Disk Defragmenter (Windows)

Windows Disk Defragmenter is a computer program included in Microsoft Windows designed to increase access speed (and sometimes increase the amount of usable space) by rearranging files stored on a disk to occupy contiguous storage locations, or defragmenting. The purpose is to optimize the time it takes to read and write files to/from the disk by minimizing head travel time and maximizing the transfer rate.


Defragmentation has been part of disk optimization since disk optimization stabilized in 1975. Few, other than Norton (Symantec) and Microsoft, have shipped defragmentation programs separate from disk optimization methods.

MS-DOS versions up to version 5 and Windows NT through version 4.0 did not come with a defragmentation utility.

When Defrag, licensed from Symantec, was shipped for free with MS-DOS 6.0, the use of the alternative commercial products became less frequent, because customers were unable to justify the additional expense.

Windows 9x

A 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. It had a limitation that if the contents of the drive changed while defragmenting, it restarted the process from the beginning.

Modern versions

Windows 2000, Windows XP, and Windows Server 2003 include a stripped-down licensed version of Diskeeper Corporation's (formerly Executive Software's) Diskeeper. Windows XP additionally shipped with a command line Defrag.exe utility.

The techniques used by the Disk Defragmenter are as follows:

  1. Moving all the index or directory information to one spot. Moving this spot into the center 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.

Windows Vista changes

In Windows Vista, Disk Defragmenter includes an option to automatically run at scheduled times using Task Scheduler and uses low CPU priority and the newly introduced low priority I/O algorithm so that it can continue to defrag using reduced resources (less CPU and disk read/write activity) when the computer is in use. The user interface has been simplified, with the color graph and progress indicator being removed entirely. It was also not possible to select which drives to defragment, though Windows Vista Service Pack 1 adds this feature.

If the fragments of a file are over 64 MB in size, the file is not defragmented if using the GUI; Microsoft has stated that this is because there is no discernible performance benefit since the time seeking such large chunks of data is negligible compared to the time required to read them. The result, however, is that Disk Defragmenter does not require a certain amount of free space in order to successfully defrag a volume, unlike performing a full defragmentation which requires at least 15% of free space on the volume. The command line utility, Defrag.exe in Windows Vista, offers more control over the defragmentation process, such as performing a full defragmentation by consolidating all file fragments regardless of size. This utility can be used to defragment specific volumes or to just analyze volumes as the defragmenter would in Windows XP.

Disk Defragmenter is currently being maintained by Microsoft's Core File Services (CFS) team. The Windows Vista version has been updated to include the improvements made in Windows Server 2008 in Windows Vista SP1. The most notable of these improvements is that the ability to select which volumes are to be defragged has been added back.


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

  • It does not defragment files residing in the Recycle Bin or files that are in use. In particular, this includes the page file and hibernation file.
  • Only one volume can be analyzed or defragmented at a time and only one instance can run.
  • The GUI version prior to Windows Vista cannot be scheduled, however the command line utility since Windows XP and later can be scheduled.
  • NTFS metadata, such as the Master File Table (MFT), or metadata that describes a directory's contents is not defragmented.

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

  • Defragmenting NTFS volumes with cluster sizes larger than 4 kilobytes (KB) is not possible.
  • 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.
  • EFS encrypted files are not defragmented.

See also


External links

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