View source for Disk image

{{Short description|Computer file consisting of the contents of a storage device}}

A '''disk image''' is a snapshot of a [[computer storage|storage]] device's content {{endash}} typically stored in a [[computer file|file]] on another storage device.<ref name=":0">{{Cite book |date=2022-11-02 |chapter=Disk Imaging as a Backup Tool for Digital Objects |chapter-url=https://www.taylorfrancis.com/chapters/edit/10.4324/9781003034865-17/disk-imaging-backup-tool-digital-objects-eddy-colloton-jonathan-farbowitz-caroline-gil-rodr%C3%ADguez |title=Conservation of Time-Based Media Art |language=en |pages=204–222 |doi=10.4324/9781003034865-17 |last1=Colloton |first1=Eddy |last2=Farbowitz |first2=Jonathan |last3=Rodríguez |first3=Caroline Gil |isbn=978-1-003-03486-5 |archive-date=2024-04-27 |access-date=2023-01-12 |archive-url=https://web.archive.org/web/20240427100331/https://www.taylorfrancis.com/chapters/edit/10.4324/9781003034865-17/disk-imaging-backup-tool-digital-objects-eddy-colloton-jonathan-farbowitz-caroline-gil-rodr%C3%ADguez |url-status=live }}</ref><ref name=":3" /> 

Traditionally, a disk image was relatively large because it was a bit-by-bit copy of every storage location of a device (i.e. every sector of a [[hard disk drive]]), but it is now common to only store allocated data to reduce storage space.<ref name=":1">{{cite conference |url=https://www.sigops.org/s/conferences/sosp/2011/posters/summaries/sosp11-final15.pdf |last1=Pullakandam |first1=R. |last2=Lin |first2=X. |last3=Hibler |first3=M. |last4=Eide |first4=E. |last5=Ricci |first5=R. |title=High-performance Disk Imaging With Deduplicated Storage |conference=23rd ACM Symposium on Operating Systems Principles |date=October 23–26, 2011 |location=Cascais, Portugal |access-date=2023-04-02 |archive-date=2023-04-02 |archive-url=https://web.archive.org/web/20230402053456/https://www.sigops.org/s/conferences/sosp/2011/posters/summaries/sosp11-final15.pdf |url-status=live }}</ref><ref>{{Citation |last=Kävrestad |first=Joakim |title=Vocabulary |date=2017 |work=Guide to Digital Forensics: A Concise and Practical Introduction |series=SpringerBriefs in Computer Science |pages=125–126 |editor-last=Kävrestad |editor-first=Joakim |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-319-67450-6_12 |isbn=978-3-319-67450-6}}</ref> [[Data compression|Compression]] and [[data deduplication|deduplication]] are commonly used to further reduce the size of image files.<ref name=":1" /><ref>{{Cite conference |last1=Lee |first1=Sang Su |last2=Kyong |first2=Un Sung |last3=Hong |first3=Do Won |date=2008 |title=A high speed disk imaging system |conference=2008 IEEE International Symposium on Consumer Electronics |pages=1–3 |doi=10.1109/ISCE.2008.4559553|s2cid=5932241 }}</ref> 

