Data storage in the context of "Disk storage"

Information theory is the mathematical study of the quantification, storage, and communication of information. The field was established and formalized by Claude Shannon in the 1940s, though early contributions were made in the 1920s through the works of Harry Nyquist and Ralph Hartley. It is at the intersection of electronic engineering, mathematics, statistics, computer science, neurobiology, physics, and electrical engineering.

A key measure in information theory is entropy. Entropy quantifies the amount of uncertainty involved in the value of a random variable or the outcome of a random process. For example, identifying the outcome of a fair coin flip (which has two equally likely outcomes) provides less information (lower entropy, less uncertainty) than identifying the outcome from a roll of a die (which has six equally likely outcomes). Some other important measures in information theory are mutual information, channel capacity, error exponents, and relative entropy. Important sub-fields of information theory include source coding, algorithmic complexity theory, algorithmic information theory and information-theoretic security.

In computing, a database is an organized collection of data or a type of data store based on the use of a database management system (DBMS), the software that interacts with end users, applications, and the database itself to capture and analyze the data. The DBMS additionally encompasses the core facilities provided to administer the database. The sum total of the database, the DBMS and the associated applications can be referred to as a database system. Often the term "database" is also used loosely to refer to any of the DBMS, the database system or an application associated with the database.

Before digital storage and retrieval of data have become widespread, index cards were used for data storage in a wide range of applications and environments: in the home to record and store recipes, shopping lists, contact information and other organizational data; in business to record presentation notes, project research and notes, and contact information; in schools as flash cards or other visual aids; and in academic research to hold data such as bibliographical citations or notes in a card file. Professional book indexers used index cards in the creation of book indexes until they were replaced by indexing software in the 1980s and 1990s.

A text file (sometimes spelled textfile; an old alternative name is flat file) is a kind of computer file that is structured as a sequence of lines of electronic text. A text file exists stored as data within a computer file system.

In operating systems such as CP/M, where the operating system does not keep track of the file size in bytes, the end of a text file is denoted by placing one or more special characters, known as an end-of-file (EOF) marker, as padding after the last line in a text file. In modern operating systems such as DOS, Microsoft Windows and Unix-like systems, text files do not contain any special EOF character, because file systems on those operating systems keep track of the file size in bytes.

DVD (digital video disc or digital versatile disc) is a digital optical disc data storage format. It was invented and developed in 1995 and first released on November 1, 1996, in Japan. The medium can store any kind of digital data and has been widely used to store video programs (watched using DVD players), software and other computer files. DVDs offer significantly higher storage capacity than compact discs (CD) while having the same dimensions. A standard single-layer DVD can store up to 4.7 GB of data, a dual-layer DVD up to 8.5 GB. Dual-layer, double-sided DVDs can store up to a maximum of 17.08 GB.

Prerecorded DVDs are mass-produced using molding machines that physically stamp data onto the DVD. Such discs are a form of DVD-ROM because data can only be read and not written or erased. Blank recordable DVD discs (DVD-R and DVD+R) can be recorded once using a DVD recorder and then function as a DVD-ROM. Rewritable DVDs (DVD-RW, DVD+RW, and DVD-RAM) can be recorded and erased many times.

Blu-ray (Blu-ray Disc or BD) is a digital optical disc data storage format designed to supersede the DVD format. It was invented and developed in 2005 and released worldwide on June 20, 2006, capable of storing several hours of high-definition video (HDTV 720p and 1080p). The main application of Blu-ray is as a medium for video material such as feature films and for the physical distribution of video games for the PlayStation 3, PlayStation 4, PlayStation 5, Xbox One, and Xbox Series X. The name refers to the blue laser used to read the disc, which allows information to be stored at a greater density than is possible with the longer-wavelength red laser used for DVDs, resulting in an increased capacity.

The polycarbonate disc is 12 centimetres (4+3⁄4 inches) in diameter and 1.2 millimetres (1⁄16 inch) thick, the same size as DVDs and CDs. Conventional (or "pre-BDXL") Blu-ray discs contain 25 GB per layer, with dual-layer discs (50 GB) being the industry standard for feature-length video discs. Triple-layer discs (100 GB) and quadruple-layer discs (128 GB) are available for BDXL re-writer drives.

The compact disc (CD) is a digital optical disc data storage format co-developed by Philips and Sony to store and play digital audio recordings. It employs the Compact Disc Digital Audio (CD-DA) standard and is capable of holding uncompressed stereo audio. First released in Japan in October 1982, the CD was the second optical disc format to reach the market, following the larger LaserDisc (LD). In later years, the technology was adapted for computer data storage as CD-ROM and subsequently expanded into various writable and multimedia formats. As of 2007, over 200 billion CDs (including audio CDs, CD-ROMs, and CD-Rs) had been sold worldwide.

Standard CDs have a diameter of 120 millimetres (4.7 inches) and typically hold up to 74 minutes of audio or approximately 650 MiB (681,574,400 bytes) of data. This was later regularly extended to 80 minutes or 700 MiB (734,003,200 bytes) by reducing the spacing between data tracks, with some discs unofficially reaching up to 99 minutes or 870 MiB (912,261,120 bytes) which falls outside established specifications. Smaller variants, such as the Mini CD, range from 60 to 80 millimetres (2.4 to 3.1 in) in diameter and have been used for CD singles or distributing device drivers and software.

In computer science, data (treated as singular, plural, or as a mass noun) is any sequence of one or more symbols; datum is a single unit of data. Data requires interpretation to become information. Digital data is data that is represented using the binary number system of ones (1) and zeros (0), instead of analog representation. In modern (post-1960) computer systems, all data is digital.

Data exists in three states: data at rest, data in transit and data in use. Data within a computer, in most cases, moves as parallel data. Data moving to or from a computer, in most cases, moves as serial data. Data sourced from an analog device, such as a temperature sensor, may be converted to digital using an analog-to-digital converter. Data representing quantities, characters, or symbols on which operations are performed by a computer are stored and recorded on magnetic, optical, electronic, or mechanical recording media, and transmitted in the form of digital electrical or optical signals. Data pass in and out of computers via peripheral devices.

Fujitsu Limited (富士通株式会社, Fujitsū kabushiki gaisha) is a Japanese multinational information and communications technology equipment and services corporation, established in 1935 and headquartered in Kawasaki, Kanagawa. It is the world's sixth-largest IT services provider by annual revenue, and it is the largest in Japan as of 2021.

Fujitsu's hardware offerings mainly consist of personal and enterprise computing products, including x86, SPARC, and mainframe-compatible server products. The corporation and its subsidiaries also offer diverse products and services in data storage, telecommunications, advanced microelectronics, and air conditioning. It has approximately 124,000 employees supporting customers in over 50 countries and regions.