Disk compression
Encyclopedia
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
designate the files to be compressed - a disk compression utility works automatically and the user doesn't need to be aware of its existence.
When information needs to be stored to the hard disk, the utility will compress
the information. When information needs to be read, the utility will decompress the information. A disk compression utility overrides the standard operating system
routines. Since all software application
s 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 megabyte
s). 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.
Note: While the most familiar disk compression utilities were designed to work on DOS
systems, the concept was not specific to DOS. The utility DiskDoubler
, for example, worked on the Apple Macintosh platform.
While Windows XP, from Microsoft, included both a native support and a command line utility named 'compact' that compresses files on NTFS
systems, that is not implemented as a separate "compressed drive" like those above.
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
.
Usually certain system file
s would not be transferred. For example, OS swap files would remain only on the host drive.
as follows:
If the system had to frequently wait for hard drive access to complete (IO bound
) converting the hard drive to compressed drives could speed up the system significantly. Compression and decompression of the data will increase the CPU utilization. If the system was already CPU bound
, disk compression will decrease overall performance.
Utility software
Utility software is system software designed to help analyze, configure, optimize or maintain a computer. A single piece of utility software is usually called a utility or tool....
increases the amount of information that can be stored on a hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
drive of given size. Unlike a file compression utility which compresses only specified files - and which requires the user
User (computing)
A user is an agent, either a human agent or software agent, who uses a computer or network service. A user often has a user account and is identified by a username , screen name , nickname , or handle, which is derived from the identical Citizen's Band radio term.Users are...
designate the files to be compressed - a disk compression utility works automatically and the user doesn't need to be aware of its existence.
When information needs to be stored to the hard disk, the utility will compress
Data compression
In computer science and information theory, data compression, source coding or bit-rate reduction is the process of encoding information using fewer bits than the original representation would use....
the information. When information needs to be read, the utility will decompress the information. A disk compression utility overrides the standard operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
routines. Since all software application
Application software
Application software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...
s 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
Microcomputer
A microcomputer is a computer with a microprocessor as its central processing unit. They are physically small compared to mainframe and minicomputers...
hard disks were still relatively small (20 to 80 megabyte
Megabyte
The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: bytes generally for computer memory; and one million bytes generally for computer storage. The IEEE Standards Board has decided that "Mega will mean 1 000...
s). 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.
Note: While the most familiar disk compression utilities were designed to work on DOS
DOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...
systems, the concept was not specific to DOS. The utility DiskDoubler
DiskDoubler
DiskDoubler is a data compression utility for compressing files on the Apple Macintosh platform. Unlike most such programs, which compress numerous files into a single archive for transmission, DiskDoubler was intended to compress single files "in place" to save space on the drive...
, for example, worked on the Apple Macintosh platform.
Standalone utilities
The initial compression utilities were sold independently. A user had to specifically choose to install and configure the software.- Stacker from Stac ElectronicsStac ElectronicsStac Electronics, originally incorporated as State of the Art Consulting and later shortened to Stac, Inc, was a technology company founded in 1983...
- XtraDrive from Integrated Information Technology ( IIT)
- SuperStor Pro from AddStor
- DoubleDisk Gold from Vertisoft Systems
- DiskDoubler from Salient Software
Bundled utilities
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:- DR DOS 6.0, from Digital Research, included a version of SuperStor.
- PalmDOS 1.0, from Novell (after acquiring Digital Research), included a DPMSDOS Protected Mode ServicesDOS Protected Mode Services is a set of extended DOS memory management services to allow DPMS-enabled DOS drivers to load and execute in extended memory and protected mode....
-enabled version of SuperStor. - MS-DOSMS-DOSMS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...
6.0 and MS-DOS 6.2, from Microsoft, included DoubleSpace, which was based in part on Vertisoft Systems' DoubleDisk. - PC DOS 6.1, from IBM, included a version of SuperStor.
- MS-DOS 6.22, from Microsoft, included DriveSpace, which was DoubleSpace with a different compression algorithmAlgorithmIn mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
. - PC DOS 6.3, from IBM, included a version of SuperStor.
- Novell DOS 7, from Novell (after acquiring Digital Research), included a DPMS-enabled version of Stacker 3.12.
- PC DOS 7.0, from IBM, included a DPMS-enabled version of Stacker 4.02 from Stac ElectronicsStac ElectronicsStac Electronics, originally incorporated as State of the Art Consulting and later shortened to Stac, Inc, was a technology company founded in 1983...
. - OpenDOS 7.01, from Caldera (after acquiring Novell DOS), included a DPMS-enabled version of Stacker 3.12.
- DR-DOS 7.02/7.03, from Caldera, included a DPMS-enabled version of Stacker 3.12.
- PC DOS 2000, from IBM, included a DPMS-enabled version of Stacker 4.02.
- Windows 95Windows 95Windows 95 is a consumer-oriented graphical user interface-based operating system. It was released on August 24, 1995 by Microsoft, and was a significant progression from the company's previous Windows products...
, from Microsoft, included an updated version of DriveSpace. - Plus!, from Microsoft, included DriveSpace 3.
