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Disk compression

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A disk compression <a href="Utility_software" title="Utility software">software utility</a> increases the amount of information that can be stored on a <a href="Hard_disk" class="mw-redirect" title="Hard disk">hard disk</a> drive of given size. Unlike a <a href="File_compression" class="mw-redirect" title="File compression">file compression</a> utility, which compresses only specified files—and which requires the <a href="User_(computing)" title="User (computing)">user</a> 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 <a href="Data_compression" title="Data compression">compresses</a> the information. When information needs to be read, the utility decompresses the information. A disk compression utility overrides the standard <a href="Operating_system" title="Operating system">operating system</a> routines. Since all <a href="Application_software" title="Application software">software applications</a> 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 <a href="Microcomputer" title="Microcomputer">microcomputer</a> hard disks were still relatively small (20 to 80 <a href="Megabyte" title="Megabyte">megabytes</a>). Hard drives were also rather expensive at the time, costing roughly 10 <a href="USD" class="mw-redirect" title="USD">USD</a> 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.

<div class="mw-heading mw-heading2"><h2 id="Common_disk_compression_solutions">Common disk compression solutions</h2></div>

<div class="mw-heading mw-heading3"><h3 id="Standalone_hardware">Standalone hardware</h3></div>

Some of the initial disk compression solutions were hardware-assisted and utilized add-on compressor/decompressor <a href="Coprocessor" title="Coprocessor">coprocessor</a> cards in addition to a software driver. Known solutions include:

<ul><li>Stacker XT/8 and Stacker AT/16 from <a href="Stac_Electronics" title="Stac Electronics">Stac Electronics</a><a href="#cite_note-IW_1992_Comparison-1">[1]</a><a href="#cite_note-PCMAG_1992_Comp-2">[2]</a></li>

<li>Expanz! and Expanz! Plus from InfoChip Systems<a href="#cite_note-IW_1992_Comparison-1">[1]</a><a href="#cite_note-PCMAG_1992_Comp-2">[2]</a><a href="#cite_note-PCMAG_1990_Expanz-3">[3]</a></li>

<li>DiskDoubler dd2000 from Datran Corporation</li>

<li><a href="Microsoft_Realtime_Compression_Interface" class="mw-redirect" title="Microsoft Realtime Compression Interface">MRCI</a> (Microsoft Real-Time Compression Interface) from <a href="Microsoft" title="Microsoft">Microsoft</a></li></ul>

<div class="mw-heading mw-heading3"><h3 id="Standalone_software">Standalone software</h3></div>

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.

<ul><li>Squish<a href="#cite_note-IW_1987_Squish-4">[4]</a><a href="#cite_note-PCMAG_1987_Squish-5">[5]</a> and Squish Plus from Sundog Software<a href="#cite_note-PP_1989_Squish-6">[6]</a></li>

<li>Stacker from <a href="Stac_Electronics" title="Stac Electronics">Stac Electronics</a><a href="#cite_note-PCMAG_1992_Comp-2">[2]</a></li>

<li>XtraDrive from <a href="Integrated_Information_Technology" class="mw-redirect" title="Integrated Information Technology">Integrated Information Technology</a> (IIT)</li>

<li>SuperStor and SuperStor Pro from AddStor<a href="#cite_note-IW_1992_Comparison-1">[1]</a><a href="#cite_note-PCMAG_1992_Comp-2">[2]</a></li>

<li>DoubleDisk and DoubleDisk Gold from Vertisoft Systems<a href="#cite_note-IW_1992_Comparison-1">[1]</a><a href="#cite_note-PCMAG_1992_Comp-2">[2]</a></li>

<li>JAM (JAM.SYS) from JAM Software, Kiev, Ukraine<a href="#cite_note-JAM-7">[7]</a></li>

<li><a href="DiskDoubler" title="DiskDoubler">DiskDoubler</a> from <a href="Salient_Software" class="mw-redirect" title="Salient Software">Salient Software</a></li>

<li>Double Density from <a href="Data_Becker" title="Data Becker">Data Becker</a></li></ul>

<div class="mw-heading mw-heading3"><h3 id="Bundled_software">Bundled software</h3></div>

The idea of <a href="Bundling_(marketing)" class="mw-redirect" title="Bundling (marketing)">bundling</a> 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):

<ul><li><a href="DR_DOS_6.0" class="mw-redirect" title="DR DOS 6.0">DR DOS 6.0</a> (1991), from <a href="Digital_Research" title="Digital Research">Digital Research</a>, included a custom version of AddStor's SuperStor.</li>

<li><a href="PalmDOS_1.0" class="mw-redirect" title="PalmDOS 1.0">PalmDOS 1.0</a> (1992), from <a href="Novell" title="Novell">Novell</a> (after acquiring Digital Research), included a <a href="DOS_Protected_Mode_Services" title="DOS Protected Mode Services">DPMS</a>-enabled version of SuperStor.</li>

<li><a href="MS-DOS_6.0" class="mw-redirect" title="MS-DOS 6.0">MS-DOS 6.0</a> (1993), from <a href="Microsoft" title="Microsoft">Microsoft</a>, included <a href="DoubleSpace" class="mw-redirect" title="DoubleSpace">DoubleSpace</a>,<a href="#cite_note-Slivka_1993-8">[8]</a> which was based in part on Vertisoft Systems' DoubleDisk. MS-DOS 6.0 introduced the so-called DOS preload API, an undocumented<a href="#cite_note-Schulman_1994_Undocumented-DOS-9">[9]</a><a href="#cite_note-Chappell_1994_DOS-Internals-10">[10]</a> <a href="Challenge%E2%80%93response" class="mw-redirect" title="Challenge–response">challenge–response</a> type interface which allowed the disk compression software (<a href="DBLSPACE.BIN" class="mw-redirect" title="DBLSPACE.BIN">DBLSPACE.BIN</a>) to be loaded automatically even before <a href="CONFIG.SYS" title="CONFIG.SYS">CONFIG.SYS</a> was processed.</li>

<li><a href="PC_DOS_6.1" class="mw-redirect" title="PC DOS 6.1">PC DOS 6.1</a> (1993), from <a href="IBM" title="IBM">IBM</a>, while the original release did not include a disk compressor, a custom version of SuperStor/DS (after acquiring AddStor) was included in a later release, also introducing the preload API in the PC DOS line.</li>

<li><a href="MS-DOS_6.2" class="mw-redirect" title="MS-DOS 6.2">MS-DOS 6.2</a> (1993), from <a href="Microsoft" title="Microsoft">Microsoft</a>, included an improved version of DoubleSpace.</li>

<li><a href="Novell_DOS_7" class="mw-redirect" title="Novell DOS 7">Novell DOS 7</a> (1993), from Novell, included a DPMS-enabled version of <a href="Stacker_3.12" class="mw-redirect" title="Stacker 3.12">Stacker 3.12</a> (<a href="STACKER.BIN" class="mw-redirect" title="STACKER.BIN">STACKER.BIN</a>) from <a href="Stac_Electronics" title="Stac Electronics">Stac Electronics</a>, which also supported compressed data exchange with remote <a href="NetWare" title="NetWare">NetWare</a> and <a href="Personal_NetWare" title="Personal NetWare">PNW</a> servers also running Stacker in order to avoid unnecessary recompression when copying files.<a href="#cite_note-Paul_1997_NWDOSTIP-11">[11]</a> It also introduced support for the DOS 6 preload API in order to maintain compatibility with Microsoft's DoubleSpace in multi-boot scenarios.<a href="#cite_note-Schulman_1994_Undocumented-DOS-9">[9]</a><a href="#cite_note-Paul_1997_NWDOSTIP-11">[11]</a> A similar mechanism was used to load the optional single- or multi-user <a href="SECURITY.BIN" class="mw-redirect" title="SECURITY.BIN">SECURITY</a> component.</li>

<li><a href="PTS-DOS_Extended_6.4" class="mw-redirect" title="PTS-DOS Extended 6.4">PTS-DOS Extended 6.4</a> (1994), from <a href="PhysTechSoft" class="mw-redirect" title="PhysTechSoft">PhysTechSoft</a>, included a disk compression component named Folder (<a href="FOLDER.SYS" class="mw-redirect" title="FOLDER.SYS">FOLDER.SYS</a>)<a href="#cite_note-CW_1994_PTS-12">[12]</a></li>

<li><a href="PC_DOS_6.3" class="mw-redirect" title="PC DOS 6.3">PC DOS 6.3</a> (1994), from IBM, included a version of SuperStor/DS.</li>

<li><a href="MS-DOS_6.22" class="mw-redirect" title="MS-DOS 6.22">MS-DOS 6.22</a> (1994), from Microsoft, included <a href="DriveSpace" title="DriveSpace">DriveSpace</a>, which was DoubleSpace with a different <a href="Compression_algorithm" class="mw-redirect" title="Compression algorithm">compression algorithm</a> (<a href="DRVSPACE.BIN" class="mw-redirect" title="DRVSPACE.BIN">DRVSPACE.BIN</a>), after <a href="MS-DOS_6.21" class="mw-redirect" title="MS-DOS 6.21">MS-DOS 6.21</a> had to ship without any disk compression software at all for legal reasons.</li>

<li><a href="PC_DOS_7.0" class="mw-redirect" title="PC DOS 7.0">PC DOS 7.0</a> (1995), from IBM, included a DPMS-enabled version of <a href="Stacker_4.02" class="mw-redirect" title="Stacker 4.02">Stacker 4.02</a> from Stac Electronics.</li>

<li><a href="Windows_95" title="Windows 95">Windows 95</a> (1995), from Microsoft, included an updated version of <a href="DriveSpace_2" class="mw-redirect" title="DriveSpace 2">DriveSpace 2</a>.</li>

<li><a href="Windows_95_OSR2" class="mw-redirect" title="Windows 95 OSR2">Windows 95 OSR2</a> (1996), from Microsoft, included a stripped-down version of <a href="DriveSpace_3" class="mw-redirect" title="DriveSpace 3">DriveSpace 3</a>.</li>

<li><a href="Windows_95_Plus!" class="mw-redirect" title="Windows 95 Plus!">Windows 95 Plus!</a> (1996), from Microsoft, included the full version of DriveSpace 3.</li>

<li><a href="OpenDOS_7.01" class="mw-redirect" title="OpenDOS 7.01">OpenDOS 7.01</a> (1997), from <a href="Caldera_UK" class="mw-redirect" title="Caldera UK">Caldera</a> (after acquiring Novell DOS), included a DPMS-enabled version of Stacker 3.12.<a href="#cite_note-Paul_1997_NWDOSTIP-11">[11]</a></li>

<li><a href="DR-DOS_7.02" class="mw-redirect" title="DR-DOS 7.02">DR-DOS 7.02</a> (1998), from Caldera, included a DPMS-enabled version of Stacker 3.12.<a href="#cite_note-Caldera_1998_USER-13">[13]</a> DR-DOS 7.02 also added support for Microsoft's DriveSpace.<a href="#cite_note-Paul_1997_OD-A3-14">[14]</a></li>

<li><a href="PC_DOS_2000" class="mw-redirect" title="PC DOS 2000">PC DOS 2000</a> (1998), from IBM, included a DPMS-enabled version of Stacker 4.02.</li>

<li><a href="Windows_98" title="Windows 98">Windows 98</a> (1998), from Microsoft, included the full version of DriveSpace 3.</li>

<li><a href="DR-DOS_7.03" class="mw-redirect" title="DR-DOS 7.03">DR-DOS 7.03</a> (1999), from Caldera, included a DPMS-enabled version of Stacker 3.12.</li>

<li><a href="Windows_98_SE" class="mw-redirect" title="Windows 98 SE">Windows 98 SE</a> (1999), from Microsoft, included the full version of DriveSpace 3.</li>

<li><a href="PTS-DOS_Pro_2000" class="mw-redirect" title="PTS-DOS Pro 2000">PTS-DOS Pro 2000</a> (1999), from <a href="Paragon_Technology_Software" class="mw-redirect" title="Paragon Technology Software">Paragon Technology Software</a>, included a disk compression component named DATACOMP.</li></ul>

<div class="mw-heading mw-heading3"><h3 id="Other_solutions">Other solutions</h3></div>

<ul><li>Squeeze, a resident DOS file compressor for <a href="Lotus_1-2-3" title="Lotus 1-2-3">Lotus 1-2-3</a> and <a href="Lotus_Symphony_(DOS)" class="mw-redirect" title="Lotus Symphony (DOS)">Lotus Symphony</a> files from Turner Hall Publishing<a href="#cite_note-Hall_1986-15">[15]</a></li>

<li>Squish Plus ROM from Sundog Software, but distributed exclusively through <a href="Personalized_Software" class="mw-redirect" title="Personalized Software">Personalized Software</a> (now <a href="Thaddeus_Computing" class="mw-redirect" title="Thaddeus Computing">Thaddeus Computing</a>), was a special <a href="Read-only_memory" title="Read-only memory">ROM</a> chip containing a custom version of Squish Plus with only 36 KB footprint adapted for the <a href="HP_Portable_Plus" class="mw-redirect" title="HP Portable Plus">HP Portable Plus</a>.<a href="#cite_note-PP_1989_Squish-6">[6]</a></li>

<li><a href="Double_Tools_for_DoubleSpace" title="Double Tools for DoubleSpace">Double Tools for DoubleSpace</a> from Addstor was an add-on product, enhancing the functionality of the <a href="DoubleSpace" class="mw-redirect" title="DoubleSpace">DoubleSpace</a> bundled with <a href="MS-DOS_6.0" class="mw-redirect" title="MS-DOS 6.0">MS-DOS 6.0</a>.</li>

<li>Multimedia Stacker from <a href="Helix_Software" class="mw-redirect" title="Helix Software">Helix Software</a> was a bundle of Stac's <a href="DOS_Protected_Mode_Services" title="DOS Protected Mode Services">DPMS</a>-enabled Stacker 4.01 combined with <a href="Helix_Cloaking" class="mw-redirect" title="Helix Cloaking">Helix Cloaking</a> and utilities, utilizing Cloaking's built-in DPMS server to relocate and run in <a href="Protected_mode" title="Protected mode">protected mode</a>.</li>

<li>THS filesystem (1994–1996 by Thomas Scheuermann), a compressed file system driver for <a href="Linux" title="Linux">Linux</a>, which could read some <a href="Compressed_Volume_File" class="mw-redirect" title="Compressed Volume File">CVFs</a>.<a href="#cite_note-Linux_DOSFS-16">[16]</a><a href="#cite_note-Debian_1996-17">[17]</a><a href="#cite_note-THSFS-18">[18]</a></li>

<li>DMSDOS (1996), a Linux kernel driver,<a href="#cite_note-DMSDOS_2005-19">[19]</a><a href="#cite_note-CMSDOS_1998-20">[20]</a> to support the reading and writing of DoubleSpace, DriveSpace 2 & 3, as well as Stacker 3 & 4 volumes.</li></ul>

While <a href="Windows_XP" title="Windows XP">Windows XP</a>, from Microsoft, included both a native support and a <a href="Command_(computing)" title="Command (computing)">command</a> named <style data-mw-deduplicate="TemplateStyles:r886049734">

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</style><a href="Compact_(computing)" class="mw-redirect" title="Compact (computing)">compact</a> that compresses files on <a href="NTFS" title="NTFS">NTFS</a> systems, that is not implemented as a separate "compressed drive" like those above.

<div class="mw-heading mw-heading2"><h2 id="Operation">Operation</h2></div>

Disk compression usually creates a single large file, which becomes a <a href="Storage_virtualization" title="Storage virtualization">virtual</a> hard drive. This is similar to how a single physical hard drive can be <a href="Disk_partitioning" title="Disk partitioning">partitioned</a> into multiple virtual drives. The compressed drive is accessed via a <a href="Device_driver" title="Device driver">device driver</a>.

<div class="mw-heading mw-heading3"><h3 id="Compressing_existing_drives">Compressing existing drives</h3></div>

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:

<ul><li>Create an empty compressed drive, stored on the existing drive.</li>

<li>Transfer existing files on the old drive to the new compressed drive.</li>

<li>Increase the size of the new compressed drive as necessary to accommodate more files and allow empty space when done.</li>

<li>When all files were transferred, the drive letters would be swapped.</li></ul>

Usually certain <a href="System_file" title="System file">system files</a> would not be transferred. For example, OS <a href="Swap_file" class="mw-redirect" title="Swap file">swap files</a> would remain only on the host drive.

