Fragmentation occurs when a computer can't or won't store a file as a single unit on its disk. The resulting issues may be minor or major, depending on the degree of fragmentation and what the computer is used for, but most of these disadvantages can be prevented or reversed.
The Causes of Fragmentation
A computer's file system divides files into blocks for disk storage. When there is adequate space and the system is working correctly, these blocks are stored together. Fragmentation occurs when, as the disk fills up, or as files are deleted and modified, the file system stores a file's blocks in multiple locations. When the file is read, the disk's read/write heads must skip around the disk to read all the file's blocks.
The greater the fragmentation, the more time it takes for the fragmented files to be read. In severe cases, files may be stored in small chunks throughout the disk, causing the read/write heads to spend more time moving around the disk than reading the file itself. Saving and opening fragmented files can be many times slower than performing the same operations on unfragmented files. For example, saving a 30 megabyte (MB) Microsoft Word document may take three seconds to an unfragmented disk, but as much as 46 seconds to an extremely fragmented disk.
Application and System Speed
Some applications, such as photography or video applications, may use disk space, or "scratch disk," for temporary storage during memory-intensive operations. If the scratch disk is fragmented, the result will be a slowdown in the application itself. Likewise, most operating systems use virtual memory (also known as "swap," "disk caching" or "paging") to temporarily store files that would otherwise use random access memory (RAM). If the virtual memory space is fragmented, the operating system itself can slow down.
Avoiding Fragmentation Problems
Fragmentation occurs most often through neglect. The easiest way to prevent fragmentation is to defragment the disk on a regular basis and remove unneeded files and applications. Microsoft Windows includes a defragmentation tool for this purpose and commercial defragmentation tools are also available. Mac OS X includes automatic file defragmentation, but Apple suggests that manual defragmentation may help for disks that are almost full. Other ways to avoid the disadvantages of fragmentation include creating a partition exclusively for large files or scratch disk space, adding a second disk or upgrading to a larger one.