Computer Storage: The fundamental components of a general-purpose computer are Input Unit, Central Processing Unit (CPU) and Output Unit. The CPU consists of Arithmetic and Logic Unit (ALU), Control Unit and Memory. If memory is removed, the device we had would be a simple digital signal processing device (e.g. calculator, media player) instead of a computer.
Computer
storage, computer memory, and often casually memory refer to computer
components, devices and recording media that retain digital data, to be used
for computing at some interval of time. It is one of the fundamental components
of all modern computers, and coupled with a CPU. Some of the commonly
associated terminologies related to computer storage are discussed below
a) Memory
Unit: Memory is the work space area
of a computer system where data and instructions are stored. Text,
numbers, pictures, audio, and nearly any other form of information can be
converted into a binary digits i.e. ‘1’ or ‘0’ and a digital computer can
understand information only in terms of ‘0’s and ‘1’s.
i) Bit: A binary digit i.e. ‘0’s and ‘1’s is
called a bit and it can be define as an electronic signal, which is either On
‘1’ or Off ‘0’. It is also
the smallest unit of information the computer uses.
ii) Byte: A group of 8 bits is called a byte.
There can be 256 different combinations possible in Byte (8 bit) and each
character typed consumes one byte. The most common unit of storage is the byte.
Bit = “0” and
“1”.
1 Byte = 8 bits
= 1 character.
1024 Byte= 1
Kilo Byte
1024 Kilo Byte=
1 Mega Byte.
1024 Mega Byte=
1 Giga Byte, and so on.
b) Storage
Capacity: It 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. 750 megabytes).
c) Storage
Density: It 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
centimeter).
d) Latency: It is the time needed 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.
e) Throughput: It is the rate at which
information can read from or written to the storage. In computer storage,
throughput is usually expressed in terms of megabytes per second or MB/s,
though bit rate may also be used. As in the case of latency, read rate and
write rate may also be differ in throughput.
f) Word
length: The number of bits
that a computer can process at a time in parallel is called its word length. It
is nothing but the measure of the computing power of a computer. Commonly used
word lengths are 8, 16, 32 or 64 bits.
1. Storage
Media: 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 need and purpose.
A) Types of
Storage Media Based on Memory Hierarchy and Distance from CPU: Based on memory hierarchy, or distance
from the central processing unit the memory or computer storage can be
categorize as primary, secondary, tertiary and network storage.
a) Primary
Storage: Primary storage or
internal memory is directly connected to the central processing unit of the
computer. It is used to store data that is likely to be in active use and is
typically very fast, as in the case of RAM. It is present for the CPU to
function correctly. Primary storage can be accessed randomly, that is,
accessing any location in storage at any moment takes the same amount of time.
A particular location in storage is selected by its physical memory address.
That address remains the same, no matter how the particular value stored there
changes. The primary storage sometimes also refers as memory (main storage /
primary storage).
Today, primary
storage is typically random access memory, a type of semiconductor memory. The
primary storage typically consists of three kinds of storage:
The processor registers is
internal to the central processing unit. Registers contain information that the
arithmetic and logic unit needs to carry out the current instruction. They are
technically the fastest of all forms of computer storage, being switching
transistors integrated on the CPU's silicon chip, and functioning as electronic
"flip-flops".
The other two types are Cache Memory and Main
Memory.
b) Secondary
and Off-Line Storage: Secondary
storage, or external memory supplements the main memory and it requires the
computer to use its input / output channels to access the information.
Secondary storage is used for long-term storage of persistent information.
Secondary storage is also known as “mass storage devices” or “auxiliary memory”
and is much slower then primary memory.
The need of secondary storage devices are
felt due to the following reasons-
i) Limited
storage capacity in primary storage devices: The capacity of primary storage
devices is limited so to store data and programs that are too large to fit into
the random-access memory at one time, we require the auxiliary storage devices.
ii) To make
blank spaces in primary storage devices: If the capacity of the primary storage
devices becomes full of information then we will be unable to use it for our
day to day activities.
iii) For easy
transportation: Primary memories are not portable in nature, so for easy
transportation we require auxiliary storage devices.
iv) Security
against physical calamities: If primary memory is destroyed due to some
physical calamities or by some accident computer crashes and the data in it
cannot be recovered then the data in the secondary storage devices can be used
for backup utility.
v) Multiple
copies: To obtain multiplied copy of our information we require secondary
storage devices.