Disk imaging is performed for a variety of purposes including [[digital forensic]]s,<ref name=":2">{{Cite conference |last=Garfinkel |first=Simson L. |date=2009 |title=Automating Disk Forensic Processing with SleuthKit, XML and Python |conference=2009 Fourth International IEEE Workshop on Systematic Approaches to Digital Forensic Engineering |pages=73–84 |doi=10.1109/SADFE.2009.12 |isbn=978-0-7695-3792-4 |s2cid=1624033 |hdl=10945/44249 |hdl-access=free }}</ref><ref name=":3">{{Cite conference |last1=Woods |first1=Kam |last2=Lee |first2=Christopher A. |last3=Garfinkel |first3=Simson |date=2011-06-13 |title=Extending digital repository architectures to support disk image preservation and access |conference=Proceedings of the 11th Annual International ACM/IEEE Joint Conference on Digital Libraries |location=New York, NY, USA |publisher=Association for Computing Machinery |pages=57–66 |doi=10.1145/1998076.1998088 |isbn=978-1-4503-0744-4|s2cid=2628912 |hdl=10945/44252 |hdl-access=free}}</ref> [[cloud computing]],<ref>{{Cite conference |last1=Kazim |first1=Muhammad |last2=Masood |first2=Rahat |last3=Shibli |first3=Muhammad Awais |date=2013-11-26 |title=Securing the virtual machine images in cloud computing |conference=Proceedings of the 6th International Conference on Security of Information and Networks |location=New York, NY, USA |publisher=Association for Computing Machinery |pages=425–428 |doi=10.1145/2523514.2523576 |isbn=978-1-4503-2498-4 |s2cid=2474546}}</ref> [[System administrator|system administration]],<ref name=":7">{{cite conference |last1=Blackham |first1=N. |last2=Higby |first2=C. |last3=Bailey |first3=M. |date=June 2004 |url=https://peer.asee.org/re-imaging-computers-for-multipurpose-labs |title=Re-Imaging Computers For Multipurpose Labs |conference=2004 American Society for Engineering Education Annual Conference |location=Salt Lake City, Utah |doi=10.18260/1-2--14125|doi-access=free }}</ref> [[backup]],<ref name=":0" /> and emulation for [[digital preservation]] strategy.<ref name=":4">{{Cite journal |last1=Day |first1=Michael |last2=Pennock |first2=Maureen |last3=May |first3=Peter |last4=Davies |first4=Kevin |last5=Whibley |first5=Simon |last6=Kimura |first6=Akiko |last7=Halvarsson |first7=Edith |date=2016 |title=The preservation of disk-based content at the British Library: Lessons from the Flashback project |url=http://journals.sagepub.com/doi/10.1177/0955749016669775 |journal=Alexandria: The Journal of National and International Library and Information Issues |language=en |volume=26 |issue=3 |pages=216–234 |doi=10.1177/0955749016669775 |s2cid=63617004 |issn=0955-7490|url-access=subscription }}</ref> 
Despite the benefits, storage costs can be high,<ref name=":1" /> management can be difficult<ref name=":2" /> and imaging can be time consuming.<ref name=":5">{{Cite book |last1=Stewart |first1=Dawid |url=http://urn.kb.se/resolve?urn=urn:nbn:se:hh:diva-47045 |title=Need for speed: A study of the speed of forensic disk imaging tools |last2=Arvidsson |first2=Alex |date=2022}}</ref><ref name=":4" />

Disk images can be made in a variety of formats depending on the purpose. [[Virtual disk]] images (such as VHD and VMDK) are intended to be used for cloud computing,<ref>{{Cite journal |last1=Arunkumar |first1=G. |last2=Venkataraman. |first2=Neelanarayanan |date=2015-01-01 |title=A Novel Approach to Address Interoperability Concern in Cloud Computing |journal=Procedia Computer Science |series=Big Data, Cloud and Computing Challenges |language=en |volume=50 |pages=554–559 |doi=10.1016/j.procs.2015.04.083 |issn=1877-0509|doi-access=free }}</ref><ref>{{Cite journal |last1=Barrowclough |first1=John Patrick |last2=Asif |first2=Rameez |date=2018-06-11 |title=Securing Cloud Hypervisors: A Survey of the Threats, Vulnerabilities, and Countermeasures |journal=Security and Communication Networks |language=en |volume=2018 |article-number=e1681908 |doi=10.1155/2018/1681908 |issn=1939-0114|doi-access=free }}</ref> [[ISO image]]s are intended to emulate optical media, such as a [[CD-ROM]].<ref name=":10">{{cite journal |last1=Colloton |first1=E. |last2=Farbowitz |first2=J. |last3=Fortunato |first3=F. |last4=Gil |first4=C. |date=2019 |url=https://resources.culturalheritage.org/emg-review/volume-6-2019-2020/colloton/ |title=Towards Best Practices In Disk Imaging: A Cross-Institutional Approach |journal=Electronic Media Review |volume=6}}</ref> [[IMG (file format)|Raw disk images]] are used for forensic purposes.<ref name=":3" /> Proprietary formats are typically used by disk imaging software.

==Background==

Disk images were originally (in the late 1960s) used for [[backup]] and [[disk cloning]] of mainframe disk media. Early ones were as small as 5 [[megabyte]]s and as large as 330 megabytes, and the copy medium was [[magnetic tape]], which ran as large as 200 megabytes per reel.<ref>{{cite web |title=IBM Mainframe Operating Systems |url=http://www.demorton.com/Tech/$OSTL.pdf |access-date=2014-06-17 |url-status=usurped |archive-url=https://web.archive.org/web/20140701185435/http://www.demorton.com/Tech/$OSTL.pdf |archive-date=2014-07-01}}</ref> Disk images became much more popular when floppy disk media became popular, where replication or storage of an exact structure was necessary and efficient, especially in the case of [[copy protected]] floppy disks.

