Disk Defragmenter is a utility in Microsoft Windows designed to increase access speed
by rearranging files stored on a disk to occupy contiguous storage locations, a technique
called defragmentation. Defragmenting a disk minimizes head travel, which reduces the time
it takes to read files from and write files to the disk. Beginning with Windows XP, Disk
Defragmenter also reduces system startup times.
History Early history
As early as the end of 1982, the IBM PC DOS operating system that shipped with early IBM
Personal Computers included a Disk Volume Organization Optimizer to defragment the 5¼-inch
floppy disks that those machines used. Microsoft's MS-DOS did not defragment hard disks. Several
third party software developers marketed defragmenters to fill this gap. MS-DOS 6.0 introduced Defrag.
Windows NT, however, did not offer a Defrag utility, and Symantec was suggested as a possible
source of the utility. Initial releases of Windows NT lacked a defragmentation
tool. Versions through Windows NT 3.51 did not have an application programming interface
for moving data clusters on hard disks. Executive Software, later renamed Diskeeper Corporation,
released Diskeeper defragmentation software for Windows NT 3.51, which shipped with a
customized version of the NT kernel and file system drivers that could move clusters.
Microsoft included file system control commands to move clusters in the Windows NT 4.0 kernel,
which worked for both NTFS and FAT32 partitions. However, Windows NT 4.0 did not provide a
graphical or command-line user interface. Introduction
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:
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.
Moving infrequently used files further from the directory area.
Obeying a user-provided table of file descriptions to emphasize or ignore.
Making files contiguous so that they can be read without unnecessary seeking.
Windows XP and Windows Server 2003 Windows Disk Defragmenter was updated to alleviate
some restrictions. It no longer relies on the Windows NT Cache Manager, which prevented
the defragmenter from moving pieces of a file that cross a 256KB boundary within the file.
NTFS metadata files can also be defragmented. A command-line tool, defrag.exe, has been
included, providing access to the defragmenter from cmd.exe and Task Scheduler. In Windows
XP, if the Master File Table is spread into multiple fragments, defrag.exe and the GUI
version can combine the MFT fragments during defragmentation.
Windows Vista and Windows Server 2008 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 when the computer is in use. The user interface has been simplified,
with the color graph, progress indicator, disk analysis and fragmentation information
being removed entirely. 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, 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 maintained by Microsoft's Core File Services team. The Windows Vista
version has been updated in Windows Vista SP1 to include the improvements made in Windows
Server 2008. The most notable of these improvements is that the ability to select which volumes
are to be defragged has been added back. Notably, the Windows Vista defragmenter is much more
effective than the version included with XP. Windows 7, 8 and 8.1
Windows 7 reintroduces the analyze function and showing percent complete of the defragmentation,
both of which were removed in Windows Vista. It can also defragment multiple volumes simultaneously.
It will not defragment a solid-state drive, as SSDs don't require defragmentation and
doing so reduces their lifespan. Limitations
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 registry, page file and hibernation file.
Prior to the Windows Vista release, only one volume could be analyzed or defragmented at
a time and only one instance could run. Only local volumes can be defragmented, network
volumes are not supported. The GUI version prior to Windows Vista cannot
be scheduled, however the command line utility since Windows XP and later can be scheduled.
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: Defragmenting NTFS volumes with cluster sizes
larger than 4 kilobytes 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
Defragmentation File system fragmentation
List of defragmentation software References
External links Troubleshooting Disk Defragmenter