To understand what defragmentation IS, a person must first understand how data files are saved onto magnetic storage disks. Whether on a floppy disk or a hard disk, data is stored in a certain format. Formatting consists of dividing a disk into organized sections so that data can be located by the computer. Formatting organizes disks into concentric rings called tracks. Tracks are divided into sectors (pie shaped wedges) in which files and parts of files are stored.
Some files are larger than others and require more storage space. Such files are often stored in several neighbouring sectors on the disk, called a cluster. As files are deleted from a disk, various sectors become available. The next time a file is saved, the computer tries to use the newly available sectors. If those sectors aren't large enough to hold the complete file, the remaining parts of the file are stored in one or more other sectors.
Obviously this results in a file being broken into pieces and scattered over the disk. This doesn't present a huge problem, since the operating system keeps track of the location of the different parts. However, when a file is fragmented, it takes a longer period of time for the computer to locate all of the pieces of information that make up the complete file. As more and more files are fragmented over the disk, the computer slows down noticeably
Defragmentation alleviates this problem by rewriting the files so that they are again stored on neighbouring clusters. This procedure results in a significant improvement in file retrieval time. DOS version 6.x includes a defragmentor for taking care of this problem. In addition to DOS 6, there are special programs (such as DiskExpress and PowerUp for the Macintosh, and DOS Rx for the PC) that can be purchased for defragmenting files. Utility packages, such as PC Tools and Norton Utilities, also include a defragmenter.