Disk image creation is called disk imaging and is often time consuming, even with a fast computer, because the entire disk must be copied.<ref name=":5" /> Typically, disk imaging requires a third party disk imaging program or backup software. The software required varies according to the type of disk image that needs to be created. For example, RawWrite and WinImage create [[IMG (file format)|floppy disk image]] files for [[MS-DOS]] and [[Microsoft Windows]].<ref>{{Cite book |last=McCune |first=Mike |url=https://books.google.com/books?id=0SM3PEH9gagC&q=rawwrite&pg=PR1 |title=Integrating Linux and Windows |date=2000 |publisher=Prentice Hall Professional |isbn=978-0-13-030670-8 |language=en}}</ref><ref>{{Cite conference |last1=Li |first1=Hongwei |last2=Yin |first2=Changhong |last3=Xu |first3=Yaping |last4=Guo |first4=Qingjun |date=2010 |title=Construction of the Practical Teaching System on Operating Systems Course |conference=2010 Second International Workshop on Education Technology and Computer Science |volume=1 |pages=405–408 |doi=10.1109/ETCS.2010.184|isbn=978-1-4244-6388-6 |s2cid=15706012 }}</ref> In [[Unix]] or [[Unix-like|similar systems]] the [[dd (Unix)|dd]] program can be used to create raw disk images.<ref name=":3" /> [[Apple Disk Copy]] can be used on [[Classic Mac OS]] and [[macOS]] systems to create and write disk image files.

Authoring software for CDs/DVDs such as [[Nero Burning ROM]] can generate and load disk images for optical media. A ''virtual disk writer'' or ''virtual burner'' is a computer program that emulates an actual disc authoring device such as a CD writer or DVD writer. Instead of writing data to an actual disc, it creates a virtual disk image.<ref>{{cite web |title=Phantom Burner Overview |url=http://www.phantombility.com/en/prod/phantomburner/ |url-status=usurped |archive-url=https://web.archive.org/web/20110819141722/http://phantombility.com/en/prod/phantomburner |archive-date=19 August 2011 |access-date=19 July 2011 |publisher=Phantombility, Inc}}</ref><ref>{{cite web |title=Virtual CD - The original for your PC |url=http://www.virtualcd.de/vcd/apps/overview/original.cfm?lg=0 |url-status=live |archive-url=https://web.archive.org/web/20110924022331/http://www.virtualcd.de/vcd/apps/overview/original.cfm?lg=0 |archive-date=24 September 2011 |access-date=19 July 2011 |work=Virtual CD website |publisher=H+H Software GmbH}}</ref> A virtual burner, by definition, appears as a disc drive in the system with writing capabilities (as opposed to conventional disc authoring programs that can create virtual disk images), thus allowing software that can burn discs to create virtual discs.<ref>{{cite web |title=Virtual CD/DVD-Writer Device |url=https://sourceforge.net/projects/virtualmedia/ |url-status=live |archive-url=https://web.archive.org/web/20110217075400/http://sourceforge.net/projects/virtualmedia/ |archive-date=17 February 2011 |access-date=19 July 2011 |work=[[SourceForge]] |publisher=Geeknet, Inc.}}</ref>

==Uses==
===Digital forensics===
Forensic imaging is the process of creating a bit-by-bit copy of the data on the drive, including files, metadata, volume information, filesystems and their structure.<ref name=":3" /> Often, these images are also [[Cryptographic hash|hashed]] to verify their integrity and that they have not been altered since being created. Unlike disk imaging for other purposes, digital forensic applications take a bit-by-bit copy to ensure forensic soundness. The purposes of imaging the disk is to not only discover evidence preserved in digital information but also to examine the drive to gather clues of how the crime was committed.