<div class="mw-heading mw-heading3"><h3 id="Compressing_the_boot_drive">Compressing the boot drive</h3></div>

A device driver had to be loaded to access the compressed drive. A compressed drive C: required changes to the <a href="Booting" title="Booting">boot process</a> as follows:

<ul><li>The <a href="System_BIOS" class="mw-redirect" title="System BIOS">System BIOS</a> loads sector 0 of the first physical hard drive (<a href="Master_Boot_Record" class="mw-redirect" title="Master Boot Record">Master Boot Record</a>).</li>

<li>The partition sector loads sector 0 of the bootable partition (<a href="Volume_Boot_Record" class="mw-redirect" title="Volume Boot Record">Volume Boot Record</a>). In this case, it's the host drive.</li>

<li>Host drive sector 0 loads (in the case of <a href="DOS" title="DOS">DOS</a>) the DOS BIOS (<a href="IO.SYS" title="IO.SYS">IO.SYS</a>, <a href="IBMBIO.COM" title="IBMBIO.COM">IBMBIO.COM</a> etc.) Depending on the version, only the first few sectors of the file or the whole file may be loaded into memory. In the first case, the loaded stub contains another loader loading the remainder of the file from disk. In the case of DR-DOS, the BIOS is packed and will decompress itself through an embedded decompression routine.<a href="#cite_note-Paul_1997_OD-A3-14">[14]</a><a href="#cite_note-Caldera_1998_NEW703-21">[21]</a></li>

<li>The DOS BIOS then loads the DOS kernel (<a href="MSDOS.SYS" title="MSDOS.SYS">MSDOS.SYS</a>, <a href="IBMDOS.COM" title="IBMDOS.COM">IBMDOS.COM</a> etc.)</li>

<li>If the DOS supports the preload API, the DOS BIOS attempts to load a preloadable compression driver<a href="#cite_note-Schulman_1994_Undocumented-DOS-9">[9]</a><a href="#cite_note-Chappell_1994_DOS-Internals-10">[10]</a> (<a href="DBLSPACE.BIN" class="mw-redirect" title="DBLSPACE.BIN">DBLSPACE.BIN</a>, <a href="DRVSPACE.BIN" class="mw-redirect" title="DRVSPACE.BIN">DRVSPACE.BIN</a> or <a href="STACKER.BIN" class="mw-redirect" title="STACKER.BIN">STACKER.BIN</a>) from the root of the host drive and attempts to communicate with it through the DOS preload API.<a href="#cite_note-Paul_1997_OD-A3-14">[14]</a><a href="#cite_note-Paul_1997_NWDOSTIP-11">[11]</a> If the driver responds as expected, the DOS BIOS will keep the driver, if not, it will get discarded.<a href="#cite_note-Slivka_1993-8">[8]</a> If kept and configured accordingly, drive letters may get swapped, so that the compressed drive becomes C:.</li>

<li>The DOS BIOS starts <a href="CONFIG.SYS" title="CONFIG.SYS">CONFIG.SYS</a> processing by looking for the file in the root of the boot drive. In the case of DR-DOS, <a href="DCONFIG.SYS" class="mw-redirect" title="DCONFIG.SYS">DCONFIG.SYS</a> takes precedence over CONFIG.SYS, if present.<a href="#cite_note-Paul_1997_NWDOSTIP-11">[11]</a><a href="#cite_note-Paul_1997_OD-A3-14">[14]</a></li>

<li>Compression drivers not loaded through the preload API may be loaded through <a href="DEVICE_(CONFIG.SYS_directive)" class="mw-redirect" title="DEVICE (CONFIG.SYS directive)">DEVICE</a> statements.</li>

<li>Depending on configuration and if this hasn't happened already, drive letters may get swapped, so that the compressed drive becomes C:.</li>

<li>Processing continues from compressed drive.</li></ul>

<div class="mw-heading mw-heading3"><h3 id="Performance_impact">Performance impact</h3></div>

On systems with slower hard drives, disk compression could actually increase system performance. This was accomplished two ways:

<ul><li>Once compressed, there was less data to be stored.</li>

<li>Disk accesses would often be batched together for efficiency.</li></ul>

If the system had to wait frequently for hard drive access to complete (<a href="I/O_bound" title="I/O bound">I/O bound</a>) 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 <a href="CPU_bound" class="mw-redirect" title="CPU bound">CPU bound</a>, disk compression decreases overall performance.<a href="#cite_note-Paul_1997_NWDOSTIP-11">[11]</a>

<div class="mw-heading mw-heading3"><h3 id="Drawbacks">Drawbacks</h3></div>

Some common drawbacks to using disk compression:

<ul><li>Not all compression utilities would confirm the absence of errors in the <a href="File_Allocation_Table" title="File Allocation Table">FAT file system</a> before compressing a disk in place. Some errors, such as <a href="Crosslinked_file" class="mw-redirect" title="Crosslinked file">crosslinked files</a>, could result in additional data loss during the transfer process.<a href="#cite_note-NB_Crosslinked-files-22">[nb 1]</a></li>

