Computer data storage , often called storage or memory , is a technology of computer components and recording media that are used to retain digital data . It is a core function and fundamental component of computers.  : 15-16
The central processing unit (CPU) is a computer manipulated data by performing computations. In practice, almost all computers use a storage hierarchy ,  : 468-473 which puts a price on the CPU and slower but larger and cheaper options farther away. Generally the fast volatile technologies are referred to as “memory”, while slower persistent technologies are referred to as “storage”.
In the Von Neumann architecture , the CPU consists of two main parts: The control unit and the arithmetic logic unit (ALU). The train controls the flow of data between CPU and memory, while the latter performs arithmetic and logical operations on data.
Without a significant amount of memory, a computer would simply be able It would have been reconfigured to change its behavior. This is acceptable for devices such as calculators , digital signal processors , and other specialized devices. Von Neumann machines differ in having a memory in which they store their operating instructions and data.  : 20 Such computers are more versatile in that they do not need to have their hardware reconfigured for each new program, but can simply be reprogrammedwith new in-memory instructions; They aussi tends to be simpler to design, en ce que a single processor Relatively May keep state successive entre computations to build up complex procedural results. Most modern computers are from Neumann machines.
Data organization and representation
A modern digital computer represents data using the binary numeral system . Text, numbers, pictures, audio, and any other form of information can be converted into a string of bits , or binary digits, each of which has a value of 1 or 0. The most common unit of storage is the byte , equal to 8 bits. A piece of information can be handled by any computer or device whose storage space is large enough to accommodate the binary representation of the piece of information , or simply data . For example, the complete works of Shakespeare , about 1250 pages in print, can be stored in five megabytes (40 million bits) with one byte per character.
Data are encoded by assigning a bit pattern to each character , digit , or multimedia object. Many standards exist for encoding (eg, encodings like ASCII , image encodings like JPEG , video encodings like MPEG-4 ).
By adding bits to each encoded unit, redundancy enables the computer to detect errors in coded data and correct them on mathematical algorithms. Errors Occur Generally in low probabilities due to random bit flipping value, or “physical bit tired,” loss of the physical bit in storage of icts Ability to Maintain a distinguishable value (0 or 1), or due to errors in inter or intra computer communication. A random bit flip (eg, due to random radiation) is typically corrected upon detection. A bit, or a group of malfunctioning physical bits is typically automatically fenced-out, taken out of use by the device, and replaced by a different functioning group. in the device, where the corrected bit values are restored (if possible). The cyclic redundancy check (CRC) is typically used in communications and storage for error detection . A detected error is then retried.
Data compression methods allow in many cases (such as a database) to represent a string of bits by a shorter string (“compress”) and reconstruct the original string (“decompress”) when needed. This utilizes substantially less storage (tens of percents) for many types of data and the cost of more computation (compress and decompress when needed). Analysis of trade-off between storage costs and costs of data processing and possible delays in data availability
For security reasons, certain types of data (eg, credit-card information) can be kept encrypted in the storage of data from the storage of snapshots.
Hierarchy of storage
Various forms of storage, divided according to their distance from the central processing unit . The fundamental components of a general-purpose computer are arithmetic and logic unit , control circuitry , storage space, and input / outputdevices. Technology and capacity have in common home computers around 2005.
Generally, the lower a storage is in the hierarchy, the lesser its bandwidth and the greater its access latency is from the CPU. This traditional division of primary, secondary and tertiary storage is also guided by cost per bit.
In contemporary usage, “memory” is usually semiconductor storage read-write random access memory , typically DRAM (dynamic RAM) or other forms of fast but temporary storage. “Storage” consists of storage devices and their media not typically accessible by the CPU ( secondary or tertiary storage ), typically hard disk drives, optical disc drives, and other devices than RAM non-volatile purpose (retaining contents when powered down). 
Historically, memory has been called core memory , main memory , real storage or internal memory . Meanwhile, non-volatile storage devices have been referred to as secondary storage , external memory or auxiliary / peripheral storage .
Primary storage (also known as main memory or internal memory ), often referred to simply as memory , is the only one directly accessible to the CPU. The CPU reads instructions stored there and executes them as required. Any data not being used in a uniform way.
Historically, early computers used delay lines , Williams tubes , or rotating magnetic drums as primary storage. By 1954, those unreliable methods were replaced by magnetic core memory . Core memory Remained dominant up to the 1970s, When advances in integrated circuitry technology allowed semiconductor memory To Become economically competitive.
This led to modern random access memory (RAM). It is small-sized, light, but quite expensive at the same time. (The particular types of RAM used for primary storage are also volatile , ie they lose the information when not powered).
As shown in the diagram, there are two more sub-layers of primary storage, besides main-capacity RAM:
- Processor registers are located inside the processor. Each register typically holds a word of data (often 32 or 64 bits). CPU instructions instruct the arithmetic logic unit to perform various calculations or other operations on this data (or with the help of it). Registers are the fastest of all forms of computer data storage.
- Processor cache is an intermediate stage between ultra-fast registers and much slower main memory. It was introduced to improve the performance of computers. Most actively used in the memory is just duplicated in the cache memory, which is faster, but of much lesser capacity. On the other hand, the memory is much slower, but has a much greater storage capacity than processor registers. Multi-level hierarchical cache setup is also commonly used- primary cache being smallest, fastest and located inside the processor; secondary cache being somewhat larger and slower.
Main memory is directly connected to the central processing unit via a memory bus . Actually it is two nozzles (not on the diagram): an address bus and a data bus . The CPU firstly sends a number through an address bus, a number called memory address , which indicates the desired location of data. Then it reads or writes the data in the memory cells using the data bus. Additionally, a memory management unit (MMU) is a small device between CPU and RAM recalculating the actual memory address, for example to provide an abstraction of virtual memory or other tasks.
As the RAM types used for primary storage are uninitialized at start up, a computer containing only such storage would not have a source of information. Hence, non-volatile primary storage containing a small startup program ( BIOS ) is used to bootstrap the computer, that is, to read a larger program from non-volatile secondary storage to RAM and start to execute it. A non-volatile technology used for this purpose s’intitule ROM for read-only memory(the terminology May be Somewhat confusing as MOST ROM kinds sont également able of random access ).
Many types of “ROM” are not literally readable , as they are possible; However, it should be deleted in large portions before it can be re-written. Some embedded systems run programs directly from ROM (or similar), because such programs are rarely changed. Standard computers do not store programs in ROM, and, rather, use large capacities of secondary storage, which is non-volatile, and not as costly.
Recently, primary storage and secondary storage in some uses refer to what was historically called, respectively, secondary storage and tertiary storage . 
Secondary storage (also known as external memory or auxiliary storage ), which is not directly accessible by the CPU. The computer usually uses ict input / output channels to access secondary storage and data transfers using the Desired intermediate area in primary storage. It is powered down-it is non-volatile. Per unit, it is also less expensive than primary storage. Modern computer systems typically have two orders of magnitude more than just storage.
In modern computers, hard drives are usually used as secondary storage. The time taken to access a given byte of information stored on a hard disk is typically a few thousandths of a second, or milliseconds. By contrast, the time taken to access a given byte of information stored in random-access memory is measured in billionths of a second, or nanoseconds. This is the significant access-time difference which distinguishes solid-state memory from rotating magnetic storage devices: hard disks are typically about a million times slower than memory. Rotating optical storage devices, such as CD and DVDdrives, have even longer access times. With disk drives, ounce of the disk reads / write head, the proper placement and the data of interest rotates under it, later data on the track is very fast to access. To reduce the search time and rotational latency, data are transferred to and from disks in large contiguous blocks.
When data reside on disk, available in an efficient way external memory algorithms . Sequential or block access to disks has been developed, and many sophisticated paradigms have been developed to design efficient algorithms based on sequential and block access. Another way to reduce the I / O bottleneck is to use multiple disks in parallel in order to increase the bandwidth between primary and secondary memory. 
Some other examples of secondary storage technologies are flash memory (eg USB flash drives or keys), floppy disks , magnetic tape , paper tape , punched cards , RAM standalone disks , and Iomega Zip drives .
The secondary storage is Often formatted selon a file system format, qui Provides the abstraction Necessary to organizes data into files and directories , providing good aussi additional information (called Expired metadata ) Describing the owner of a some file, the access time, the access permissions, and other information.
Most computer operating systems use the concept of virtual memory , which is used in the system. As the primary memory fills up, the system moves the least-used chunks ( pages ) to secondary storage devices (to a swap file or page file ), retrieving them later when they are needed. These are the most important features of degraded performance.
