What Are the Causes of Drive Fragmentation?

A fragmented hard disk can slow down your computer.

Over time, your computer's hard disk can become fragmented, meaning that the files stored on the disk are no longer in a contiguous order. There are two types of fragmentation: file, where sections of a file are located in different areas of the disk; and space, where the free space is no longer allocated to one area of the disk. Fragmentation causes reduced efficiency as disk access times are increased.

File System

Computers use file systems to allocate space on a hard disk and to store files in that space. There are several different types of file system, two of which are FAT and NTFS. The file system divides the hard disk into sectors, or blocks, and stores information, including what data is stored and where. When you use an application to save a file, the file system finds a location on the hard disk to store it.

File Size

When you save a large file such as a video, for example, the file system may need many segments, or blocks, of space to store it -- these are referred to as clusters. If the file system cannot find a cluster of blocks large enough to save the file in a contiguous -- or adjacent -- area of the disk, it breaks the file into smaller pieces that can be stored across clusters of blocks in different areas of the disk. When this happens, the file is described as fragmented.

Saving Files

As you save more and more files, the free disk space can also become fragmented. This happens because only data for a particular file can be saved in any one block. If the file is large enough to fill multiple blocks plus a fraction of a block, the remaining free space in that block cannot be used to store any more data. There may be 1GB of free space on the hard disk, for example, but the file system can only use 800MB of this space for storage.

Deleting Files

When a file is deleted, the clusters used to store it become free and can be used to store new files. However, if the deleted file used three clusters of disk space, for example, and the next file you create requires four clusters, the file system may have to break the file up. This happens if the file system cannot find the four contiguous clusters needed, in this example, in which to save the file as a whole.

References

• Disk Keeper: Introduction
• The PC Guide: Fragmentation and Defragmentation