Other utilities
- Double Tools for DoubleSpaceDouble Tools for DoubleSpaceDouble Tools for DoubleSpace is a software utility released in 1993 by the Menlo Park-based company Addstor, Inc. The utility functioned as an add-on to the disk compression software DoubleSpace, supplied with MS-DOS 6.0, adding a number of features not available in the standard...
from Addstor, Inc., was an add-on product, enhancing the functionality of the DoubleSpaceDoubleSpaceDriveSpace is a disk compression utility supplied with MS-DOS starting from version 6.0. The purpose of DriveSpace is to increase the amount of data the user could store on disks, by transparently compressing and decompressing data on-the-fly. It is primarily intended for use with hard drives, but...
bundled with MS-DOSMS-DOSMS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...
6.0.
While Windows XP, from Microsoft, included both a native support and a command line utility named 'compact' that compresses files on NTFS
NTFS
NTFS is the standard file system of Windows NT, including its later versions Windows 2000, Windows XP, Windows Server 2003, Windows Server 2008, Windows Vista, and Windows 7....
systems, that is not implemented as a separate "compressed drive" like those above.
How disk compression works
Disk compression usually creates a single large file, which becomes a virtualStorage Virtualization
Storage virtualization or storage virtualisation is a concept and term used within computer science. Specifically, storage systems may use virtualization concepts as a tool to enable better functionality and more advanced features within the storage system.Broadly speaking, a 'storage system' is...
hard drive. This is similar to how a single physical hard drive can be partitioned
Disk partitioning
Disk partitioning is the act of dividing a hard disk drive into multiple logical storage units referred to as partitions, to treat one physical disk drive as if it were multiple disks. Partitions are also termed "slices" for operating systems based on BSD, Solaris or GNU Hurd...
into multiple virtual drives. The compressed drive is accessed via a device driver
Device driver
In computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....
.
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:- Create an empty compressed drive, stored on the existing drive.
- Transfer existing files on the old drive to the new compressed drive.
- Increase the size of the new compressed drive as necessary to accommodate more files and allow empty space when done.
- When all files were transferred, the drive letters would be swapped.
Usually certain system file
System file
A system file is a computer file important to the operating system. More specifically, it may refer to:* .sys — a Microsoft Windows file extension for system-related files* The System suitcase on Mac OS* Any file marked with a "system" attribute...
s would not be transferred. For example, OS swap files would remain only on the host drive.
Compressing the boot drive
Note that the device driver had to be loaded to access the compressed drive. A compressed C drive required changes to the boot processBooting
In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...
as follows:
- BIOS loads sector 0 of the first physical hard drive (partition sector)
- Partition sector loads sector 0 of the bootable partition. In this case, it's the host drive.
- Host drive sector 0 loads (in the case of MS-DOS) IO.SYS and begins Config.Sys processing
- Compression device driver is loaded. Compressed drive becomes C; host drive usually became F.
- Processing continues from compressed drive.
Performance Impacts
On systems with slower hard drives, disk compression could actually increase system performance. This was accomplished two ways:- Once compressed, there was less data to be stored.
- Disk accesses would often be batched together for efficiency.
If the system had to frequently wait for hard drive access to complete (IO bound
IO bound
In computer science, I/O bound refers to a condition in which the time it takes to complete a computation is determined principally by the period of time spent waiting for input/output operations to be completed. This is the opposite of a task being CPU bound...
) converting the hard drive to compressed drives could speed up the system significantly. Compression and decompression of the data will increase the CPU utilization. If the system was already CPU bound
CPU bound
In computer science, CPU bound is when the time for a computer to complete a task is determined principally by the speed of the central processor: processor utilization is high, perhaps at 100% usage for many seconds or minutes...
, disk compression will decrease overall performance.
Drawbacks
Some common drawbacks to using disk compression:- Not all compression utilities would confirm the absence of errors in the file systemFile Allocation TableFile Allocation Table is a computer file system architecture now widely used on many computer systems and most memory cards, such as those used with digital cameras. FAT file systems are commonly found on floppy disks, flash memory cards, digital cameras, and many other portable devices because of...
before compressing a disk in place. Some errors, such as crosslinked files, could result in additional data loss during the transfer process. - The compressed drive is only visible if the device driver is loaded and the compressed drive is mounted. A boot diskBoot diskA boot disk is a removable digital data storage medium from which a computer can load and run an operating system or utility program. The computer must have a built-in program which will load and execute a program from a boot disk meeting certain standards.Boot disks are used for:* Operating...
, for example, might not contain the driver. - Users did not always realize that the large file on the host drive contained the compressed drive. While it was usually "hidden" by default, users who did find the large file curious or suspicious were able to delete it. This would normally result in data loss.
See also
- Stac ElectronicsStac ElectronicsStac Electronics, originally incorporated as State of the Art Consulting and later shortened to Stac, Inc, was a technology company founded in 1983...
- DriveSpace
- DR-DOSDR-DOSDR-DOS is an MS-DOS-compatible operating system for IBM PC-compatible personal computers, originally developed by Gary Kildall's Digital Research and derived from Concurrent PC DOS 6.0, which was an advanced successor of CP/M-86...
- PC-DOSPC-DOSIBM PC DOS is a DOS system for the IBM Personal Computer and compatibles, manufactured and sold by IBM from the 1980s to the 2000s....
- MS-DOSMS-DOSMS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...
- DiskDoublerDiskDoublerDiskDoubler is a data compression utility for compressing files on the Apple Macintosh platform. Unlike most such programs, which compress numerous files into a single archive for transmission, DiskDoubler was intended to compress single files "in place" to save space on the drive...
External links
- BYTE Magazine, How Safe is Disk Compression?, February, 1994.
- Smart Computing, How To. . . Compress Hard Drives, April, 1999.
- MS-DOS 6 Technical Reference