Some of the characteristics of secondary storage
devices are-
i) Storage
medium can be easily removed from the computer system.
ii) It uses
input / output channel of the computer system to access the information.
iii) Mainly used
for data transfer and archival purposes.
iv) The cost of
secondary memory is very less as compared to primary memory.
v) The secondary storage is more
permanent in nature, non volatile and secures method for storing programs and
data compared to RAM memory.
vi) The
Secondary or mass storage is typically of much greater capacity than primary
storage (main memory).
In
modern computers, Hard Disks, CDs, DVDs, memory cards, flash memory devices
including "USB drives", Zip disks and magnetic tapes are commonly
used for off-line mass storage purposes. "Hot-pluggable" USB hard
disks are also available. Off-line storage devices used in the past include
punched cards, microforms, and removable Winchester disk drums.
i) Hard
Disks: A hard disk is a fixed
unit placed within the cabinet of the computer system and it can not be removed
like a CD. It consists of rigid circular platters of magnetizable material
sealed in a metal box with associated read/write heads. In modern computers,
hard disks are usually used for mass storage and it is no removable magnetic
media as it is usually internal to the computer. 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. So, hard disks are typically
about a million times slower than memory. This also illustrates the very
significant speed difference which distinguishes solid-state memory from
rotating magnetic storage devices. In today’s context 160 GB is of hard disk
storage is minimal for personnel computer; one can also go for even 1
terabytes.
ii) Optical
Storage Devices: The computer
storage devices in which data is placed and / or retrieved by means of a
focused optical beam is called as optical storage device. Optical technology
involves the use of lasers (i.e. by burning microscopic “pits” to represent 1s
and 0s) to enable or recode the data from an optical laser disk. In case of
optical disk nothing touches the encoded portion and so not worm out by the
playing process and last long. Rotating optical storage devices, such as CD and
DVD drives, are typically even slower than hard disks, although their access
speeds are likely to improve with advances in technology. The term Worm drives/
device, optical disk libraries, jukebox / optical jukebox, write once read only
device/ drives etc. are synonymous with optical storage devices.
- Optical
Disk: The laser optical disks
are hard metal disk ranging in size from 4.72 inches to 14 inches. They were
originally developed as a compact disk for audio and video application. Most
optical disks are read only storage devices. Eg. of optical disk includes
Jukebox, DVD, CD, etc.
- Jukebox: A jukebox is an optical disk
device that can automatically load and unload optical disk and provide as much
as 500 GB of near line information. The device is often called as optical disk
– libraries, Robotic drives or autochangers.
- DVD: DVD (commonly known as "Digital
Versatile Disc" or "Digital Video Disc") is an optical disc
storage media format that can be used for data storage, including movies with
high video and sound quality. DVDs resemble Compact Discs as their diameter is
the same (120 mm (4.72 inches) or occasionally 80 mm (3.15 inches) in
diameter), but they are encoded in a different format and at a much higher
density. A DVD typically may contain at least 4.4 GiB of data, nearly 7 times
the amount of a CD-ROM and about 20 times faster than a CD-ROM. There are
several possible successors to DVD being developed by different consortiums:
Sony/Panasonic's Blu-ray Disc (BD), Toshiba's HD DVD and Maxell's Holographic
Versatile Disc (HVD). However, as reported in a mid 2005 issue of Popular
Mechanics, it is not yet clear which technology will win the format war over
DVD. HD DVD discs have a lower capacity than Blu-ray Discs (15 GB vs. 25 GB for
single layer, 30 GB vs. 50 GB for dual layer). Other speculations as to which
format will win include Blu-ray Disc's larger hardware vendor and movie studio
support, and HD-DVD's faster read times.
- CD-ROM: CD-ROM is an abbreviation for
"Compact Disc Read-only memory. It is a small plastic disk used to store
information digitally. The disk is covered with a transparent plastic coating
and is played on machine that uses laser to read the pattern of pitted and
unpitted areas on the disk’s surface. Since nothing touches the encoded portion
the CD’s is not worn out by the playing process. It is possible to produce
composite CDs containing both data and audio with the latter capable of being
played on a CD player, whilst data or perhaps video can be viewed on a
computer. These are called Enhanced CDs. The standard CD-ROM holds 650 or 700
MiB of data. The CD-ROM is popular for distribution of software, especially multimedia
applications, and large databases. A CD weighs less than 30 grams.