<li>The compressed drive is only visible if the device driver is loaded and the compressed drive is mounted. A <a href="Boot_disk" title="Boot disk">boot disk</a>, for example, might not contain the driver. (This was not normally an issue for solutions that came bundled with the operation systems unless booting foreign operating systems, as system utilities like <a href="SYS_(DOS_command)" class="mw-redirect" title="SYS (DOS command)">SYS</a> were modified to copy the compression drivers to boot disks by default,<a href="#cite_note-Paul_1997_NWDOSTIP-11">[11]</a><a href="#cite_note-Caldera_1998_NEW703-21">[21]</a> and the corresponding operation systems would also fall back to look for them on drives other than the boot disk if they couldn't be found there.)</li>

<li>Uninstallation was problematic if the expanded size of data on the compressed disk was larger than the normal disk capacity.</li>

<li>Users did not always realize that the large file on the host drive contained the compressed drive. While it was usually "hidden" by default,<a href="#cite_note-NB_FAT-attributes-23">[nb 2]</a> users who did find the large file curious or suspicious were able to delete it. This would normally result in data loss.</li></ul>

<ul><li><a href="RAM_compression" class="mw-redirect" title="RAM compression">RAM compression</a></li>

<li><a href="Data_compression" title="Data compression">Data compression</a></li>

<li><a href="Disk_fragmentation" class="mw-redirect" title="Disk fragmentation">Disk fragmentation</a></li>

<li><a href="Executable_compression" title="Executable compression">Executable compression</a></li>

<li><a href="Self-extracting_archive" title="Self-extracting archive">Self-extracting archive</a></li>

<li><a href="Cramfs" title="Cramfs">cramfs</a></li>

<li><a href="E2compr" class="mw-redirect" title="E2compr">e2compr</a></li>

<li><a href="SquashFS" title="SquashFS">SquashFS</a></li>

<li><a href="MDFAT" class="mw-redirect" title="MDFAT">MDFAT</a> (DoubleSpace)</li>

<li><a href="BitFAT" class="mw-redirect" title="BitFAT">BitFAT</a> (DoubleSpace)</li>

<li><a href="MDBPB" class="mw-redirect" title="MDBPB">MDBPB</a> (DoubleSpace)</li></ul>

<div class="mw-heading mw-heading2"><h2 id="Notes">Notes</h2></div>

<li id="cite_note-NB_Crosslinked-files-22"><a href="#cite_ref-NB_Crosslinked-files_22-0">^</a> In <a href="Crosslinked_file" class="mw-redirect" title="Crosslinked file">crosslinked files</a>, two files are storing at least part of their data in the same location. At least part of one file (the "bad" file) is always lost in this instance. However, if the "bad" file is copied and then deleted, part of the "good" file is deleted as well. Microsoft <a href="SCANDISK" class="mw-redirect" title="SCANDISK">SCANDISK</a> was created, in part, to perform a better check of the file system prior to compression than the <a href="MS-DOS" title="MS-DOS">MS-DOS</a> <a href="CHKDSK" title="CHKDSK">CHKDSK</a> utility.

</li>

<li id="cite_note-NB_FAT-attributes-23"><a href="#cite_ref-NB_FAT-attributes_23-0">^</a> For example, DOS associated up to four <a href="FAT_attributes" class="mw-redirect" title="FAT attributes">attributes</a> with files: System, Hidden, Read-Only, and Archive. Files with the System or Hidden attributes are often not displayed by default. Files with the System or Read-Only attribute cannot be deleted with the <a href="Del_(command)" title="Del (command)">ERASE</a> or <a href="DEL_(DOS_command)" class="mw-redirect" title="DEL (DOS command)">DEL</a> DOS command. Most compression utilities would mark the drive file with at least one or more of the System, Hidden, and Read-Only attributes (many would use all three). However, files marked with such attributes can be viewed and deleted by other means. In addition, the user can also remove attributes.

</li>

</ol></div>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFSlovick1992" class="citation journal cs1">Slovick, Linda (1992-02-17). <a rel="nofollow" class="external text" href="https://books.google.com/books?id=_z0EAAAAMBAJ&pg=PA62">"The big squeeze"</a>. <a href="InfoWorld" title="InfoWorld">InfoWorld</a>. 14 (7): 57–63. Retrieved 2015-04-06.</cite>

</li>

<li id="cite_note-PCMAG_1992_Comp-2">^ <a href="#cite_ref-PCMAG_1992_Comp_2-0">a</a> <a href="#cite_ref-PCMAG_1992_Comp_2-1">b</a> <a href="#cite_ref-PCMAG_1992_Comp_2-2">c</a> <a href="#cite_ref-PCMAG_1992_Comp_2-3">d</a> <a href="#cite_ref-PCMAG_1992_Comp_2-4">e</a> <cite id="CITEREFPastrick1992" class="citation journal cs1">Pastrick, Greg (1992-01-28). <a rel="nofollow" class="external text" href="https://books.google.com/books?id=uEkrL23rU98C&pg=PT280">"Double - no - triple your hard disk space with on-the-fly data compression utilities"</a>. <a href="PC_Magazine" class="mw-redirect" title="PC Magazine">PC Magazine</a>. 11 (2): 261. Retrieved 2015-04-06.</cite>

</li>

<li id="cite_note-PCMAG_1990_Expanz-3"><a href="#cite_ref-PCMAG_1990_Expanz_3-0">^</a> <cite id="CITEREFMendelson1990" class="citation journal cs1"><a href="Edward_Mendelson" title="Edward Mendelson">Mendelson, Edward</a> (1990-11-13). <a rel="nofollow" class="external text" href="https://books.google.com/books?id=WIeyaksLI8gC&pg=PT44">"Expanz! board quickly compresses files on all disk types but forfeits some DOS compatibilities"</a>. <a href="PC_Magazine" class="mw-redirect" title="PC Magazine">PC Magazine</a>. 9 (19): 13. Retrieved 2015-04-06.</cite>

