Computer storage overview

Computer storage, computer memory, and oftencapacities shown in the diagram will inevitably be
casually memory refer to computer components,exceeded with advances in technology over time.)
devices and recording media that retain data forSecondary and off-line storage
some interval of time. Computer storage providesSecondary storage requires the computer to use its
one of the core functions of the modern computer,input/output channels to access the information, and
that of information retention. It is one of theis used for long-term storage of persistent
fundamental components of all modern computers,information. However most computer operating
and coupled with a central processing unit (CPU),systems also use secondary storage devices as
implements the basic Von Neumann computer modelvirtual memory - to artificially increase the apparent
used since the 1940s.amount of main memory in the computer. Secondary
In contemporary usage, memory usually refers to astorage is also known as "mass storage", as shown in
form of solid state storage known as random accessthe diagram above. Secondary or mass storage is
memory (RAM) and sometimes other forms of fasttypically of much greater capacity than primary
but temporary storage. Similarly, storage morestorage (main memory), but it is also much slower. In
commonly refers to mass storage - optical discs,modern computers, hard disks are usually used for
forms of magnetic storage like hard disks, and othermass storage. The time taken to access a given
types of storage which are slower than RAM, but ofbyte of information stored on a hard disk is typically
a more permanent nature. These contemporarya few thousandths of a second, or milliseconds. By
distinctions are helpful, because they are alsocontrast, the time taken to access a given byte of
fundamental to the architecture of computers ininformation stored in random access memory is
general. As well, they reflect an important andmeasured in thousand-millionths of a second, or
significant technical difference between memory andnanoseconds. This illustrates the very significant
mass storage devices, which has been blurred by thespeed difference which distinguishes solid-state
historical usage of the terms "main storage" (andmemory from rotating magnetic storage devices:
sometimes "primary storage") for random accesshard disks are typically about a million times slower
memory, and "secondary storage" for mass storagethan memory. Rotating optical storage devices, such
devices. This is explained in the following sections, inas CD and DVD drives, are typically even slower than
which the traditional "storage" terms are used ashard disks, although their access speeds are likely to
sub-headings for convenience.improve with advances in technology. Therefore, the
The fundamental components of a general-purposeuse of virtual memory, which is millions of times
computer are arithmetic and logic unit, controlslower than "real" memory, significantly degrades the
circuitry, storage space, and input/output devices. Ifperformance of any computer. Virtual memory is
storage was removed, the device we had would beimplemented by many operating systems using terms
a simple digital signal processing device (e.g. calculator,like swap file or "cache file". The main historical
media player) instead of a computer. The ability toadvantage of virtual memory was that it was much
store instructions that form a computer program, andless expensive than real memory. That advantage is
the information that the instructions manipulate isless relevant today, yet surprisingly most operating
what makes stored program architecture computerssystems continue to implement it, despite the
versatile.significant performance penalties.
A digital computer represents information using theOff-line storage is a system where the storage
binary numeral system. Text, numbers, pictures,medium can be easily removed from the storage
audio, and nearly any other form of information candevice. Off-line storage is used for data transfer and
be converted into a string of bits, or binary digits,archival purposes. In modern computers, CDs, DVDs,
each of which has a value of 1 or 0. The mostmemory cards, flash memory devices including "USB
common unit of storage is the byte, equal to 8 bits.drives", floppy disks, Zip disks and magnetic tapes
A piece of information can be manipulated by anyare commonly used for off-line mass storage
computer whose storage space is large enough topurposes. "Hot-pluggable" USB hard disks are also
accommodate the corresponding data, or the binaryavailable. Off-line storage devices used in the past
representation of the piece of information. Forinclude punched cards, microforms, and removable
example, a computer with a storage space of eightWinchester disk drums.
million bits, or one megabyte, could be used to edit aTertiary and database storage
small novel.Tertiary storage is a system where a robotic arm will
Various forms of storage, based on various natural"mount" (connect) or "dismount" off-line mass
phenomena, have been invented. So far, no practicalstorage media (see the next item) according to the
universal storage medium exists, and all forms ofcomputer operating system's demands. Tertiary
storage have some drawbacks. Therefore astorage is used in the realms of enterprise storage
computer system usually contains several kinds ofand scientific computing on large computer systems
storage, each with an individual purpose, as shown inand business computer networks, and is something a
the diagram.typical personal computer user never sees firsthand.
Primary storageDatabase storage is a system where information in
Primary storage is directly connected to the centralcomputers is stored in large databases, data banks,
processing unit of the computer. It must be presentdata warehouses, or data vaults. It involves packing
for the CPU to function correctly, just as in aand storing large amounts of storage devices
biological analogy the lungs must be present (forthroughout a series of shelves in a room, usually an
oxygen storage) for the heart to function (to pumpoffice, all linked together. The information in database
and oxygenate the blood). As shown in the diagram,storage systems can be accessed by a
primary storage typically consists of three kinds ofsupercomputer, mainframe computer, or personal
storage:computer. Databases, data banks, and data
Processor registers are internal to the centralwarehouses, etc, can only be accessed by authorized
processing unit. Registers contain information that theusers.
arithmetic and logic unit needs to carry out theNetwork storage
current instruction. They are technically the fastest ofNetwork storage is any type of computer storage
all forms of computer storage, being switchingthat involves accessing information over a computer
transistors integrated on the CPU's silicon chip, andnetwork. Network storage arguably allows to
functioning as electronic "flip-flops".centralize the information management in an
Cache memory is a special type of internal memoryorganization, and to reduce the duplication of
used by many central processing units to increaseinformation. Network storage includes:
their performance or "throughput". Some of theNetwork-attached storage is secondary or tertiary
information in the main memory is duplicated in thestorage attached to a computer which another
cache memory, which is slightly slower but of muchcomputer can access at file level over a local-area
greater capacity than the processor registers, andnetwork, a private wide-area network, or in the case
faster but much smaller than main memory. Multi-levelof online file storage, over the Internet.
cache memory is also commonlyStorage area network provides other computers
used—"primary cache" being smallest,with storage capacity over a network, the crucial
fastest and closest to the processing device;difference between network-attached storage (NAS)
"secondary cache" being larger and slower, but stilland storage area Networks (SAN) is the former
faster and much smaller than main memory.presents and manages file systems to client
Main memory contains the programs that arecomputers, whilst a SAN provides access to disks at
currently being run and the data the programs areblock addressing level, leaving it to attaching systems
operating on. In modern computers, the mainto manage data or file systems within the provided
memory is the electronic solid-state random accesscapacity.
memory. It is directly connected to the CPU via aNetwork computers are computers that do not
"memory bus" (shown in the diagram) and a "datacontain internal secondary storage devices. Instead,
bus". The arithmetic and logic unit can very quicklydocuments and other data are stored on a
transfer information between a processor registernetwork-attached storage.
and locations in main storage, also known as aConfusingly, these terms are sometimes used
"memory addresses". The memory bus is also calleddifferently. Primary storage can be used to refer to
an address bus or front side bus and both busses arelocal random-access disk storage, which should
high-speed digital "superhighways". Access methodsproperly be called secondary storage. If this type of
and speed are two of the fundamental technicalstorage is called primary storage, then the term
differences between memory and mass storagesecondary storage would refer to offline,
devices. (Note that all memory sizes and storagesequential-access storage like tape media.