Editing File system (section)

=== Storage space organization ===

A local file system tracks which areas of storage belong to which file and which are not being used.

When a file system creates a file, it allocates space for data. Some file systems permit or require specifying an initial space allocation and subsequent incremental allocations as the file grows.

To delete a file, the file system records that the file's space is free; available to use for another file.

[[File:100 000-files 5-bytes each -- 400 megs of slack space.png|frame|An example of slack space, demonstrated with 4,096-[[byte]] NTFS clusters: 100,000 files, each five bytes per file, which equal to 500,000 bytes of actual data but require 409,600,000 bytes of disk space to store <!-- The size listing shown in Explorer is oddly doubly-wrong. The example files are 5 bytes each, not 0.1K, and the clusters are a minimum of 4K not 1K.-->]]

A local file system 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''.{{Citation needed|date=September 2012}}

The granular nature results in unused space, sometimes called [[slack space]], for each file except for those that have the rare size that is a multiple of the granular allocation.{{Sfn|Carrier|2005|pp=187–188}} For a 512-byte allocation, the average unused space is 256 bytes. For 64&nbsp;KB clusters, the average unused space is 32&nbsp;KB.

Generally, the allocation unit size is set when the storage is configured.
Choosing a relatively small size compared to the files stored, results in excessive access overhead.
Choosing a relatively large size results in excessive unused space.
Choosing an allocation size based on the average size of files expected to be in the storage tends to minimize unusable space.