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however, the term '''disk image''' has been generalized to cover any file, whether originated from an actual physical storage device or not, capable of producing the relative structure and data file contents of the intended destination medium. Disk imaging utilities, when not replicating an entire original source medium, such as omitting unused file space on the source media, are actually creating an [[archive file]] and not a true '''disk image'''.
== Uses of disk images ==
Originally disk images were used for [[backup]] and [[disk cloning]], where replication or storage of an exact structure was necessary or efficient. With the advent of [[optical drive]]s such as [[CD-ROM]] and [[DVD]], a more commonly encountered type of disk image is a CD/DVD image, often in the form of an [[ISO image|.ISO]] file (or sometimes a .BIN/.CUE file), referring to the [[ISO 9660]] file system commonly used on such disks. These provide an exact digital replica of a CD/DVD, whereby all of the data is stored in one file to completely preserve the data structure and integrity of the CD/DVD. The .ISO format is the most common format for software disk images, but does not support multi-track data or [[Red Book (audio CD standard)|audio CDs]]. In general, disk imaging is essential for retaining copy-protection data and multi-track data/audio on CD/DVD.▼
=== Historical ==
Originally disk images were used for [[backup]] and [[disk cloning]] of floppy media, where replication or storage of an exact structure was necessary or efficient.
With the advent of the Internet, it became convenient to use [[archive files]] to distribute "Disk images" of [[optical drive]]s such as [[CD-ROM]] and later [[DVD]].
[[proprietary software|Proprietary]] formats include [[Nero Burning ROM|.NRG]] ([[Nero Burning ROM]]), .MDS/.MDF ([[Alcohol 120%]]), [[Direct Access Archive|.DAA]] ([[PowerISO]]), .PQI ([[DriveImage]]), and .CCD/.IMG/.SUB/.DVD ([[CloneCD]]). Other disk image formats include: [[.img]] (commonly used for [[USB stick]] and hard disks) and [[.dmg|.DMG]] (used by [[Apple]]). ▼
=== Data storage ===▼
An ordinary backup program only backs up the files it can access; [[booting|boot]] information and files locked by the operating system (such as those in use at the time of the backup) may not be saved. A full disk image contains all these, faithfully replicating all data. For this reason, it is commonly used for backing up disks with operating systems, or [[bootable]] [[CD]]s and [[DVD]]s. ▼
=== Software distribution ===▼
A common use of disk images is for distribution of large programs or software systems over the [[internet]]. Installation disks and [[LiveCD]]s for operating systems such as [[Linux]] or [[Berkeley Software Distribution|BSD]] systems are typically available as .ISO images. Installation [[floppy disk]]s or CD-ROMs are stored as disk image files, transferred over the [[internet]], and burned or copied onto actual disks by end users.▼
Disk images, most often .ISO or .BIN/.CUE files, are also popular for [[peer-to-peer]] [[file sharing]] of software.▼
===Rapid deployment of clone systems ===▼
Large enterprises often need to buy or replace new computer systems in large numbers. Installing operating system and programs in to each of them one by one requires a lot of time and effort and has a significant possibility of human error. Therefore, system administrators use disk imaging to quickly clone the fully-prepared software environment of a reference system. This method saves time and effort and allows administrators to focus on unique distinctions that each system must bear.▼
===Universal archive formats===▼
As images are unaffected by the [[file system]] or contents of the original data source, and most operating systems allow disk images to be mounted as if they were physical [[disc storage|disc]]s, image formats such as .ISO may be used as a universal archive format. ▼
=== Virtual disks ===▼
Another common use is to provide virtual disk drive space to be used by [[emulator]]s (e.g., [[QEMU]]) and [[virtual machine]]s. This can prevent the CD from getting damaged. It can also reduce bulk when one wishes to carry the contents of the CD along with oneself: one can store disk images to a relatively lightweight and [[bootable]] storage device which has a higher storage capacity than that of a CD (e.g., a USB [[keydrive]]).▼
Emulation also provides a performance boost, as hard disks transfer data at much higher rates than optical disk drives. [[Console emulator]]s, such as [[ePSXe]], and many other emulators that read from CD/DVD, are able to read ISO/BIN (and other similar formats) instead of running directly from the CD drive, obtaining better performance.▼
Another reason for some people is that CD/DVD drives are noisy, a virtual disk on a hard disk is already a lot less noisy, and a USB stick is silent.▼
==Imaging process==
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Once created, an image file can be treated as a form of [[compressed archive]], containing all the information needed to replicate the original media: its contents and its physical layout on the relevant device. As such, disk images can be restored onto a suitable physical device (a hard drive or a CD/DVD) to replicate the original. Alternatively, if the structural information is unnecessary, programs such as [[ISOBuster]] and [[WinRAR]] which handle file compression generally, can unpack the individual files from a disk image.
==
Disk image files are created using an [[archive formats]] determined by the imaging software. Below are examples of [[archive formats]] specific to disk image files.
▲=== Data storage ===
▲An ordinary backup program only backs up the files it can access; [[booting|boot]] information and files locked by the operating system (such as those in use at the time of the backup) may not be saved. A full disk image contains all these, faithfully replicating all data. For this reason, it is commonly used for backing up disks with operating systems, or [[bootable]] [[CD]]s and [[DVD]]s.
