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((Short description|Accumulation of data corruption on a storage device over time))
((Distinguish|Software rot))
((Multiple issues|
((Original research|date=April 2015))
((More citations needed|date=February 2018))
))

'''Data degradation''' is the gradual [[Data corruption|corruption]] of [[Data (computing)|computer data]] due to an accumulation of non-critical failures in a [[data storage device]]. The [[phenomenon]] is also known as '''data decay''', '''data rot''' or '''bit rot'''. This process leads to the slow deterioration of data quality over time, even if the data is not actively being used or accessed.

==Example==
Below are several digital images illustrating data degradation, all consisting of 326,272 bits. The original photo is displayed first. In the next image, a single bit was changed from 0 to 1. In the next two images, two and three bits were flipped. On [[Linux]] systems, the binary difference between files can be revealed using ((code|cmp)) command (e.g. ((code|cmp -b bitrot-original.jpg bitrot-1bit-changed.jpg))).

<gallery>
File:Bitrot in JPEG files, 0 bits flipped.jpg|0 bits flipped
File:Bitrot in JPEG files, 1 bit flipped.jpg|1 bit flipped
File:Bitrot in JPEG files, 2 bits flipped.jpg|2 bits flipped
File:Bitrot in JPEG files, 3 bits flipped.jpg|3 bits flipped
</gallery>

==Primary storages ==
Data degradation in [[dynamic random-access memory]] (DRAM) can occur when the [[electric charge]] of a [[bit]] in DRAM disperses, possibly altering program code or stored data. DRAM may be altered by [[cosmic ray]]s<ref>((Cite web|url=https://www.lanl.gov/science/NSS/issue1_2012/story4full.shtml|title=The Invisible Neutron Threat ((!)) National Security Science Magazine|website=Los Alamos National Laboratory|access-date=2020-03-13))</ref> or other high-energy particles. Such data degradation is known as a [[soft error]].<ref name=":0">((cite journal |last1=O'Gorman |first1=T. J. |last2=Ross |first2=J. M. |last3=Taber |first3=A. H. |last4=Ziegler |first4=J. F. |last5=Muhlfeld |first5=H. P. |last6=Montrose |first6=C. J. |last7=Curtis |first7=H. W. |last8=Walsh |first8=J. L. |title=Field testing for cosmic ray soft errors in semiconductor memories|journal=IBM Journal of Research and Development |date=January 1996 |volume=40 |issue=1 |pages=41–50 |doi=10.1147/rd.401.0041 ))</ref> [[ECC memory]] can be used to mitigate this type of data degradation.<ref name="Boeing1" />

==Secondary storages==

Data degradation results from the gradual decay of [[storage media]] over the course of years or longer. Causes vary by medium:

