After defining disk defragmentation, I'll provide a synopsis of the history of NT defragmentation products, and describe the support Microsoft added to NT 4.0's file systems specifically for use by defragmentation tools. When you create a file on a drive, the file system assigns clusters of storage for the file's data. Later, you add a few pages to the document, and the file grows by six more clusters.
Fragmentation can negatively affect performance.
When the disk head reads contiguous data, the level of throughput is high. When the system reads a fragmented disk file, the disk head must move from fragment to fragment. On a disk with many fragmented files, moving the disk head across fragments can consume significant time. Disk defragmenters are products that aim to optimize system performance by reorganizing a drive's clusters to make file clusters contiguous, and in some cases, to make the free clusters contiguous. Defragmenters use heuristic algorithms that implement a fragment-shifting strategy to defragment the drive's files as quickly as possible.