In computer
main memory,
auxiliary storage
Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers.
The central processing unit (CPU) of a compute ...
and
computer buses, data redundancy is the existence of data that is additional to the actual data and permits correction of errors in stored or transmitted data. The additional data can simply be a complete copy of the actual data (a type of
repetition code
In coding theory, the repetition code is one of the most basic error-correcting codes. In order to transmit a message over a noisy channel that may corrupt the transmission in a few places, the idea of the repetition code is to just repeat the me ...
), or only select pieces of data that allow
detection of errors and reconstruction of lost or damaged data up to a certain level.
For example, by including additional data
checksum
A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. By themselves, checksums are often used to verify dat ...
s,
ECC memory is capable of detecting and correcting single-bit errors within each
memory word, while
RAID 1 combines two
hard disk drive
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magn ...
s (HDDs) into a logical storage unit that allows stored data to survive a complete failure of one drive. Data redundancy can also be used as a measure against
silent data corruption; for example,
file systems
In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...
such as
Btrfs
Btrfs (pronounced as "better F S", "butter F S", "b-tree F S", or simply by spelling it out) is a computer storage format that combines a file system based on the copy-on-write (COW) principle with a logical volume manager (not to be confused ...
and
ZFS use data and
metadata checksumming in combination with copies of stored data to detect silent data corruption and repair its effects.
In database systems
While different in nature, data redundancy also occurs in
database systems that have values repeated unnecessarily in one or more records or
fields, within a
table, or where the field is replicated/repeated in two or more tables. Often this is found in
unnormalized database designs and results in the complication of database management, introducing the risk of corrupting the data, and increasing the required amount of
storage
Storage may refer to:
Goods Containers
* Dry cask storage, for storing high-level radioactive waste
* Food storage
* Intermodal container, cargo shipping
* Storage tank
Facilities
* Garage (residential), a storage space normally used to store car ...
. When done on purpose from a previously normalized database schema, it ''may'' be considered a form of
database denormalization
Denormalization is a strategy used on a previously- normalized database to increase performance. In computing, denormalization is the process of trying to improve the read performance of a database, at the expense of losing some write performance, ...
; used to improve performance of database queries (shorten the database response time).
For instance, when customer data are duplicated and attached with each product bought, then redundancy of data is a known source of
inconsistency since a given customer might appear with different values for one or more of their attributes.
Data redundancy leads to
data anomalies and corruption and generally should be avoided by design;
applying
database normalization prevents redundancy and makes the best possible usage of storage.
See also
*
Data maintenance
*
Data deduplication
*
Data scrubbing
*
End-to-end data protection
*
Redundancy (engineering)
*
Redundancy (information theory)
In information theory, redundancy measures the fractional difference between the entropy of an ensemble , and its maximum possible value \log(, \mathcal_X, ). Informally, it is the amount of wasted "space" used to transmit certain data. Data co ...
References
{{DEFAULTSORT:Data Redundancy
Computer memory
Data
Data modeling
Databases
Fault-tolerant computer systems