Defrag

File:Windows XP-Disk Defragmenter.png

Microsoft Windows XP Disk Defragmenter

DEFRAG or Disk Defragmenter is a program included with most versions of the MS-DOS and Microsoft Windows operating systems since MS-DOS 5.0, ca 1989. Defrag.exe defragments a file system such as a hard drive.

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

Disk optimization is, briefly, a method of optimizing the efficiency of disk caching to minimize head travel and maximize effective speed:

1. Move all the index or directory information to one spot. Move this spot into the center of the data, e.g. one third of the way in, so that head travel to data is halved compared to having directory information at the front.
2. Cluster files around the directory area.
3. Optionally: Move infrequently used files further from the directory area.
4. Optionally: Obey a user provided table of file descriptions to emphasize or ignore.
5. While moving files around, be sure that all are contiguous (defragmentation).

File:Microsoft Defrag for MS-DOS.png

Microsoft Defrag for MS-DOS by Symantec

By 1980 disk optimization programs were shipped with almost all computer systems using a hard drive.

MS-DOS up to version 5 and Windows NT through version 4 did not come with any defragmentation utility much less disk optimization. Why Dave Cutler, chief architect of NT did not include disk optimization remains a mystery, as apparently he had included disk optimization in all his very similar operating systems from VMS back to his work supporting OSs at Carnegie Mellon.

In the MS-DOS world there were several after-market disk optimizers. The most famous, the Norton 'Defrag' utility, or 'Speed Disk', only managed the defragmentation step and was considered incompetent by some people. Norton's defrag was also very slow, used memory inefficiently and often got caught up moving one cluster at a time. A shareware program called Disk Organizer was much faster than most other program and became quite popular for several years.

When defrag was shipped for free with MS-DOS 5.0, competing commercial products were forced out of the market, which is often cited as one of Microsoft's key strategies (see: Embrace, extend and extinguish).

See Defragmentation for other information about this process.