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(computer science)
A procedure in which portions of files on a computer disk are moved until all parts of each file occupy continuous sectors, resulting in a substantial improvement in disk access times.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


To reorganize the disk by putting files into contiguous order. Because the operating system stores new data in whatever free space is available, data files become spread out across the disk as they are updated. Large files may be broken into thousands of fragments, causing the read/write head to move back and forth numerous extra times to read the data. A "defragger" or "optimizer program" rewrites all the files and stores them in adjacent sectors.

Windows comes with the DEFRAG.EXE utility, which can be activated by the Run dialog from the Start menu. Other popular defraggers, such as Diskeeper, offer enhanced capabilities.

Users Run the Gamut
Some users are fanatical about defragging their disks on a regular basis. Others rarely do it, if ever. After defragmenting, performance increases are most noticeable on very large databases that had been heavily fragmented. Because today's computers are so fast, users may not notice any increase in speed with routine applications such as word processing and spreadsheets, especially if the files are small.

A Good Habit
Nevertheless, defragmenting disks regularly is good practice because it reduces wear and tear on the drive mechanism. In addition, should the hard disk ever crash and you did not back up important files, data recovery experts will tell you that a defragged disk is much easier to restore.

Before and After
Diskeeper is one of the most popular defraggers on the market. The red in the drive map at the top shows all the fragmented files. After defragmenting, the new drive map was pasted on the bottom for comparison. Diskeeper can be set up to defrag the disk whenever the machine is idle. As soon as the user clicks the mouse or presses a key, the defragging is suspended.
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