</li>

<li id="cite_note-IW_1987_Squish-4"><a href="#cite_ref-IW_1987_Squish_4-0">^</a> <cite id="CITEREFWalkenbach1987" class="citation journal cs1">Walkenbach, John (1987-02-02). <a rel="nofollow" class="external text" href="https://books.google.com/books?id=8jsEAAAAMBAJ&pg=PA55">"Squish - File compressor crams data into smaller space, but slowly"</a>. <a href="InfoWorld" title="InfoWorld">InfoWorld</a>. 9 (6): 55. Retrieved 2015-04-05.</cite>

</li>

<li id="cite_note-PCMAG_1987_Squish-5"><a href="#cite_ref-PCMAG_1987_Squish_5-0">^</a> <cite id="CITEREFDavis1987" class="citation journal cs1">Davis, Stephen Randy (1987-02-24). <a rel="nofollow" class="external text" href="https://books.google.com/books?id=phxlBt4dX3oC&pg=PA46">"Squish Puts a Squeeze on Files Before They Are Written to Disk"</a>. <a href="PC_Magazine" class="mw-redirect" title="PC Magazine">PC Magazine</a>. 6 (4): 46. Retrieved 2015-04-06.</cite>

</li>

<li id="cite_note-PP_1989_Squish-6">^ <a href="#cite_ref-PP_1989_Squish_6-0">a</a> <a href="#cite_ref-PP_1989_Squish_6-1">b</a> <cite id="CITEREFGoldstein1989" class="citation journal cs1">Goldstein, Hal (1989-07-08). <a rel="nofollow" class="external text" href="http://www.hpmuseum.net/pdf/ThePortablePaperV4N4_48pages_Jul-Aug%2089_OCR.pdf">"50% more file space for as little as $89.95"</a> (PDF). <a href="The_Portable_Paper" class="mw-redirect" title="The Portable Paper">The Portable Paper</a>. 4 (4). <a href="Personalized_Software" class="mw-redirect" title="Personalized Software">Personalized Software</a>, now <a href="Thaddeus_Computing" class="mw-redirect" title="Thaddeus Computing">Thaddeus Computing</a>: 46. Retrieved 2015-04-05.</cite>

</li>

<li id="cite_note-JAM-7"><a href="#cite_ref-JAM_7-0">^</a> <a rel="nofollow" class="external free" href="http://files.mpoli.fi/unpacked/software/dos/compress/jam125sw.zip/readme.doc">http://files.mpoli.fi/unpacked/software/dos/compress/jam125sw.zip/readme.doc</a>

</li>

<li id="cite_note-Slivka_1993-8">^ <a href="#cite_ref-Slivka_1993_8-0">a</a> <a href="#cite_ref-Slivka_1993_8-1">b</a> <cite id="CITEREFSlivkaStraubFreedman1993" class="citation journal cs1">Slivka, Benjamin W.; Straub, Eric; Freedman, Richard (July 1993). <a rel="nofollow" class="external text" href="https://archive.org/stream/BYTEVol1808199307PentiumPCs/BYTE%20Vol%2018-08%201993-07%20Pentium%20PCs#page/n203/mode/1up">"Hands on - Under the Hood: Inside MS-DOS 6 - The developers of MS-DOS describe the operating system's memory optimization and disk-compression technology"</a>. <a href="BYTE_Magazine" class="mw-redirect" title="BYTE Magazine">BYTE Magazine</a>. 18 (8): 197–202. Retrieved 2016-11-01.</cite> <a rel="nofollow" class="external autonumber" href="https://archive.org/stream/BYTEVol1808199307PentiumPCs/BYTE%20Vol%2018-08%201993-07%20Pentium%20PCs_djvu.txt">[1]</a>

</li>

<li id="cite_note-Schulman_1994_Undocumented-DOS-9">^ <a href="#cite_ref-Schulman_1994_Undocumented-DOS_9-0">a</a> <a href="#cite_ref-Schulman_1994_Undocumented-DOS_9-1">b</a> <a href="#cite_ref-Schulman_1994_Undocumented-DOS_9-2">c</a> <cite id="CITEREFSchulmanBrownMaxeyMichels1994" class="citation book cs1">Schulman, Andrew; <a href="Ralf_D._Brown" class="mw-redirect" title="Ralf D. Brown">Brown, Ralf D.</a>; Maxey, David; Michels, Raymond J.; Kyle, Jim (1994) [November 1993]. <a rel="nofollow" class="external text" href="https://archive.org/details/undocumenteddosp00andr_0">Undocumented DOS: A programmer's guide to reserved MS-DOS functions and data structures - expanded to include MS-DOS 6, Novell DOS and Windows 3.1</a> (2 ed.). <a href="Addison_Wesley" class="mw-redirect" title="Addison Wesley">Addison Wesley</a>. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>0-201-63287-X</bdi>.</cite> (xviii+856+vi pages, 3.5"-floppy) Errata: <a rel="nofollow" class="external autonumber" href="https://web.archive.org/web/20190417215556/http://www.cs.cmu.edu/afs/cs/user/ralf/pub/books/UndocumentedDOS/errata.ud2">[2]</a><a rel="nofollow" class="external autonumber" href="https://web.archive.org/web/20190417212906/https://www.pcjs.org/pubs/pc/programming/Undocumented_DOS/#errata-2nd-edition">[3]</a>

