Disk compression
Automatic on-the-fly compression to double disk space.
Disk compression works by automatically shrinking data as it's saved to a disk and expanding it again when it's read back, effectively letting you store more information in the same physical space. The software that does this—called a disk compression utility—inserts itself into the system's file access layer, so every program on the computer automatically uses compression without needing any special setup. When a program writes a file, the utility compresses the data before putting it on the disk; when a program reads a file, it decompresses the data before handing it over.
This kind of utility typically stores all the user-visible files of a disk volume inside a single, large normal file on the host drive. That large file acts like a virtual drive, and accessing it usually requires a device driver. There are also similar utilities that compress and decompress each file individually instead of using one big container file, but those aren't considered disk compression.
Unlike a file compression utility, which requires the user to manually pick which files to compress and when, a disk compression utility works automatically as part of normal file use. This automatic behavior is often described as on-the-fly, transparent, real-time, or online.
Disk compression was especially popular in the early 1990s, when microcomputer hard drives were small—typically 20 to 80 megabytes—and expensive, costing roughly $10 per megabyte. For many users, buying a compression utility was a more economical way to get more storage space than buying a larger hard drive. A good utility could roughly double the available space with only a minor impact on speed. By the late 1990s, disk compression had fallen out of use because advances in hard drive technology and manufacturing had driven down the cost per unit of storage.
**Use**
To start using disk compression on an existing drive, the utility would typically follow these steps: create an empty, compressed virtual drive (a normal file on the host drive); transfer files from the host drive to the compressed drive, leaving behind certain system files like the swap file; and then increase the size of the compressed drive to accommodate the files and leave some empty space.
Because accessing a compressed drive requires a device driver, booting from a compressed drive (such as C:) required changes to the boot process.
- Popularity period
- early 1990s
- Typical hard disk size
- 20 to 80 megabytes
- Cost per megabyte
- roughly 10 USD per megabyte
- Average space doubling
- double the available space
- Performance impact
- negligible speed loss
- Decline period
- late 1990s
Lore & Background
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.
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 a 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.
To start using disk compression on an existing drive, the utility would create an empty, compressed, virtual drive (a normal file on the host drive), transfer files from the host drive to the compressed drive except for certain system files such as the swap file, and increase the size of the compressed drive to accommodate files and allow empty space when complete. A compressed boot drive required changes to the boot process, involving the System BIOS, partition sector, host drive sector, DOS BIOS, DOS kernel, and a preloadable compression driver such as DBLSPACE.BIN, DRVSPACE.BIN or STACKER.BIN.
Reader's Guide
Disk compression's significance lies in its role as an economic alternative to purchasing larger hard drives during a period when storage was expensive and limited. By automatically doubling available space with negligible speed loss, it allowed users and resellers to effectively increase capacity without hardware upgrades. The technology was widely bundled with operating systems, including DR DOS 6.0, MS-DOS 6.0, Windows 95, and Windows 98, making it a standard feature for many users. However, disk compression had notable considerations: performance could decrease on CPU-bound systems but increase on I/O-bound systems; data loss could occur from crosslinked files if the utility did not confirm FAT file system errors before compression; boot drive accessibility depended on the device driver being loaded; uninstallation was problematic if the expanded data size exceeded normal disk capacity; and users could accidentally delete the virtual drive file, losing all stored files. The technology fell into disuse by the late 1990s as hard drive costs dropped dramatically, making compression less necessary. Examples of disk compression utilities include Stacker, DoubleSpace, DriveSpace, SuperStor, and DiskDoubler, with both hardware-assisted (using a coprocessor) and software-only solutions available.
Did You Know?
- Disk compression utilities could double available space with negligible speed loss.
- A compressed drive is stored as a single, large normal file on the host drive.
- Disk compression was popular in the early 1990s when hard disks were 20 to 80 megabytes and cost roughly 10 USD per megabyte.
- Accidentally deleting the virtual drive file resulted in the loss of all files stored in it.
More in Computer Storage, Part 3 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced