Computer data storage

1 GiB of SDRAM mounted in a personal computer. An example of primary storage.

40 GB hard disk drive (HDD); when connected to a computer it serves as secondary storage.

160 GB SDLT tape cartridge, an example of off-line storage. When used within a robotic tape library, it is classified as tertiary storage instead.

Computer data storage, computer memory, and often casually storage or memory refer to computer components, devices and recording media that retain digital data used for computing for some interval of time. Computer data storage provides one of the core functions of the modern computer, that of information retention. It is one of the fundamental components of all modern computers, and coupled with a central processing unit (CPU, a processor), implements the basic computer model used since the 1940s.

In contemporary usage, memory usually refers to a form of semiconductor storage known as random access memory (RAM) and sometimes other forms of fast but temporary storage. Similarly, storage today more commonly refers to mass storage - optical discs, forms of magnetic storage like hard disks, and other types slower than RAM, but of a more permanent nature. Historically, memory and storage were respectively called primary storage and secondary storage.

The contemporary distinctions are helpful, because they are also fundamental to the architecture of computers in general. As well, they reflect an important and significant technical difference between memory and mass storage devices, which has been blurred by the historical usage of the term storage. Nevertheless, this article uses the traditional nomenclature.

Purpose of storage

Various forms of storage, based on various natural phenomena, have been invented. So far, no practical universal storage medium exists, and all forms of storage have some drawbacks. Therefore a computer system usually contains several kinds of storage, each with an individual purpose.

A digital computer represents information using the binary numeral system. Text, numbers, pictures, audio, and nearly 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 whose storage space is large enough to accommodate the binary representation of the piece of information, or simply data. For example, using eight million bits, or about one megabyte, a typical computer could store a small novel.

Traditionally the most important part of every computer is the central processing unit (CPU, or simply a processor), because it actually operates on data, performs any calculations, and controls all the other components.

Processor without a memory would not be a computer, merely a simple digital signal processing device, able to perform a fixed operation and immediately output the result. It would have to be re-built to change its behaviour, like in case of a calculator. The ability to store and change both instructions and data, the important von Neumann's idea, makes computers versatile. It basically introduces the concept of computer programming, as opposed to re-building the hardware.

A computer can exist that uses the single type of storage for all the data. However, to provide acceptable computer performance at a lower cost, computers usually use a whole storage hierarchy. The traditional division of storage to primary, secondary, tertiary and off-line storage is based on the speed and cost per bit. The lower a storage is in hierarchy, the bigger is its distance from the CPU.

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/output devices. Technology and capacity as in common home computers around 2005.

Primary storage

Direct links to this section: Primary storage, Main memory.

Primary storage, presently known as memory, is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them. Any data actively operated on is also stored there in uniform manner.

Historically, early computers used delay lines, Williams tubes, or rotating magnetic drums as primary storage. By 1954, those unreliable methods were mostly replaced by magnetic core memory, which was still rather cumbersome. Undoubtedly, a revolution was started with the invention of a transistor, that soon enabled then-unbelievable miniaturization of electronic memory via solid-state silicon chip technology.

This led to a modern random access memory (RAM). It is small-sized, light, but quite expensive at the same time. It also loses the stored information when not electrically powered—it is volatile.

As shown in the diagram, traditionally there are two more sub-layers of the primary storage, besides main large-capacity RAM:

• Processor registers are located inside every processor. Each of the registers holds only several bits of data, for example 64 bits. The arithmetic and logic unit uses this data to carry out the current instruction. Registers are technically the fastest of all forms of computer data storage, being switching transistors integrated on the CPU's chip, and functioning as electronic "flip-flops".
• Processor cache is an intermediate stage between ultra-fast registers and much slower main memory. It's introduced solely to increase performance of the computer. Most actively used information in the main memory is just duplicated in the cache memory, which is faster, but of much lesser capacity. On the other hand it is much slower, but much larger 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 CPU via a memory bus, or front side bus, a high-speed digital "superhighway". It is actually comprised of two buses (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, that indicates the desired location of data. Then it reads or writes the data itself 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.

Any data stored in the RAM is cleared as soon as it receives no electricity. Therefore, if a computer contained only RAM, the CPU would not have a source to read any instructions from in order to start the computer. Hence, a non-volatile primary storage containing a small startup program (BIOS) is used, both to perform a hardware power-on self test, and to bootstrap, that is, to read the larger program from non-volatile secondary storage to RAM and execute it. A non-volatile technology used for this purpose is commonly, but somewhat confusingly, called read-only memory (ROM).

Current ROM implementation, an EEPROM, is not literally read only, an update is possible; however it is slow and memory is completely erased before it can be re-written. Some embedded systems work without RAM, exclusively on ROM, because their programs and data are rarely changed. Standard computers do not store non-rudimentary programs in ROM, rather use large capacities of secondary storage, which is non-volatile as well, 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

A hard disk drive with protective cover removed.

Direct link to this section: Secondary storage.

Secondary storage, or storage in popular usage, differs from primary storage in that it is not directly accessible by the CPU. The computer usually uses its input/output channels to access secondary storage and transfers desired data using intermediate area in primary storage. Secondary storage does not lose the data when the device is powered down—it is non-volatile. Per unit, it is typically also an order of magnitude less expensive than primary storage. Consequently, modern computer systems typically have an order of magnitude more secondary storage than primary storage and data is kept for a longer time there.

In modern computers, hard disks 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 thousand-millionths of a second, or nanoseconds. This illustrates the very 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 DVD drives, typically have somewhat longer access times than hard disks.

Some other examples of secondary storage technologies are: flash memory (e.g. USB sticks or keys), floppy disks, magnetic tape, paper tape, punch cards, standalone RAM disks, and Zip drives.

The secondary storage is often formatted according to a filesystem format, which provides the abstraction necessary to organize data into files and directories, providing also additional information (called metadata) describing the owner of a certain file, the access time, the access permissions, and other information.

Most computer operating systems use the concept of virtual memory, allowing utilization of more primary storage capacity than is physically available 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. As more of these retrievals from slower secondary storage are necessary, the more the overall system performance is degraded.

Tertiary storage

Large tape library. Tape cartridges placed on shelves in the front, robotic arm moving in the back. Visible height of the library is about 180 cm.

Tertiary storage or tertiary memory, is a system where a robotic arm will mount (insert) and dismount removable mass storage media into a storage device according to the system's demands. It is primarily cheese used for archival of rarely accessed information, since it is much slower than secondary storage (e.g. 5-60 seconds vs. 1-10 milliseconds). This is primarily useful for extraordinarily 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 which tape or disc contains the information. Next, the computer will instruct a 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.

Off-line storage

Off-line storage, also known as disconnected 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 or connected by a human operator before a computer can access it again. Unlike tertiary storage, it cannot be accessed without human interaction.

Off-line storage is used to transfer information, since the detached medium can be easily physically transported. Additionally in case a disaster, for example a fire, destroys the original data, a medium in a remote location will be probably unaffected, enabling disaster recovery. Off-line storage increases a general information security, since it is physically unaccessible from a computer, and data confidentiality or integrity cannot be affected by computer-based attack techniques. Also, if the information stored for archival purposes is accessed seldom or never, off-line storage is less expensive than tertiary storage.

In modern personal computers, most secondary and tertiary storage media are also used for off-line storage. Optical discs and flash memory devices are most popular, and to much lesser extent removable hard disk drives. In enterprise uses, magnetic tape is predominant. Older examples are floppy disks, Zip disks, or punched cards.

Characteristics of storage

A 1GB DDR RAM memory module

There are also other ways to characterize various types of storage.

Volatility of information

Main article: Volatile memory

• Non-volatile memory will retain the stored information even if it is not constantly supplied with electric power. It is suitable for long-term storage of information. Nowadays used for most of secondary, tertiary, and off-line storage. In 1950s and 1960s, it was also used for primary storage, in the form of magnetic core memory.
• Volatile memory requires constant power to maintain the stored information. The fastest memory technologies of today are volatile ones (not a universal rule). Since primary storage is required to be very fast, it predominantly uses volatile memory.
  • Dynamic memory, is a kind of volatile memory which also requires the stored information to be periodically re-read and re-written, or refreshed, otherwise it would vanish.

Ability to access non-contiguous information

• Random access means that any location in storage can be accessed at any moment in approximately the same amount of time. Such characteristic is well suited for primary storage.
• Sequential access means that the accessing a piece of information will take a varying amount of time, depending on which piece of information was accessed last. The device may need to seek (e.g. to position the read/write head correctly), or cycle (e.g. to wait for the correct location in a revolving medium to appear below the read/write head).

Ability to change information

• 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 purposes would be useless for many tasks. Modern computers typically use read/write storage also for secondary storage.
• Read only storage retains the information stored at the time of manufacture, and write once storage (WORM) allows the information to be written only once at some point after manufacture. These are called immutable storage. Immutable storage is used for tertiary and off-line storage. Examples include CD-R.
• Slow write, fast read storage is read/write storage which allows information to be overwritten multiple times, but with the write operation being much slower than the read operation. Examples include CD-RW.

Addressability of information

• In location-addressable storage, each individually accessible unit of information in storage is selected 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.
• In file system storage, 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, but 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 use file systems.
• In content-addressable storage, each individually accessible unit of information is selected with a hash value, or a short identifier with number? pertaining to the memory address the information is stored on. Content-addressable storage can be implemented using software (computer program) or hardware (computer device), with hardware being faster but more expensive option.

Capacity and performance

• Storage capacity is 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).
• Storage density refers to 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 is 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, and in case of sequential access storage, minimum, maximum and average latency.
• Throughput is 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 or 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.

Fundamental storage technologies

Semiconductor storage

Semiconductor memory uses semiconductor-based integrated circuits to store information. A semiconductor memory chip may contain millions of tiny transistors or capacitors. Both volatile and non-volatile forms of semiconductor memory exist. In modern computers, primary storage almost exclusively 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 memory has steadily gained share as off-line storage for home computers. Non-volatile semiconductor memory is also used for secondary storage in various advanced electronic devices and specialized computers.

Magnetic storage

Magnetic storage uses different patterns of magnetization on a magnetically coated surface to store information. Magnetic storage is non-volatile. The information is accessed using one or more read/write heads. Since the read/write head only covers a part of the surface, magnetic storage is sequential access and must seek, cycle or both. In modern computers, the magnetic surface will take these forms:

• Magnetic disk
  • Floppy disk, used for off-line storage
  • Hard disk, used for secondary storage
• Magnetic tape data storage, used for tertiary and off-line storage

In early computers, magnetic storage was also used for primary storage in a form of magnetic drum, or core memory, core rope memory, thin film memory, twistor memory or bubble memory. Also unlike today, magnetic tape was often used for secondary storage.

Optical storage

Optical storage, in the case of typical Optical discs, uses tiny pits etched on the surface of a circular disc to store information, and reads this information by illuminating the surface with a laser diode and observing the reflection. Optical disc storage is non-volatile and sequential access. The following forms are currently in common use:

• CD, CD-ROM, DVD: Read only storage, used for mass distribution of digital information (music, video, computer programs)
• CD-R, DVD-R, DVD+R: Write once storage, used for tertiary and off-line storage
• CD-RW, DVD-RW, DVD+RW, DVD-RAM: Slow write, fast read storage, used for tertiary and off-line storage

Magneto-optical disc storage is optical disc storage where the magnetic state on 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, fast read storage used for tertiary and off-line storage.

3D optical data storage has also been proposed.

Other early methods

Paper data storage

Main article: Paper data storage

Paper tape and punch cards have been used to store information for automatic processing since the 1890s, long before general-purpose computers existed. Information was recorded by punching holes into the paper or cardboard medium, and was read by mechanically (or, later, optically) sensing whether a particular location on the medium was solid or contained a hole.

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 Williams tube was unreliable and 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.

Proposed methods

Phase-change memory uses different mechanical phases of phase change material to store information, and reads the information by observing the varying electric 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.

Holographic 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 polymers that can store electric charge. Molecular memory might be especially suited for primary storage.

Related technologies

Network connectivity

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 in a much 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 term was coined lately, together with NAS and SAN.
• Network-attached storage (NAS) is mass storage attached to a computer which another computer can access at file level over a local-area network, a private wide-area network, or in the case of online file storage, over the Internet.
  • Network computers are computers that do not contain internal mass storage devices. Instead, all data are stored exclusively on a NAS.
• Storage area network (SAN) is a specialized network, that provides other computers with storage capacity. The crucial difference between NAS and SAN is the former presents and manages file systems to client computers, whilst a latter provides access at block-addressing (raw) level, leaving it to attaching systems to manage data or file systems within the provided capacity.

Robotic storage

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 tape libraries, and in optical storage field optical jukeboxes, or optical disk libraries per analogy. Smallest forms of either technology containing just one drive device are referred to as autoloaders or autochangers.

Robotic-access storage devices may have a number of slots, each holding individual media, and usually one or more picking robots that traverse the slots and load media to built-in drives. The arrangement of the slots and picking devices affects performance. Important characteristics 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 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.

See also

Primary storage topics

• Data proliferation
• Memory management
  • Virtual memory
  • Memory allocation
    • Dynamic memory
    • Memory leak
• Memory protection
• Dynamic random access memory (DRAM)
• Static random access memory (SRAM)

Secondary, tertiary and off-line storage topics

• Data proliferation
• File system
  • List of file formats
• Flash memory
• Information repository
• Solid state drive
• Virtual Tape Library
• Wait state
• Write protection

Data storage conferences

• Storage Decisions
• Storage Networking World
• Storage World Conference

References

• Estimates of the quantities of data contained by the various media
• Early storage media explained