▲=== Software distribution ===
▲A common use of disk images is for distribution of large programs or software systems over the [[internet]]. Installation disks and [[LiveCD]]s for operating systems such as [[Linux]] or [[Berkeley Software Distribution|BSD]] systems are typically available as .ISO images. Installation [[floppy disk]]s or CD-ROMs are stored as disk image files, transferred over the [[internet]], and burned or copied onto actual disks by end users.
▲Disk images, most often .ISO or .BIN/.CUE files, are also popular for [[peer-to-peer]] [[file sharing]] of software.
▲===Rapid deployment of clone systems ===
▲Large enterprises often need to buy or replace new computer systems in large numbers. Installing operating system and programs in to each of them one by one requires a lot of time and effort and has a significant possibility of human error. Therefore, system administrators use disk imaging to quickly clone the fully-prepared software environment of a reference system. This method saves time and effort and allows administrators to focus on unique distinctions that each system must bear.
▲===Universal archive formats===
▲As images are unaffected by the [[file system]] or contents of the original data source, and most operating systems allow disk images to be mounted as if they were physical [[disc storage|disc]]s, image formats such as .ISO may be used as a universal archive format.
▲=== Virtual disks ===
▲Another common use is to provide virtual disk drive space to be used by [[emulator]]s (e.g., [[QEMU]]) and [[virtual machine]]s. This can prevent the CD from getting damaged. It can also reduce bulk when one wishes to carry the contents of the CD along with oneself: one can store disk images to a relatively lightweight and [[bootable]] storage device which has a higher storage capacity than that of a CD (e.g., a USB [[keydrive]]).
▲Emulation also provides a performance boost, as hard disks transfer data at much higher rates than optical disk drives. [[Console emulator]]s, such as [[ePSXe]], and many other emulators that read from CD/DVD, are able to read ISO/BIN (and other similar formats) instead of running directly from the CD drive, obtaining better performance.
▲Another reason for some people is that CD/DVD drives are noisy, a virtual disk on a hard disk is already a lot less noisy, and a USB stick is silent.
=== CD Specific ===
Images of [[CD-ROM]]s most often carry the filename extension .[[ISO image|ISO]], referring to the [[ISO 9660]] file system commonly used on such disks. The .ISO format is the most common format for [[Linux distribution]]s and other disk images distributed over the internet, but it is not a multi-track image format and thus cannot be used for [[Red Book (audio CD standard)|Audio CDs]]. Other common CD image formats are .NRG ([[Nero Burning ROM]]'s proprietary format), .BIN/.CUE, .MDF/.MDS ([[Alcohol 120%]]'s proprietary format[can be read by Magic Iso, and converted to .ISO format]), [[Direct Access Archive|.DAA]] ([[PowerISO]]'s proprietary format), and .CCD/.IMG/.SUB ([[CloneCD]]'s proprietary format). All of these support multi-track CDs.
=== (relocated chunk) ===
▲[[proprietary software|Proprietary]] formats include [[Nero Burning ROM|.NRG]] ([[Nero Burning ROM]]), .MDS/.MDF ([[Alcohol 120%]]), [[Direct Access Archive|.DAA]] ([[PowerISO]]), .PQI ([[DriveImage]]), and .CCD/.IMG/.SUB/.DVD ([[CloneCD]]). Other disk image formats include: [[.img]] (commonly used for [[USB stick]] and hard disks) and [[.dmg|.DMG]] (used by [[Apple]]).
===
==== DCF ====
.DCF is a file extension used by [http://www.fdos.org/ripcord/rawrite/trial/DCF53.ZIP/ Disk Copy Fast (DCF)] and '''Disk Copy Plus''' written by Chang Ping Lee. It contains a raw image of a floppy disk. This format is not compatible with Rawrite (.IMG) format.
====.IMA ====
.IMA is a filename extention used by [[Nero (computer software)|Nero]] and [[WinImage]] for disk image of floppy disks. A .IMA file contains the raw image of the floppy disk and metadata including file comments.▼
=== .ISO ===
{{main|ISO image}}
.ISO files are often used to distribute large programs or software systems over the internet. [[Operating system]]s such as [[Linux]] or [[Berkeley Software Distribution|BSD]] systems installation disks and [[LiveCD]]s are typically available as .ISO images. The format is also popular for [[peer-to-peer]] [[file sharing]] of pirated commercial software.
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On [[Mac OS X]], ISO images often have the .cdr extension.
====
▲
▲.IMA is a filename extention used by [[Nero (computer software)|Nero]] and [[WinImage]] for disk image of floppy disks. A .IMA file contains the raw image of the floppy disk and metadata including file comments.
===
.IMG is the filename extension usually used for the disk image of floppy disks, optical media and hard disks. A .IMG file contains a raw [[dump]] of the content of a disk. First popularized by the [[DOS]]-based programs HD-Copy (for hard drives) and DiskDupe (for floppy disks) and now utilized by [[GNU Project|GNU]] [[RaWrite2]], [[RawWrite for Windows]], and [[Winrawrite|WinRawrite]], the .IMG format is a handy way to archive a floppy disk completely, including [[bootable]] ones.
The .img extension was also used by the Macintosh application ShrinkWrap by Aladdin Systems (now Allume Systems), for Mac OS 9 and older, however this Macintosh format is not compatible with the .IMG format used on Windows and it stores metadata in the resource fork. .IMG files are frequently used on Mac OS X to store images of optical disks.
Some newer software supports a [[ZIP (file format)|zipped]] version of the format, such as [[WinImage]]'s '''.IMZ'''.
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