Disk compression

Disk compression increases the amount of information that can be stored on a disk by automatically compressing and decompressing data as it is written to and read from it. A disk compression utility is software that provides disk compression by overriding the system's file access layer so that all programs participate in compression. When a program writes a file, the utility compresses the data before storage. When a program reads a file, the utility decompresses the stored data before providing it to the program.

Disk compression uses a single, normal file for storing the logical, user-visible files of a disk volume. This file tends to be relatively large and acts as a virtual drive. Typically, access to the compressed storage is via device driver. Similar utilities automatically compress and decompress each file instead of using a single, large file, but such a utility is not classified as disk compression.

Unlike a file compression utility which is generally interactive (user specifies which files to compress and when to compress them), a disk compression utility is noninteractive (files are processed automatically as part of normal file use). This automatic behavior is sometimes described as on-the-fly, transparent, real-time and online.

Disk compression was popular especially in the early 1990s, when microcomputer hard disks were relatively small (20 to 80 megabytes) and rather expensive (roughly 10 USD per megabyte). Such utilities tended to be a more economic means of having more disk space as opposed to buying a disk with more space. A good disk compression utility could, on average, double the available space with negligible speed loss. Disk compression fell into disuse by the late 1990s, as advances in hard drive technology and manufacturing led to lower cost per unit of space.