</li>

<li id="cite_note-Chappell_1994_DOS-Internals-10">^ <a href="#cite_ref-Chappell_1994_DOS-Internals_10-0">a</a> <a href="#cite_ref-Chappell_1994_DOS-Internals_10-1">b</a> <cite id="CITEREFChappell1994" class="citation book cs1">Chappell, Geoff (1994). DOS Internals. <a href="Addison_Wesley" class="mw-redirect" title="Addison Wesley">Addison Wesley</a>. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-0-201-60835-9</bdi>.</cite> <a rel="nofollow" class="external autonumber" href="https://gopher.tildeverse.org/gopher.viste.fr/9/programming/PC/DOS/DOS%20Internals/DOS_Internals.zip">[4]</a><a rel="nofollow" class="external autonumber" href="https://www.pcjs.org/pubs/pc/programming/DOS_Internals/">[5]</a> Errata: <a rel="nofollow" class="external autonumber" href="http://www.geoffchappell.com/notes/dos/internals/">[6]</a><a rel="nofollow" class="external autonumber" href="https://gopher.tildeverse.org/gopher.viste.fr/1/programming/PC/DOS/DOS%20Internals/crtdrvr">[7]</a><a rel="nofollow" class="external autonumber" href="https://gopher.tildeverse.org/gopher.viste.fr/1/programming/PC/DOS/DOS%20Internals/xmswatch">[8]</a>

</li>

<li id="cite_note-Paul_1997_NWDOSTIP-11">^ <a href="#cite_ref-Paul_1997_NWDOSTIP_11-0">a</a> <a href="#cite_ref-Paul_1997_NWDOSTIP_11-1">b</a> <a href="#cite_ref-Paul_1997_NWDOSTIP_11-2">c</a> <a href="#cite_ref-Paul_1997_NWDOSTIP_11-3">d</a> <a href="#cite_ref-Paul_1997_NWDOSTIP_11-4">e</a> <a href="#cite_ref-Paul_1997_NWDOSTIP_11-5">f</a> <a href="#cite_ref-Paul_1997_NWDOSTIP_11-6">g</a> <cite id="CITEREFPaul1997" class="citation book cs1 cs1-prop-foreign-lang-source">Paul, Matthias R. (1997-07-30). <a rel="nofollow" class="external text" href="http://www.antonis.de/dos/dos-tuts/mpdostip/html/nwdostip.htm">NWDOS-TIPs — Tips & Tricks rund um Novell DOS 7, mit Blick auf undokumentierte Details, Bugs und Workarounds</a>. MPDOSTIP (in German) (3 ed.). <a rel="nofollow" class="external text" href="https://web.archive.org/web/20170910194752/http://www.antonis.de/dos/dos-tuts/mpdostip/html/nwdostip.htm">Archived</a> from the original on 2017-09-10. Retrieved 2015-04-05.</cite> NWDOSTIP.TXT is a comprehensive work on <a href="Novell_DOS_7" class="mw-redirect" title="Novell DOS 7">Novell DOS 7</a> and <a href="OpenDOS_7.01" class="mw-redirect" title="OpenDOS 7.01">OpenDOS 7.01</a>, including the description of many undocumented features and internals. It is part of the author's yet larger MPDOSTIP.ZIP collection maintained up to 2001 and distributed on many sites at the time. The provided link points to a HTML-converted older version of the NWDOSTIP.TXT file.

</li>

<li id="cite_note-CW_1994_PTS-12"><a href="#cite_ref-CW_1994_PTS_12-0">^</a> <cite class="citation journal cs1"><a rel="nofollow" class="external text" href="http://www.computerwoche.de/a/ms-dos-clone-made-in-russia,1121205">"MS-DOS-Clone made in Russia"</a>. <a href="Computerwoche" title="Computerwoche">Computerwoche</a>. 1994-05-06. Retrieved 2015-04-12.</cite>

</li>

<li id="cite_note-Caldera_1998_USER-13"><a href="#cite_ref-Caldera_1998_USER_13-0">^</a> <cite class="citation book cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20161105165004/http://www.drdos.net/documentation/usergeng/uglontoc.htm">Caldera DR-DOS 7.02 User Guide</a>. <a href="Caldera%2C_Inc." class="mw-redirect" title="Caldera, Inc.">Caldera, Inc.</a> 1998 [1993, 1997]. Archived from <a rel="nofollow" class="external text" href="http://www.drdos.net/documentation/usergeng/uglontoc.htm">the original</a> on 2016-11-05. Retrieved 2013-08-10.</cite>

</li>

<li id="cite_note-Paul_1997_OD-A3-14">^ <a href="#cite_ref-Paul_1997_OD-A3_14-0">a</a> <a href="#cite_ref-Paul_1997_OD-A3_14-1">b</a> <a href="#cite_ref-Paul_1997_OD-A3_14-2">c</a> <a href="#cite_ref-Paul_1997_OD-A3_14-3">d</a> <cite id="CITEREFPaul1997" class="citation web cs1">Paul, Matthias R. (1997-10-02). <a rel="nofollow" class="external text" href="https://web.archive.org/web/20031004074600/http://www-student.informatik.uni-bonn.de/~frinke/ibmbioa3.zip">"Caldera OpenDOS 7.01/7.02 Update Alpha 3 IBMBIO.COM README.TXT"</a>. Archived from <a rel="nofollow" class="external text" href="http://www.uni-bonn.de/~uzs180/download/ibmbioa3.zip">the original</a> on 2003-10-04. Retrieved 2009-03-29.</cite> <a rel="nofollow" class="external autonumber" href="https://web.archive.org/web/20181225154705/http://mirror.macintosharchive.org/max1zzz.co.uk/+Windows%20&%20DOS/DOS/System/Novell/Support/Bins/Op702src.zip">[9]</a>