; Solid-state media
: [[EPROM]]s, [[flash memory]] and other [[solid-state drive]] store data using electrical charges, which can slowly leak away due to imperfect insulation. Modern flash controller chips account for this leak by trying several lower threshold voltages (until ECC passes), prolonging the age of data. [[Multi-level cell]]s with much lower distance between voltage levels cannot be considered stable without this functionality.<ref>((cite journal |last1=Li |first1=Qianhui |last2=Wang |first2=Qi |last3=Yang |first3=Liu |last4=Yu |first4=Xiaolei |last5=Jiang |first5=Yiyang |last6=He |first6=Jing |last7=Huo |first7=Zongliang |title=Optimal read voltages decision scheme eliminating read retry operations for 3D NAND flash memories |journal=Microelectronics Reliability |date=April 2022 |volume=131 |pages=114509 |doi=10.1016/j.microrel.2022.114509))</ref>
: The chip itself is not affected by this, so reprogramming it approximately once per decade prevents decay. An undamaged copy of the master data is required for the reprogramming. A [[checksum]] can be used to assure that the on-chip data is not yet damaged and ready for reprogramming.
; [[Magnetic storage|Magnetic media]]
: Magnetic media, such as [[hard disk drive]]s, [[floppy disk]]s and [[magnetic tape]]s, may experience data decay as bits lose their magnetic orientation. Higher temperature speeds up the rate of magnetic loss. As with solid-state media, re-writing is useful as long as the medium itself is not damaged (see below).<ref name=NAA/> Modern hard drives use [[Giant magnetoresistance]] and have a higher magnetic lifespan on the order of decades. They also automatically correct any errors detected by ECC through rewriting. The reliance on a factory servo track can complicate data recovery if it becomes unrecoverable, however.
: Floppy disks and tapes are poorly protected against ambient air. In warm/humid conditions, they are prone to the physical [[decomposition]] of the storage medium.<ref>((cite web|last=Riss|first=Dan|date=July 1993|title=Conserve O Gram (number 19/8) Preservation Of Magnetic Media|url=https://www.nps.gov/museum/publications/conserveogram/19-08.pdf|website=nps.gov|publisher=National Park Service / Department of the Interior (US)|page=2|publication-place=Harpers Ferry, West Virginia|quote=The longevity of magnetic media is most seriously affected by processes that attack the binder resin. Moisture from the air is absorbed by the binder and reacts with the resin. The result is a gummy residue that can deposit on tape heads and cause tape layers to stick together. Reaction with moisture also can result in breaks in the long molecular chains of the binder. This weakens the physical properties of the binder and can result in a lack of adhesion to the backing. These reactions are greatly accelerated by the presence of acids. Typical sources would be the usual pollutant gases in the air, such as sulphur dioxide (SO2) and nitrous oxides (NOx), which react with moist air to form acids. Though acid inhibitors are usually built into the binder layer, over time they can lose their effectiveness.))</ref><ref name=NAA>((Cite web|title=Preserving magnetic media|url=https://www.naa.gov.au/information-management/storing-and-preserving-information/preserving-information/preserving-magnetic-media|access-date=3 November 2020|website=National Archives of Australia|quote=High temperature and humidity and fluctuations may cause the magnetic and base layers in a reel of tape to separate, or cause adjacent loops to block together. High temperatures may also weaken the magnetic signal, and ultimately de-magnetise the magnetic layer.))</ref>
; [[Optical storage|Optical media]]
: Optical media such as [[CD-R]], [[DVD-R]] and [[BD-R]], may experience data decay from the [[disc rot|breakdown]] of the storage medium. This can be mitigated by storing discs in a dark, cool, low humidity location. "Archival quality" discs are available with an extended lifetime, but are still not permanent. However, [[Optical disc#Surface error scanning|data integrity scanning]] that measures the rates of various types of errors is able to predict data decay on optical media well ahead of uncorrectable data loss occurring.<ref name=qpx-g>((cite web |title=QPxTool glossary |url=https://qpxtool.sourceforge.io/glossar.html |website=qpxtool.sourceforge.io |publisher=QPxTool |access-date=22 July 2020 |date=2008-08-01 |ref=QPx-Glossary))</ref>
: Both the disc dye and the disc backing layer are potentially susceptible to breakdown. Early cyanine-based dyes used in CD-R were notorious for their lack of UV stability. Early CDs also suffered from [[CD bronzing]], and is related to a combination of bad lacquer material and failure of the aluminum reflection layer.<ref name="IASA">((cite web
|url=http://www.iasa-web.org/content/information-bulletin-no-22-july-1997
|title=Bronzed CD alert!
|work=IASA Information Bulletin no. 22
|date=July 1997
|accessdate=3 August 2007
|archive-url=https://web.archive.org/web/20110722224026/http://www.iasa-web.org/content/information-bulletin-no-22-july-1997
|archive-date=22 July 2011
|url-status=dead
))</ref> Later discs use more stable dyes or forgo them for an inorganic mixture. The aluminum layer is also commonly swapped out for gold or silver alloy.
; [[Paper data storage|Paper media]]
: Paper media, such as [[punched cards]] and [[punched tape]], may literally [[Decomposition|rot]]. [[Mylar]] punched tape is another approach that does not rely on electromagnetic stability. Degradation of [[books]] and [[Printing_and_writing_paper|printing paper]] is primarily driven by [[acid hydrolysis]] of [[glycosidic bonds]] within the [[cellulose]] molecule as well as by [[oxidation]];<ref>((cite journal |last1=Małachowska |first1=Edyta |last2=Pawcenis |first2=Dominika |last3=Dańczak |first3=Jacek |last4=Paczkowska |first4=Joanna |last5=Przybysz |first5=Kamila |title=Paper Ageing: The Effect of Paper Chemical Composition on Hydrolysis and Oxidation |journal=Polymers |date=26 March 2021 |volume=13 |issue=7 |page=1029 |doi=10.3390/polym13071029 |pmid=33810293 |pmc=8036582 |doi-access=free ))</ref> degradation of paper is accelerated by high [[relative humidity]], high temperature, as well as by exposure to acids, oxygen, light, and various pollutants, including various [[volatile organic compounds]] and [[nitrogen dioxide]].<ref>((cite journal |last1=Menart |first1=Eva |last2=De Bruin |first2=Gerrit |last3=Strlič |first3=Matija |title=Dose–response functions for historic paper |journal=Polymer Degradation and Stability |date=9 September 2011 |volume=96 |issue=12 |pages=2029–2039 |doi=10.1016/j.polymdegradstab.2011.09.002 |url=https://discovery.ucl.ac.uk/1335848/1/Menart_PDSt_2011_EPS.pdf |access-date=5 June 2023))</ref>

