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Data degradation, also known as data decay or data rot, is a colloquial computing phrase for the gradual decay of storage media. It should not be confused with "bit rot", defined in the Jargon File as a jocular explanation for the degradation of a software program over time even if "nothing has changed".
Decay of data in memory
Data degradation in dynamic random-access memory (DRAM) can occur when the small electric charge of a bit in DRAM disperses, possibly altering program code or stored data. Semiconductor RAM may also occasionally be altered by cosmic rays or other high-energy particles; such data degradation is known as soft errors.
Decay of storage media
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Data degradation can also be used to describe the phenomenon of storage media gradually decaying over the duration of many years. The cause of data decay varies depending on the medium:
- Solid-state media, such as EPROMs, flash memory and other solid-state drives, stores data using electrical charges, which can slowly leak away due to imperfect insulation. The chip itself is not affected by this, so reprogramming it once per decade or so will prevent data decay. The biggest problem can be finding a clean copy of the master data from which the chip may be reprogrammed; frequently, by the time the user discovers the data decay, the master data may be lost.
- Magnetic media, such as hard disk drives, floppy disks and magnetic tapes, may experience data decay as bits lose their magnetic orientation. Periodic refreshing by rewriting the data can alleviate this problem. Also, in warm and humid conditions these media, especially the ones poorly protected against aggressive air conditions, are prone to the decomposition of the very material they are fabricated from.
- Optical media, such as CD-R, DVD-R and BD-R, may experience data decay from the breakdown of the material onto which the data is stored. This can be mitigated by storing discs in a dark, cool location with low humidity. "Archival quality" discs are also available, but do not necessarily provide a permanent solution to the onset of data decay or other types of data corruption beyond a certain amount of time. Some media (such as M-DISC) are designed to improve longevity over DVD-R and BD-R.
- Paper media, such as punched cards and punched tape, may also experience literal rotting. Mylar punched tape is available for use in such situations.
Component and system failures
Most disk, disk controller and higher-level systems are subject to a small degree 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.
Higher-level software systems may be employed to mitigate the risk of such underlying failures by increasing redundancy and implementing integrity checking and self-repairing algorithms. The ZFS file system was designed to address many of these data corruption issues. The Btrfs file system also includes data protection and recovery mechanisms, and so does ReFS.
- Raymond, Eric. "Bit rot". The Jargon File. Retrieved 3 March 2013.
- O'Gorman, T. J.; Ross, J. M.; Taber, A. H.; Ziegler, J. F.; Muhlfeld, H. P.; Montrose, C. J.; Curtis, H. W.; Walsh, J. L. (January 1996). "Field testing for cosmic ray soft errors in semiconductor memories". IBM Journal of Research and Development 40 (1): 41–50. doi:10.1147/rd.401.0041. Retrieved 4 March 2013.
- Gray, Jim; van Ingen, Catharine (December 2005). "Empirical Measurements of Disk Failure Rates and Error Rates" (PDF). Microsoft Research Technical Report MSR-TR-2005-166. Retrieved 4 March 2013.
- Salter, Jim (15 January 2014). "Bitrot and atomic COWs: Inside "next-gen" filesystems". Ars Technica. Archived from the original on 23 March 2015. Retrieved 15 January 2014.
- Bonwick, Jeff. "ZFS: The Last Word in File Systems" (PDF). Storage Networking Industry Association (SNIA). Retrieved 4 March 2013.
- "btrfs Wiki: Features". The btrfs Project. Retrieved 19 September 2013.
- Wlodarz, Derrick. "Windows Storage Spaces and ReFS: is it time to ditch RAID for good?". Betanews. Retrieved 2014-02-09.