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.
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.
The techniques used by the Disk Defragmenter are as follows:
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 addition, the Windows 2000 version has the following limitations which were removed in Windows XP: