Below are several digital images illustrating data degradation, all consisting of 326,272 bits. The original photo is displayed on the left. In the next image to the right, 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
cmp command (e.g.
cmp -b bitrot-original.jpg bitrot-1bit-changed.jpg).
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 rays or other high-energy particles. Such data degradation is known as a soft error. ECC memory can be used to mitigate this type of data degradation.
Data degradation results from the gradual decay of storage media over the course of years or longer. Causes vary by medium:
- Solid-state media, such as EPROMs, flash memory and other solid-state drives, store 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 prevents decay. An undamaged copy of the master data is required for the reprogramming; by the time reprogramming is attempted, 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. In warm/humid conditions these media, especially those poorly protected against ambient air, are prone to the physical decomposition of the storage medium.
- Optical media, such as CD-R, DVD-R and BD-R, may experience data decay from the 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 truly permanent.
- Paper media, such as punched cards and punched tape, may literally rot. Mylar punched tape is another approach that does not rely on electromagnetic stability.
Component and system 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.
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, as does ReFS.
- 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.
- 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 6 March 2015. Retrieved 15 January 2014.
- Bonwick, Jeff. "ZFS: The Last Word in File Systems" (PDF). Storage Networking Industry Association (SNIA). Archived from the original (PDF) on 21 September 2013. 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.