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Data degradation is the gradualcorruption ofcomputer data due to an accumulation of non-critical failures in adata storage device. It is also referred to asdata decay,data rot,digital decay, orbit rot.[1] This results in a decline in data quality over time, even when the data is not being utilized.
Data degradation indynamic random-access memory (DRAM) can occur when theelectric charge of abit in DRAM disperses, possibly altering program code or stored data. DRAM may be altered bycosmic rays[2] or other high-energy particles. Such data degradation is known as asoft error.[3]ECC memory can be used to mitigate this type of data degradation.[4]
Data degradation results from the gradual decay ofstorage media over the course of years or longer. Causes vary by medium.
EPROMs,flash memory and othersolid-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 (untilECC passes), prolonging the age of data.Multi-level cells with much lower distance between voltage levels cannot be considered stable without this functionality.[5]
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. Achecksum 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] 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] Refresh cycles should be no longer than 6 months to be sure the device is legible.
Magnetic media, such ashard disk drives,floppy disks andmagnetic tapes, 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).[6] Modern hard drives useGiant 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 aservowriter 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 physicaldecomposition of the storage medium.[7][6]
Optical media such asCD-R,DVD-R andBD-R, may experience data decay from thebreakdown 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,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.[8]
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 fromCD bronzing, and is related to a combination of bad lacquer material and failure of the aluminum reflection layer.[9] 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, such aspunched cards andpunched tape, may literallyrot.Mylar punched tape is another approach that does not rely on electromagnetic stability. Degradation ofbooks andprinting paper is primarily driven byacid hydrolysis ofglycosidic bonds within thecellulose molecule as well as byoxidation;[10] degradation of paper is accelerated by highrelative humidity, high temperature, as well as by exposure to acids, oxygen, light, and various pollutants, including variousvolatile organic compounds andnitrogen dioxide.[11]
Data degradation occurs instreaming media transmission, causing data quality issues.[12]
One manifestation of data degradation is when one or a few bits are randomly flipped over a long period of time.[13] 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. OnLinux systems, the binary difference between files can be revealed using thecmp command (e.g.cmp -b bitrot-original.jpg bitrot-1bit-changed.jpg).
This deterioration can be caused by a variety of factors that impact the reliability and integrity of digital information, including physical factors,software errors, security breaches,human error, obsolete technology, and unauthorized access incidents.[14][15][16][17]
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 undetecteddata corruption increases.[18]
Low-level disk controllers typically employerror correction codes (ECC) to correct erroneous data.[19]
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.[20] TheZFSfile system was designed to address many of these data corruption issues.[21] TheBtrfs file system also includes data protection and recovery mechanisms,[22][better source needed] as doesReFS.[23]
There is no solution that completely eliminates the threat of data degradation,[24] but various measures exist that can stave it off. One of these is toreplicate the data asbackups. Both the original and backed data are thenaudited for any faults due to storage media errors bychecksumming the data or comparing it with that of other copies. This is the only way to detectlatent faults proactively,[25] which might otherwise go unnoticed until the data is actually accessed.[26] Current storage systems such as those based onRAID already employ such measures internally.[27] Ideally, and especially for data that must bepreserved digitally, the replicas should be distributed across multiple administrative sites that function autonomously and deploy various hardware and software, increasing resistance to failure, as well as human error and cyberattacks.[28]
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.
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.