Disk compression
A disk compression software utility increases the amount of information that can be stored on a hard disk drive of given size. Unlike a file compression utility, which compresses only specified files—and which requires the user to designate the files to be compressed—an on-the-fly disk compression utility works automatically through resident software without the user needing to be aware of its existence. On-the-fly disk compression is therefore also known as transparent, real-time or online disk compression. When information needs to be stored to the hard disk, the utility compresses the information . When information needs to be read, the utility decompresses the information. A disk compression utility overrides the standard operating system routines. Since all software applications access the hard disk using these routines, they continue to work after disk compression has been installed. Disk compression utilities were popular especially in the early 1990s, when microcomputer hard disks were still relatively small (20 to 80 megabytes). Hard drives were also rather expensive at the time, costing roughly 10 USD per megabyte. For the users who bought disk compression applications, the software proved to be in the short term a more economic means  of acquiring more disk space as opposed to replacing their current drive with a larger one. 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 increased capacities and lower prices. Common disk compression solutions. Standalone hardware. Some of the initial disk compression solutions were hardware-assisted and utilized add-on compressor/decompressor coprocessor cards in addition to a software driver. Known solutions include: Standalone software. With increasing PC processor power software-only solutions began  to reach or even outperform the performance of hardware-assisted solutions in most scenarios. These compression utilities were sold independently. A user had to specifically choose to install and configure the software. Bundled software. The idea of bundling disk compression into new machines appealed to resellers and users. Resellers liked that they could claim more storage space; users liked that they did not have to configure the software. Bundled utilities included (in chronological order): Other solutions. While Windows XP, from Microsoft, included both a native support and a command named that compresses files on NTFS systems, that is not implemented  as a separate "compressed drive" like those above. Operation. Disk compression usually creates a single large file, which becomes a virtual hard drive. This is similar to how a single physical hard drive can be partitioned into multiple virtual drives. The compressed drive is accessed via a device driver. Compressing existing drives. All drives would initially be empty. The utility to create a drive would usually offer to "compress a current drive". This meant the utility would: Usually certain system files would not be transferred. For example, OS swap files would remain only on the host drive. Compressing the boot  drive. A device driver had to be loaded to access the compressed drive. A compressed drive C: required changes to the boot process as follows: Performance impact. On systems with slower hard drives, disk compression could actually increase system performance. This was accomplished two ways: If the system had to wait frequently for hard drive access to complete (I/O bound) converting the hard drive to compressed drives could speed up the system significantly. Compression and decompression of the data increases the CPU utilization. If the system was already CPU bound, disk compression decreases overall performance. Drawbacks. Some common drawbacks  to using disk compression: