Computer data storage

Raw capacity

The total amount of stored information that a storage device or medium can hold. It is expressed as a quantity of bits or bytes (e.g. 10.4 megabytes).

Memory storage density

The compactness of stored information. It is the storage capacity of a medium divided with a unit of length, area or volume (e.g. 1.2 megabytes per square inch).

Latency

The time it takes to access a particular location in storage. The relevant unit of measurement is typically nanosecond for primary storage, millisecond for secondary storage, and second for tertiary storage. It may make sense to separate read latency and write latency (especially for non-volatile memory) and in case of sequential access storage, minimum, maximum and average latency.

Throughput

The rate at which information can be read from or written to the storage. In computer data storage, throughput is usually expressed in terms of megabytes per second (MB/s), though bit rate may also be used. As with latency, read rate and write rate may need to be differentiated. Also accessing media sequentially, as opposed to randomly, typically yields maximum throughput.

Granularity

The size of the largest "chunk" of data that can be efficiently accessed as a single unit, e.g. without introducing additional latency.

Reliability

The probability of spontaneous bit value change under various conditions, or overall failure rate.

Utilities such as hdparm and sar can be used to measure IO performance in Linux.

• Storage devices that reduce fan usage automatically shut-down during inactivity, and low power hard drives can reduce energy consumption by 90 percent.^[10][11]
• 2.5-inch hard disk drives often consume less power than larger ones.
  memory wall
  , may also consume a large amount of power.

Full disk encryption, volume and virtual disk encryption, andor file/folder encryption is readily available for most storage devices.^[17]

Hardware memory encryption is available in Intel Architecture, supporting Total Memory Encryption (TME) and page granular memory encryption with multiple keys (MKTME).^[18][19] and in SPARC M7 generation since October 2015.^[20]

Distinct types of data storage have different points of failure and various methods of predictive failure analysis.

Vulnerabilities that can instantly lead to total loss are head crashing on mechanical hard drives and failure of electronic components on flash storage.

Impending failure on hard disk drives is estimable using S.M.A.R.T. diagnostic data that includes the hours of operation and the count of spin-ups, though its reliability is disputed.^[21]

Flash storage may experience downspiking transfer rates as a result of accumulating errors, which the flash memory controller attempts to correct.

The health of

As of 2011[update], the most commonly used data storage media are semiconductor, magnetic, and optical, while paper still sees some limited usage. Some other fundamental storage technologies, such as all-flash arrays (AFAs) are proposed for development.

As early as 2006, notebook and desktop computer manufacturers started using flash-based solid-state drives (SSDs) as default configuration options for the secondary storage either in addition to or instead of the more traditional HDD.^{[23][24][25][26][27]}

• Magnetic disk;
  • Floppy disk, used for off-line storage;
  • Hard disk drive, used for secondary storage.
• Magnetic tape, used for tertiary and off-line storage;
• Carousel memory (magnetic rolls).

In early computers, magnetic storage was also used as:

• Primary storage in a form of
  core memory, core rope memory, thin-film memory and/or twistor memory
  ;
• Tertiary (e.g. NCR CRAM) or off line storage in the form of magnetic cards;
• Magnetic tape was then often used for secondary storage.

Magnetic storage does not have a definite limit of rewriting cycles like flash storage and re-writeable optical media, as altering magnetic fields causes no physical wear. Rather, their life span is limited by mechanical parts.^[28][29]

Optical storage, the typical optical disc, stores information in deformities on the surface of a circular disc and reads this information by illuminating the surface with a laser diode and observing the reflection. Optical disc storage is non-volatile. The deformities may be permanent (read only media), formed once (write once media) or reversible (recordable or read/write media). The following forms are in common use as of 2009^[update]:^[30]

• BD-ROM
  : Read only storage, used for mass distribution of digital information (music, video, computer programs);
• BD-R
  : Write once storage, used for tertiary and off-line storage;
• BD-RE
  : Slow write, fast read storage, used for tertiary and off-line storage;
• Ultra Density Optical or UDO is similar in capacity to BD-R or BD-RE and is slow write, fast read storage used for tertiary and off-line storage.

3D optical data storage has also been proposed.

Light induced magnetization melting in magnetic photoconductors has also been proposed for high-speed low-energy consumption magneto-optical storage.^[31]

Vacuum-tube memory

A Williams tube used a cathode-ray tube, and a Selectron tube used a large vacuum tube to store information. These primary storage devices were short-lived in the market, since the Williams tube was unreliable, and the Selectron tube was expensive.

Electro-acoustic memory

Delay-line memory used sound waves in a substance such as mercury to store information. Delay-line memory was dynamic volatile, cycle sequential read/write storage, and was used for primary storage.

Optical tape

is a medium for optical storage, generally consisting of a long and narrow strip of plastic, onto which patterns can be written and from which the patterns can be read back. It shares some technologies with cinema film stock and optical discs, but is compatible with neither. The motivation behind developing this technology was the possibility of far greater storage capacities than either magnetic tape or optical discs.

Phase-change memory

uses different mechanical phases of

electrical resistance

of the material. Phase-change memory would be non-volatile, random-access read/write storage, and might be used for primary, secondary and off-line storage. Most rewritable and many write-once optical disks already use phase-change material to store information.

Holographic data storage

stores information optically inside crystals or photopolymers. Holographic storage can utilize the whole volume of the storage medium, unlike optical disc storage, which is limited to a small number of surface layers. Holographic storage would be non-volatile, sequential-access, and either write-once or read/write storage. It might be used for secondary and off-line storage. See Holographic Versatile Disc (HVD).

Molecular memory

stores information in polymer that can store electric charge. Molecular memory might be especially suited for primary storage. The theoretical storage capacity of molecular memory is 10 terabits per square inch (16 Gbit/mm²).^[34]

Magnetic photoconductors

store magnetic information, which can be modified by low-light illumination.^[31]

DNA

stores information in DNA nucleotides. It was first done in 2012, when researchers achieved a ratio of 1.28 petabytes per gram of DNA. In March 2017 scientists reported that a new algorithm called a DNA fountain achieved 85% of the theoretical limit, at 215 petabytes per gram of DNA.^{[35][36][37][38]}

While a group of bits malfunction may be resolved by error detection and correction mechanisms (see above), storage device malfunction requires different solutions. The following solutions are commonly used and valid for most storage devices:

• Device mirroring (replication) – A common solution to the problem is constantly maintaining an identical copy of device content on another device (typically of the same type). The downside is that this doubles the storage, and both devices (copies) need to be updated simultaneously with some overhead and possibly some delays. The upside is the possible concurrent reading of the same data group by two independent processes, which increases performance. When one of the replicated devices is detected to be defective, the other copy is still operational and is being utilized to generate a new copy on another device (usually available operational in a pool of stand-by devices for this purpose).
• Redundant array of independent disks (RAID) – This method generalizes the device mirroring above by allowing one device in a group of devices to fail and be replaced with the content restored (Device mirroring is RAID with n=2). RAID groups of n=5 or n=6 are common. n>2 saves storage, when compared with n=2, at the cost of more processing during both regular operation (with often reduced performance) and defective device replacement.

Device mirroring and typical RAID are designed to handle a single device failure in the RAID group of devices. However, if a second failure occurs before the RAID group is completely repaired from the first failure, then data can be lost. The probability of a single failure is typically small. Thus the probability of two failures in the same RAID group in time proximity is much smaller (approximately the probability squared, i.e., multiplied by itself). If a database cannot tolerate even such a smaller probability of data loss, then the RAID group itself is replicated (mirrored). In many cases such mirroring is done geographically remotely, in a different storage array, to handle recovery from disasters (see disaster recovery above).

A secondary or tertiary storage may connect to a computer utilizing computer networks. This concept does not pertain to the primary storage, which is shared between multiple processors to a lesser degree.

• Direct-attached storage (DAS) is a traditional mass storage, that does not use any network. This is still a most popular approach. This retronym was coined recently, together with NAS and SAN.
• CIFS/SMB
  protocols.
• Storage area network (SAN) is a specialized network, that provides other computers with storage capacity. The crucial difference between NAS and SAN, is that NAS presents and manages file systems to client computers, while SAN provides access at block-addressing (raw) level, leaving it to attaching systems to manage data or file systems within the provided capacity. SAN is commonly associated with Fibre Channel networks.

Large quantities of individual magnetic tapes, and optical or magneto-optical discs may be stored in robotic tertiary storage devices. In tape storage field they are known as

Robotic storage is used for backups, and for high-capacity archives in imaging, medical, and video industries. Hierarchical storage management is a most known archiving strategy of automatically migrating long-unused files from fast hard disk storage to libraries or jukeboxes. If the files are needed, they are retrieved back to disk.

Wikiversity has learning resources about Computer data storage

• Aperture (computer memory)
• Dynamic random-access memory (DRAM)
• Memory latency
• Mass storage
• Memory cell (disambiguation)
• Memory management
  • Memory leak
  • Virtual memory
• Memory protection
• Page address register
• Stable storage
• Static random-access memory (SRAM)

• Cloud storage^[39]
• Hybrid cloud storage
• Data deduplication
• Data proliferation
• Data storage tag used for capturing research data
• Disk utility
• File system
  • List of file formats
  • Global filesystem
• Flash memory
• Geoplexing
• Information repository
• Noise-predictive maximum-likelihood detection
• Object(-based) storage
• Removable media
• Solid-state drive
• Spindle
• Virtual tape library
• Wait state
• Write buffer
• Write protection
• Cold data

• Storage Networking World
• Storage World Conference

 This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 22 January 2022.