</li>

<li id="cite_note-Hall_1986-15"><a href="#cite_ref-Hall_1986_15-0">^</a> <cite class="citation journal cs1"><a rel="nofollow" class="external text" href="https://books.google.com/books?id=nuXmVNll5JEC&pg=PA155">"Squeeze 10 times as many spreadsheets on a disk - SQZ! - The data squeezer for Lotus"</a>. <a href="PC_Magazine" class="mw-redirect" title="PC Magazine">PC Magazine</a>. Turner Hall Publishing: 155. 1986-10-14. Retrieved 2016-11-18.</cite>

</li>

<li id="cite_note-Linux_DOSFS-16"><a href="#cite_ref-Linux_DOSFS_16-0">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="ftp://sunsite.unc.edu/pub/Linux/system/Filesystems/dosfs/thsfs.tgz">"FTP link"</a>. sunsite.unc.edu (<a href="FTP" class="mw-redirect" title="FTP">FTP</a>).</cite> (To view documents see Help:FTP)

</li>

<li id="cite_note-Debian_1996-17"><a href="#cite_ref-Debian_1996_17-0">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://lists.debian.org/debian-user/1996/09/msg01042.html">"Re: Disk Compression - Stacker - Drivespace, etc"</a>.</cite>

</li>

<li id="cite_note-THSFS-18"><a href="#cite_ref-THSFS_18-0">^</a> <a rel="nofollow" class="external free" href="https://ftp.nluug.nl/ftp/ibiblio/system/filesystems/thsfs.lsm">http://ftp.nluug.nl/ftp/ibiblio/system/filesystems/thsfs.lsm</a>

</li>

<li id="cite_note-DMSDOS_2005-19"><a href="#cite_ref-DMSDOS_2005_19-0">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://cmp.felk.cvut.cz/~pisa/dmsdos/">"DMSDOS - Linux kernel driver"</a>. 2005-03-18. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20161111124320/http://cmp.felk.cvut.cz/~pisa/dmsdos/">Archived</a> from the original on 2016-11-11. Retrieved 2016-05-25.</cite>

</li>

<li id="cite_note-CMSDOS_1998-20"><a href="#cite_ref-CMSDOS_1998_20-0">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://cmp.felk.cvut.cz/~pisa/dmsdos/dmsdos.html">"DMSDOS CVF module"</a> (dmsdoc.doc). 0.9.2.0. 1998-11-19. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20161102123812/http://cmp.felk.cvut.cz/~pisa/dmsdos/dmsdos.html">Archived</a> from the original on 2016-11-02. Retrieved 2016-11-01.</cite>

</li>

<li id="cite_note-Caldera_1998_NEW703-21">^ <a href="#cite_ref-Caldera_1998_NEW703_21-0">a</a> <a href="#cite_ref-Caldera_1998_NEW703_21-1">b</a> <cite class="citation book cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20190408142232/http://www.lookas.net/ftp/incoming/darbui/Justas/DRDOS/WHATSNEW.TXT">DR-DOS 7.03 WHATSNEW.TXT — Changes from DR-DOS 7.02 to DR-DOS 7.03</a>. <a href="Caldera%2C_Inc." class="mw-redirect" title="Caldera, Inc.">Caldera, Inc.</a> 1998-12-24. Archived from <a rel="nofollow" class="external text" href="http://www.lookas.net/ftp/incoming/darbui/Justas/DRDOS/WHATSNEW.TXT">the original</a> on 2019-04-08. Retrieved 2019-04-08.</cite>

</li>

</ol></div></div>

<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

<ul><li><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.microsoft.com/technet/archive/msdos/01_intro.mspx?mfr=true">"MS-DOS 6 Technical Reference"</a>. <a href="Microsoft" title="Microsoft">Microsoft</a>.</cite></li>

<li><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.techhelpmanual.com/805-doublespace_compressed_volume_file_layout.html">"DoubleSpace CVF file layout"</a>.</cite> ff.</li>

<li><cite class="citation journal cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20080619060529/http://www.byte.com/art/9402/sec6/art1.htm">"How Safe is Disk Compression?"</a>. <a href="BYTE_Magazine" class="mw-redirect" title="BYTE Magazine">BYTE Magazine</a>. February 1994. Archived from <a rel="nofollow" class="external text" href="http://www.byte.com/art/9402/sec6/art1.htm">the original</a> on 2008-06-19.</cite></li>

<li><cite class="citation journal cs1"><a rel="nofollow" class="external text" href="http://www.smartcomputing.com/editorial/article.asp?article=articles/archive/l0504/9834/9834.asp&guid=">"How To... Compress Hard Drives"</a>. <a href="Smart_Computing" class="mw-redirect" title="Smart Computing">Smart Computing</a>. April 1999.</cite></li></ul></div><div><div class="zim-footer">

This article is issued from <a class="external text" title="Last edited on 2025-05-27" href="https://en.wikipedia.org/wiki/?title=Disk_compression&oldid=1292511292">Wikipedia</a>. The text is available under <a class="external text" href="https://creativecommons.org/licenses/by-sa/4.0/deed.en">Creative Commons Attribution-Share Alike 4.0</a> unless otherwise noted. Additional terms may apply for the media files.

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