==Hardware failures==
Most disk, [[disk controller]] and higher-level systems are subject to a slight chance of unrecoverable failure. With ever-growing disk capacities, file sizes, and increases in the amount of data stored on a disk, the likelihood of the occurrence of data decay and other forms of uncorrected and undetected [[data corruption]] increases.<ref>((cite journal |last1=Gray |first1=Jim |last2=van Ingen |first2=Catharine |title=Empirical Measurements of Disk Failure Rates and Error Rates |journal=Microsoft Research Technical Report MSR-TR-2005-166 |date=December 2005 |url=http://research.microsoft.com/pubs/64599/tr-2005-166.pdf |access-date=4 March 2013))</ref>

Low-level disk controllers typically employ [[error correction code]]s (ECC) to correct erroneous data.<ref>((cite web |title=ECC and Spare Blocks help to keep Kingston SSD data protected from errors |url=https://www.kingston.com/en/ssd/data-protection |website=Kingston Technology Company |access-date=30 March 2021))</ref>

Higher-level software systems may be employed to mitigate the risk of such underlying failures by increasing redundancy and implementing integrity checking, error correction codes and self-repairing algorithms.<ref>((cite web |last=Salter |first=Jim |title=Bitrot and atomic COWs: Inside "next-gen" filesystems |date=15 January 2014 |publisher=[[Ars Technica]] |url=https://arstechnica.com/information-technology/2014/01/bitrot-and-atomic-cows-inside-next-gen-filesystems/ |access-date=15 January 2014 |archive-url=https://web.archive.org/web/20150306225935/http://arstechnica.com/information-technology/2014/01/bitrot-and-atomic-cows-inside-next-gen-filesystems/ |archive-date=6 March 2015 |url-status=dead ))</ref> The [[ZFS]] [[file system]] was designed to address many of these data corruption issues.<ref>((cite web|last=Bonwick |first=Jeff |title=ZFS: The Last Word in File Systems |publisher=Storage Networking Industry Association (SNIA) |url=http://www.snia.org/sites/default/files2/sdc_archives/2009_presentations/monday/JeffBonwickzfs-Basic_and_Advanced.pdf |access-date=4 March 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130921055345/http://www.snia.org/sites/default/files2/sdc_archives/2009_presentations/monday/JeffBonwickzfs-Basic_and_Advanced.pdf |archive-date=21 September 2013 ))</ref> The [[Btrfs]] file system also includes data protection and recovery mechanisms,<ref>((cite web | author=<!--wiki--> | title = btrfs Wiki: Features | publisher = The btrfs Project | url = https://btrfs.wiki.kernel.org/index.php/Main_Page#Features | access-date = 19 September 2013))</ref> as does [[ReFS]].<ref>((cite web | last = Wlodarz | first = Derrick | title = Windows Storage Spaces and ReFS: is it time to ditch RAID for good? | date = 15 January 2014 | publisher = Betanews | url = http://betanews.com/2014/01/15/windows-storage-spaces-and-refs-is-it-time-to-ditch-raid-for-good/ | access-date = 2014-02-09))</ref>

==See also==
((Div col|colwidth=25em))
* [[Checksum]]
* [[Database integrity]]
* [[Data curation]]
* [[Data preservation]]
* [[Data scrubbing]]
* [[Digital permanence]]
* [[Digital preservation]]
* [[Disc rot]]
* [[Error detection and correction]]
* [[Link rot]]
* [[Media preservation]]
* [[RAR (file format)|RAR]] archive file format has optional recovery
* [[Parchive|PAR2]] recovery file format
((div col end))

==References==
((Reflist|30em|refs=<ref name="Boeing1">[https://web.archive.org/web/20131021190327/http://pdf.yuri.se/files/art/2.pdf Single Event Upset at Ground Level, Eugene Normand, Member, IEEE, Boeing Defense & Space Group, Seattle, WA 98124-2499]</ref>))

((Data))

[[Category:Computer jargon]]
[[Category:Data quality]]

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