What is the difference between file extents and fragmentation?

When a file is initially created there are several default parameters associated with that file. There is a parameter that determines how many contiguous blocks are allocated each time a file grows larger. As a file expands, VMS looks at the spaces immediately following the file. If enough space is found, then that is where the next file extention will be located. That file is contiguous. But if not enough space is immediately available, then VMS must search the disk for space to record the new information. Hence, a file that is not contiguous but fragmented. Fragmentation is a condition in which individual files on a disk are not contiguous, but rather are broken up into pieces scattered around the disk, or the available free space on a disk consists of little bits of space here and there rather than in a few large spaces. Here is how Diskeeper works. If the file is already contiguous it is not moved unless moving the file would improve the arrangement of free spaces on the disk. As long as the file is not excluded for one of several reaons, it will be selected for movement if doing so would reduce that file's fragmentation or increase contiguous free space on the disk.  

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