===Virtualization===
Creating a virtual disk image of optical media or a [[hard disk drive]] is typically done to make the content available to [[Virtualization|one or more virtual machines]]. [[Virtual machine]]s emulate a CD/DVD drive by reading an [[ISO image]]. This can also be faster than reading from the physical optical medium.<ref>{{cite web |url=http://www.pcguide.com/ref/cd/perfAccess-c.html |title=Access Time |at=CD-ROM Performance and Reliability |website=The PC Guide |archive-url=https://web.archive.org/web/20190110014217/http://www.pcguide.com/ref/cd/perfAccess-c.html |archive-date=10 January 2019 |date=17 April 2001 |first=Charles M. |last=Kozierok}}</ref> Further, there are less issues with wear and tear. A [[hard disk drive]] or [[solid-state drive]] in a [[virtual machine]] is implemented as a disk image (i.e. either the [[VHD (file format)|VHD]] format used by Microsoft's [[Hyper-V]], the [[VDI (file format)|VDI]] format used by [[Oracle Corporation]]'s [[VirtualBox]], the [[VMDK]] format used for [[VMware]] virtual machines, or the [[QCOW]] format used by [[QEMU]]). Virtual hard disk images tend to be stored as either a collection of files (where each one is typically 2GB in size), or as a single file. Virtual machines treat the image set as a physical drive.

=== Rapid deployment of systems ===
Educational institutions and businesses can often need to buy or replace computer systems in large numbers. Disk imaging is commonly used to rapidly deploy the same configuration across workstations.<ref name=":7" /> Disk imaging software is used to create an image of a completely-configured system (such an image is sometimes called a golden image).<ref>{{Cite journal |last=Bowling |first=Jeramiah |date=2011-01-01 |title=Clonezilla: build, clone, repeat |url=https://dl.acm.org/doi/10.5555/1924401.1924407 |journal=Linux Journal |volume=2011 |issue=201 |pages=6:6 |issn=1075-3583 |archive-date=2023-01-12 |access-date=2023-01-12 |archive-url=https://web.archive.org/web/20230112102630/https://dl.acm.org/doi/10.5555/1924401.1924407 |url-status=live }}</ref><ref>{{cite web |title=Create a golden image in Azure |url=https://learn.microsoft.com/en-us/azure/virtual-desktop/set-up-golden-image |website=learn.microsoft.com}}</ref> This image is then written to a computer's hard disk (which is sometimes described as restoring an image).<ref name=":9">{{Cite journal |last1=Shiau |first1=Steven J. H. |last2=Huang |first2=Yu-Chiang |last3=Tsai |first3=Yu-Chin |last4=Sun |first4=Chen-Kai |last5=Yen |first5=Ching-Hsuan |last6=Huang |first6=Chi-Yo |date=2021 |title=A BitTorrent Mechanism-Based Solution for Massive System Deployment |journal=IEEE Access |volume=9 |pages=21043–21058 |doi=10.1109/ACCESS.2021.3052525 |bibcode=2021IEEEA...921043S |s2cid=231851821 |issn=2169-3536|doi-access=free }}</ref> 

==== Network-based image deployment ====
Image restoration can be done using network-based image deployment. This method uses a [[Preboot Execution Environment|PXE]] server to boot an operating system over a computer network that contains the necessary components to image or restore storage media in a computer.<ref name=":8">{{Cite journal |last1=Shiau |first1=Steven J. H. |last2=Sun |first2=Chen-Kai |last3=Tsai |first3=Yu-Chin |last4=Juang |first4=Jer-Nan |last5=Huang |first5=Chi-Yo |date=2018 |title=The Design and Implementation of a Novel Open Source Massive Deployment System |journal=Applied Sciences |language=en |volume=8 |issue=6 |page=965 |doi=10.3390/app8060965 |issn=2076-3417 |doi-access=free}}</ref> This is usually used in conjunction with a [[Dynamic Host Configuration Protocol|DHCP]] server to automate the configuration of network parameters including IP addresses. [[Multicast]]ing, [[Broadcasting (networking)|broadcasting]] or [[unicast]]ing tend to be used to restore an image to many computers simultaneously.<ref name=":8" /><ref name=":9" /> These approaches do not work well if one or more computers experience packet loss.<ref name=":9" /> As a result, some imaging solutions use the [[BitTorrent]] protocol to overcome this problem.