Tertiary storage or tertiary memory  provides a third level of storage. Typically, it involves a robotic mechanism that will mount (insert) and dismount removable mass storage media into a storage device according to the system’s demands; These data are often copied to secondary storage before use. It is primarily used for secondary storage (eg 5-60 seconds vs. 1-10 milliseconds). This is primarily useful for large data stores, accessed without human operators. Typical examples include tape libraries and optical jukeboxes .
When a computer needs to read information from the tertiary storage, it will first consult a catalog database to determine qui tape or disc contains the information. Next, the computer will be instructed to robotic arm to fetch the medium and place it in a drive. When the computer has finished reading the information, the robotic arm will return the medium to its place in the library.
Tertiary storage is also known as nearline storage because it is “near to online”. The formal distinction between online, nearline, and offline storage is: 
- Online storage is immediately available for I / O.
- Nearline storage is not immediately available, but can be made online quickly without human intervention.
- Offline storage is not immediately available, and requires some human intervention to become online.
For example, always-on spinning hard disk drives are online storage, while spinning drives that spin down automatically, such as in massive arrays of idle disks ( MAID ), are nearline storage. Removable media such as tape cartridges that can be automatically loaded, are in tape libraries , are nearline storage, while tape cartridges that must be manually loaded are offline storage.
Off-line storage is a computer data storage on a medium or a device that is not under the control of a processing unit .  The medium is recorded, usually in a secondary or tertiary storage device, and then physically removed or disconnected. It must be inserted by a human operator before a computer can access it again. Unlike tertiary storage, it can not be accessed without human interaction.
Off-line storage is used to transfer information , since it can be easily transported. Additionally, in the case of a disaster, the destruction of the original data, a medium in a remote location will probably be unaffected, enabling disaster recovery . Off-line storage increases general information security , since it is inaccessible from a computer, and data confidentiality or integrity can not be affected by computer-based attack techniques. Also, if the information stored for archival purposes is rarely accessed, it is less expensive than tertiary storage.
In modern personal computers, the most secondary and tertiary storage media are also used for off-line storage. Optical discs and flash memory devices are most popular, and much more. In enterprise uses, magnetic tape is predominant. Older examples are floppy disks, Zip disks, or punched cards.
Characteristics of storage
Storage technologies can be differentiated by a number of different aspects. These core characteristics are volatility, mutability, accessibility, and addressability. For any particular implementation of any storage technology, the characteristics worth measuring are capacity and performance.
Non-volatile memory retains the stored information.  It is suitable for long-term storage of information. Volatile memory requires constant power to maintain the stored information. The fastest memory technologies are volatile ones, which is not a universal rule. Since the primary storage is required, it predominantly uses volatile memory.
Dynamic random-access memory is a form of memory that also requires periodic rewriting, or refreshed , otherwise it would vanish. Static random-access memory is a form of volatile memory; it loses its content when the power supply is lost.
An uninterruptible power supply (UPS) can be used to provide a computer that can not be used to recover energy. Some systems, for example EMC Symmetrix , have integrated batteries that maintain volatile storage for several minutes.
- Read / write storage or mutable storage
- Allows information to be overwritten at any time. A computer without some amount of read / write storage for primary storage would be useless for many tasks. Modern computers typically use read / write storage for secondary storage.
- Read only storage
- Retains the information stored at the time of manufacture, and write once ( read once in a while ). These are called immutable storage . Immutable storage is used for tertiary and off-line storage. Examples include CD-ROMand CD-R .
- Slow write, fast read storage
- Read / write storage which allows you to read multiple times, but with the operation being much easier than the read operation. Examples include CD-RW and swayne memory
- Random access
- Any location in storage can be accessed at any time in the same amount of time. Such characteristic is well suited for primary and secondary storage. Random access and random access.
- Sequential access
- The accessing of pieces of information in a serial order, one after the other; the time to access a particular piece of information. Such characteristic is typical of off-line storage.
- Each individually accessible unit of information is stored with its numerical memory address . In modern computers, location-addressable storage usually limits to primary storage, accessed internally by computer programs, since location-addressability is very efficient, but burdensome for humans.
- Addressable file
- Information is divided into files of variable length, and a Particular file is selected with human-readable directory and file names. The Underlying device is still location-addressable, order the operating system of a computer Provides the file system abstraction to make the operation more understandable. In modern computers, secondary, tertiary and off-line storage.
- Each individually accessible information is stored on the basis of the contents stored there. Content-addressable storage can be implemented with software (computer program) or hardware (computer device), with hardware being faster but more expensive option. Hardware content addressable memory is often used in a computer’s CPU cache .
- Raw capacity
- The total amount of stored information that can hold. It is expressed as a quantity of bits or bytes (eg 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 (eg 1.2 megabytes per square inch).
- The time it takes to access a particular location in storage. The 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.
- The rate at which information can be read from or written to the storage. In computer data storage, these terms are usually expressed in 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.
- The size of the largest “chunk” of data that can be easily accessed as a single unit, but without introducing additional latency.
- The probability of spontaneous rate change under various conditions, or overall failure rate .
- Storage devices that reduce fan usage, can be reduced by 90%. 
- 2.5-inch hard disk drive often consumes less power than larger ones.   Low capacity solid-state drives have no moving parts and consume less power than hard disks.    Also, remember that  Large caches, which are used to avoid hitting memory wall , may also consume a large amount of power. 
As of 2011 , the most commonly used data storage media are semiconductor, magnetic, and optical. Some other fundamental storage technologies have been used in the past or are proposed for development.
Semiconductor memory uses semiconductor -based integrated circuits to store information. A semiconductor chip can contain millions of tiny transistors or capacitors . Both volatile and nonvolatile forms of semiconductor memory exist. In modern computers, primary storage almost always consists of dynamic volatile semiconductor memory or dynamic random-access memory . Since the turn of the century, a type of non-volatile semiconductor memory known as flash memoryhas steadily gained share as offline storage for home computers. Non-volatile semiconductor memory is also used for secondary storage in various advanced electronic devices and specialized computers that are designed for them.
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 or more traditional HDD.     
Magnetic storage uses different patterns of magnetization was magnetically coated area to store information. Magnetic storage is non-volatile . The information is accessed by one or more read / write heads which may contain one or more recording transducers. A read / write head only covers a part of the surface so that the head or medium or both must be moved relative to another in order to access data. In modern computers, magnetic storage will take these forms:
- 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 magnetic memory , or core memory , core rope memory , thin-film memory and / or twistor memory .
- Tertiary (eg NCR CRAM ) or off line storage in the form of magnetic cards.
- Magnetic tape was then often used for secondary storage.
Optical storage , the typical optical disc , blinds information in the field 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 currently in common use: 
- CD , CD-ROM , DVD , BD-ROM : Read only storage, for mass distribution of digital information (music, video, computer programs)
- CD-R , DVD-R , DVD + R , BD-R : Write once storage, used for tertiary and off-line storage
- CD-RW , DVD-RW , DVD + RW , DVD-RAM , 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.
Magneto-optical disc storage is optical disc storage where the magnetic state has a ferromagnetic surface stores information. The information is read optically and written by combining magnetic and optical methods. Magneto-optical disc storage is non-volatile , sequential access , slow write, and fast read storage used for tertiary and off-line storage.
3D optical data storage has also been proposed.
Light induced magnetization has been proposed for high-speed low-energy magneto-optical storage. 
Paper data storage , typically in the form of paper tape or punched cards , has been used to store information for automatic processing, especially before general-purpose computers existed. Information was recorded by punching holes in the paper or medium and was read mechanically (or later optically). A few technologies allow people to make marks on paper that are easily read by machine-these are widely used for tabulating and grading standardized tests. Barcodes made it possible for any object to be securely attached to it.
Other storage media or substrates
- Vacuum tube memory
- A tube used in cathode ray tube , and a tube used in a large vacuum tube to store information. These primary storage devices were short-lived in the market, since Williams tube was unreliable and the Selectron tube was expensive.
- Electro-acoustic memory
- Delay line memory used sound waves in a substance like mercury to store information. Delay line memory was dynamic volatile, sequential read / write storage cycle, and was used for primary storage.
- Optical tape
- It is a medium for optical storage that can be read and written. It shares some technologies with movie film and optical discs, but is compatible with neither. The motivation behind developing this technology is the possibility of greater capabilities than either magnetic tape or optical discs.
- Phase-change memory
- uses different mechanical phases of phase-change material in an XY addressable matrix, and reads the information by observing the varying electrical resistance of the material. Phase-change memory would be nonvolatile, random-access read / write storage, and might be used for primary, secondary and off-line storage. Most rewritable and many write optical disks.
- Holographic data storage
- blinds information optically inside crystals gold photopolymers . Holographic storage can be used to reduce the size of the storage space, which is limited to a small number of surface layers. Holographic storage would be non-volatile, sequential access, and write or read / write storage. It could be used for secondary and off-line storage. See Holographic Versatile Disc (HVD).
- Molecular memory
- blinds in polymer that can store electric charge. Molecular memory might be particularly suited for primary storage. The theoretical storage capacity of molecular memory is 10 terabits per square inch. 
- Magnetic photoconductors
- which can be modified by low-light illumination. 
- blinds information in DNA nucleotides . It was first done in 2012 when researchers achieved a rate 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.    
Malfunction can be solved by error detection and correction mechanisms (see above). The following solutions are commonly used and valid for most storage devices:
- Device mirroring (replication) – A common solution to the problem is always present in a specific device (typically of a same type). The downside is that this double the storage, and both devices (copies) need to be updated with some overhead and possibly some delays. The upside is possible concurrent read of a data by two independent processes, which increases performance. When one of the devices is detected, the other copy is still operational, and is used to generate a new copy on another device (usually available in a pool of stand-by devices for this purpose).
- Redundant Array of Independent Disks ( RAID ) – This method is used in a group of devices that are not compatible with the content of the device (RAID mirroring is N = 2). RAID groups of N = 5 or N = 6 are common. N> 2 storage saves, when comparing with N = 2, and the performance of 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 then the RAID group is completely recovered 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 group is much smaller (ie, the probability squared, ie, multiplied by itself). If a database can not tolerate even such 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 also recovery from disasters (see disaster recovery above).
A secondary or tertiary storage can connect to a computer utilizing computer networks . This concept does not pertain to primary storage, which is shared between multiple processors to a lesser degree.
- Direct-attached storage (DAS) is a traditional mass storage, which does not use any network. This is still a most popular approach. This has been recently cornered, together with NAS and SAN.
- Network-attached storage (NAS) is a mass storage device that is connected to a network at a local area network , or a private wide area network , or in the case of online storage , over the Internet . NAS is commonly associated with the NFS and CIFS / SMB protocols.
- Storage area network (SAN) is a specialized network, which provides other computers with storage capacity. The crucial difference between NAS and SAN is that of SANs and SANs, and provides SANs with access to block-addressing (raw) levels. SAN is commonly associated with Fiber Channel networks.
Large quantities of individual magnetic tapes, and optical or magneto-optical discs can be stored in robotic tertiary storage devices. In tape storage field Known As They Are tape libraries , and optical storage field in optical jukeboxes , or optical disk libraries per analogy. Smallest forms of technology Either Containing just one drive device are Referred to as autoloaders or autochangers .
Robotic-access storage devices can have a number of slots, each holding individual media, and usually one or more picking robots that crosses the slots and load media to built-in drives. The arrangement of the slots Important features of such storage are possible expansion options: adding slots, modules, drives, robots. Tape libraries May-have from 10 to more than 100,000 slots, and Provide terabytes or petabytes of near-line information. Optical jukeboxes are somewhat smaller solutions, up to 1,000 slots.
Robotic storage is used for backups , and for high-capacity archives in imaging, medical, and video industries. Hierarchical storage management is one of the most known archiving strategies of automatically migrating files from fast hard disk storage to libraries or jukeboxes. If the files are needed, they are retrieved back to disk.
Primary storage topics
- Aperture (computer memory)
- Dynamic random-access memory (DRAM)
- Memory latency
- Mass storage
- Memory cell (disambiguation)
- Memory management
- Dynamic memory allocation
- Memory leak
- Virtual memory
- Dynamic memory allocation
- Memory protection
- Page address register
- Static random-access memory (SRAM)
- Stable storage
Secondary, tertiary and off-line storage topics
- Cloud storage
- Data deduplication
- Data proliferation
- Data storage tag used for capturing research data
- File system
- List of file formats
- Flash memory
- Information repository
- Removable media
- Solid-state drive
- Virtual tape library
- Wait state
- Write buffer
- Write protection
- Noise-predictive maximum-likelihood detection
- Object (-based) storage
Data storage conferences
- Storage Networking World
- Storage World Conference
- USENIX Conference on File and Storage Technologies
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