is not possible to perform fine-grained movement of uncompressed NTFS file data in Windows 2000. Moving a single file cluster also moves the 4 KB part of the file that contains the cluster.
3. EFS encrypted files are not defragmented.

In addition, the Windows 7, 8 and 10 Versions has following Limitations

1. Disk Defragmenter does not defragment files in the Recycle Bin. It is best to run Disk Cleaner first and then empty the Recycle Bin before defragmenting
2. Disk Defragmenter will also not defragment files which are in use. Best to try and shut down as many processes as possible and then defragment.
3. Disk Defragmenter does not defragment the following files: Bootsect DOS, Safeboot fs, Safeboot CSV, Safeboot RSV, Hiberfil sys, Memory dump and the Windows page file. However using the -b parameter, as mentioned below, will optimize the boot files