Example of slack space, demonstrated with 4,096-byte NTFS clusters: 100,000 files, each 5 bytes per file, equals 500,000 bytes of actual data, but requires 409,600,000 bytes of disk space to store
File systems allocate space in a granular manner, usually multiple physical units on the device. The file system is responsible for organizing files and directories, and keeping track of which areas of the media belong to which file and which are not being used. For example, in Apple DOS of the early 1980s, 256-byte sectors on 140 kilobyte floppy disk used a track/sector map.
This results in unused space when a file is not an exact multiple of the allocation unit, sometimes referred to as slack space. For a 512-byte allocation, the average unused space is 255 bytes. For a 64KB clusters, the average unused space is 32KB. The size of the allocation unit is chosen when the file system is created. Choosing the allocation size based on the average size of the files expected to be in the filesystem can minimize the amount of unusable space. Frequently the default allocation may provide reasonable usage. If it can be anticipated that a file system will contain mostly small files a small cluster size should be chosen. Choosing an allocation size that is too small results in excessive overhead if the file system will contain mostly very large files.
File system fragmentation occurs when unused space or single files are not contiguous. As a filesystem is used, files are created, modified and deleted. When a file is created the filesystem allocates space for the data. Some filesystems permit or require specifying an initial space allocation and subsequent incremental allocations as the file grows. As files are deleted the space they were allocated eventually is considered available for use by other files. This creates alternating used and unused areas of various sizes. This is free space fragmentation. When a file is created and there is not an area of contiguous space available for its initial allocation the space must be assigned in fragments. When a file is modified such that it becomes larger it may exceed the space initially allocated to it, another allocation must be assigned elsewhere and the file becomes fragmented.
A file system may not make use of a storage device but can be used to organize and represent access to any data, whether it is stored or dynamically generated (e.g., procfs).
 File names
Main article: Filename
A file name (or filename) is used to reference the storage location in the filesystem. Most filesystems have restrictions on the length of the filename. Some filesystems have case insensitive filenames.
Most file system interface utilities place restrictions on the characters permitted in the filename