Editing Data degradation (section)

==Manifestations==

===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="O'Gorman_1996"/> [[ECC memory]] can be used to mitigate this type of data degradation.<ref name="Normand_1996"/>

===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 [[Error correction code|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 name="Li_2022"/>

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.

The typical SD card, USB stick and M.2 NVMe all have a limited endurance. Power on can usually recover data{{Citation Needed|date=January 2025}} but error rates will eventually degrade the media to illegibility. Writing zeros to a degraded NAND device can revive the storage to close to new condition for further use.{{Citation needed|date=February 2025}} Refresh cycles should be no longer than 6 months to  be sure the device is legible.

====Magnetic media====
[[Magnetic storage|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 [[servowriter]] 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 name="Riss_1993"/><ref name="NAA"/>

====Optical media====
[[Optical storage|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"/>

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_1997"/> 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 media====
[[Paper data storage|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 name="Malachowska_2021"/> 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 name="Menart_2011"/>

====Streaming media====
Data degradation in [[streaming media]] acquisition modules, as addressed by the repair algorithms, reflects real-time data quality issues caused by device limitations. However, a more general form of data degradation refers to the gradual decay of storage media over extended periods, influenced by factors like physical wear, environmental conditions, or technological obsolescence. Causes of such degradation can vary depending on the medium, such as magnetic fields in hard drives, moisture or temperature for tape storage, or electronic failure over time.<ref name="Yu_2022"/>

===Example===
One manifestation of data degradation is when one or a few bits are randomly flipped over a long period of time.{{Sfn|Rosenthal|2010|p=50}} This is illustrated by several digital images below, 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 the {{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>