manages storage space to provide a level of reliability and efficiency. Generally, it allocates storage device space in a granular manner, usually multiple physical units (i.e. bytes). For example, in Apple DOS of the early 1980s, 256-byte sectors on 140 kilobyte floppy disk used a track/sector map.

The granular nature results in unused space, sometimes called slack space, for each file except for those that have the rare size that is