Network-based image deployment reduces the need to maintain and update individual systems manually. Imaging is also easier than automated setup methods because an administrator does not need to have knowledge of the prior configuration to copy it.<ref name=":9" /> 

===Backup strategy===
{{See also-text|[[System image]]|[[Backup and Restore]] (for Windows Vista and later)}}
A disk image contains all files and data (i.e., [[file attribute]]s and the [[file fragmentation]] state). For this reason, it is also used for backing up [[optical media]] ([[CD]]s and [[DVD]]s, etc.), and allows the exact and efficient recovery after [[Sandbox (software development)|experimenting]] with modifications to a system or [[virtual machine]]. Typically, disk imaging can be used to quickly restore an entire system to an operational state after a disaster.<ref>{{Cite web |title=Fast, Scalable Disk Imaging with Frisbee |url=https://www.cs.utah.edu/flux/papers/frisbee-usenix03/frisbee-www.html |access-date=2023-01-12 |website=www.cs.utah.edu |archive-date=2023-01-12 |archive-url=https://web.archive.org/web/20230112102631/https://www.cs.utah.edu/flux/papers/frisbee-usenix03/frisbee-www.html |url-status=live }}</ref>

===Digital preservation===
Libraries and museums are typically required to archive and digitally preserve information without altering it in any manner.<ref name=":4" /><ref name=":6">{{Cite journal |last1=Durno |first1=John |last2=Trofimchuk |first2=Jerry |date=2015-01-21 |title=Digital forensics on a shoestring: a case study from the University of Victoria |url=https://journal.code4lib.org/articles/10279 |journal=The Code4Lib Journal |issue=27 |issn=1940-5758}}</ref> [[Emulator]]s frequently use disk images to emulate floppy disks that have been preserved. This is usually simpler to program than accessing a real floppy drive (particularly if the disks are in a format not supported by the host operating system), and allows a large library of software to be managed. Emulation also allows existing disk images to be put into a usable form even though the data contained in the image is no longer readable without emulation.<ref name=":10" />

== Limitations ==
Disk imaging is time consuming, the space requirements are high and reading from them can be slower than reading from the disk directly because of a performance overhead.<ref name=":1" /> 

Other limitations can be the lack of access to software required to read the contents of the image. For example, prior to Windows 8, third party software was required to mount disk images.<ref>{{cite web |date=30 August 2011 |title=Accessing data in ISO and VHD files |url=http://blogs.msdn.com/b/b8/archive/2011/08/30/accessing-data-in-iso-and-vhd-files.aspx |url-status=dead |archive-url=https://web.archive.org/web/20120419080359/http://blogs.msdn.com/b/b8/archive/2011/08/30/accessing-data-in-iso-and-vhd-files.aspx |archive-date=19 April 2012 |access-date=27 April 2012 |work=Building Windows 8 (TechNet Blogs) |publisher=Microsoft}}</ref><ref>{{cite web |title=Mount-DiskImage |url=https://learn.microsoft.com/en-us/powershell/module/storage/mount-diskimage |publisher=Microsoft}}</ref>  When imaging multiple computers with only minor differences, much data is duplicated unnecessarily, wasting space.<ref name=":1" />

=== Speed and failure ===
Disk imaging can be slow, especially for older storage devices. A typical 4.7 GB DVD can take an average of 18 minutes to duplicate.<ref name=":4" /> Floppy disks read and write much slower than hard disks. Therefore, despite their small size, it can take several minutes to copy a single disk. In some cases, disk imaging can fail due to bad sectors or physical wear and tear on the source device.<ref name=":10" /> Unix utilities (such as [[Dd (Unix)|dd]]) are not designed to cope with failures, causing the disk image creation process to fail.<ref name=":6" /> When [[data recovery]] is the end goal, it is instead recommended to use more specialised tools (such as [[ddrescue]]).

==See also==
* {{Annotated link|Boot image}}
* {{Annotated link|Card image}}
* {{Annotated link|Comparison of disc image software}}
* {{Annotated link|Disk cloning}}
* {{Annotated link|El Torito (CD-ROM standard)}}
* {{Annotated link|ISO image}}, an archive file of an optical media volume
* {{Annotated link|Loop device}}
* {{Annotated link|Mtools}}
* {{Annotated link|no-CD crack}}
* {{Annotated link|Protected Area Run Time Interface Extension Services}} (PARTIES)
* {{Annotated link|ROM image}}
* {{Annotated link|Software cracking}}

==References==
{{Reflist}}

==External links==
*[http://www.ibiblio.org/pub/Linux/utils/dos/ Software repository including RAWRITE2]

{{Virtual drive software}}
{{Disk images}}
{{Film piracy}}

[[Category:Disk images| ]]
[[Category:Archive formats]]
[[Category:Compact Disc and DVD copy protection]]
[[Category:Computer file formats]]
[[Category:Disk image emulators]]
[[Category:Hacker culture]]
[[Category:Hardware virtualization]]
[[Category:Optical disc authoring]]
[[Category:Warez]]