- Optical
Tape: The optical tape is
similar in appearance to a magnetic tape, but data are stored by optical laser
technique. Like other optical media the optical tape is also read only data
storage device.
- Optical
Card: Optical cards are also
called as laser card. It is in the size of a credit card and has an optical
laser encoded strip that can store approximately 2 MB of data.
- USB
Pen Drive: USB Pen Drive is a small keyring-sized device that
can be used to easily transfer files between USB-compatible systems. It comes
with a very different size and capacities.
c) Tertiary
and Database Storage: Database
storage is a system where information in computers is stored in large databases,
data banks, data warehouses, or data vaults. It involves packing and storing
large amounts of storage devices throughout a series of shelves in a room,
usually an office, all linked together. The information in database storage
systems can be accessed by a supercomputer, mainframe computer, or personal
computer. Databases, data banks, and data warehouses, etc, can only be accessed
by authorized users. In Tertiary or database storage a robotic arm will
"mount" (connect) or "dismount" off-line mass storage media
according to the computer operating system's demands. Tertiary storage is used
in the realms of enterprise storage and scientific computing on large computer
systems and business computer networks, and is something a typical personal
computer user never sees firsthand.
d) Network
Storage: Network storage is
any type of computer storage that involves accessing information over a
computer network. Network storage arguably allows to centralize the information
management in an organization, and to reduce the duplication of information.
Network storage includes:
i)
Network-Attached Storage: It is secondary or tertiary
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.
ii) Storage
Area Network: It provides other computers with storage capacity over a
network, the crucial difference between Network-Attached Storage (NAS) and
Storage Area Networks (SAN) is the former presents and manages file systems to
client computers, whilst a SAN provides access to disks at block addressing
level, leaving it to attaching systems to manage data or file systems within
the provided capacity.
iii) Network
Computers: Network computers
are computers that do not contain internal secondary storage devices. Instead,
documents and other data are stored on a network-attached storage.
Confusingly
sometimes primary storage can be used to refer to local random-access disk
storage, which should properly be called secondary storage. If this type of
storage is called primary storage, then the term secondary storage would refer
to offline, sequential-access storage like tape media.
B) Based on
Volatility of Information: Based on
volatile and non volatile nature of information stored in the memory, memory
can be of the following types-
a) Volatile
Memory: It requires constant
power to maintain the stored information. It is typically used only for primary
storage, but at the same time primary storage is not necessarily volatile, even
though today's most cost-effective primary storage technologies are volatile. Non-volatile
technologies have been widely used for primary storage in the past and may
again be in the future.
b) Dynamic
Memory: It is volatile memory
that demands to be periodically refreshed, or read and rewritten to store
information without modifications.
c) Non-volatile
Memory: The Non volatile
memory retains the stored information even if it is not constantly supplied
with electric power. It is suitable for long-term storage of information, and
therefore used for secondary, tertiary, and off-line storage.
C) Based on Access: Based on the access provision, memory can be
divided into the following categories-
a) Random
Access: In Random access one
can access any point at random i.e. without passing through intervening points.
It means that any location in storage can be accessed at any moment without
wasting much time. This makes random access memory well suited for primary
storage.Example: Magnetic disk, Optical disk, Zip disks
b) Sequential
Access: In sequential access
the data stored in the media can only be read in sequence and to get to a
particular point on the media one has to go through all the preceding points.
It means to access a piece of information takes 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). Example includes magnetic tapes and such other media.
D) Based on
Ability to Change Information: Based
on the provision of modifying the information, computer memory can be of the
following types-
a) Read /
Write Storage, or Mutable Storage: It
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. 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.
b) Read Only
Storage: It 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.
E) Addressability
of Information: Based on the
provision of addressability of information, computer memory can be of the
following types-
a) Location-addressable
Storage: Here, 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.
b) Content-addressable
Storage: Here, 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.
c) File
System Storage: Here,
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.
a) 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 takes the forms
of 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.)
b) 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.
c) Optical
Disc Storage: Optical disks
are non-magnetic auxiliary storage devices that resemble audio compact disks.
Optical disc storage 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. It takes the forms of CD, CD-ROM (or
compact disk, read-only memory), DVD (Read only storage, used for mass
distribution of digital information such as music, video, computer programs
etc), 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), Blu-ray Disc (BD), HD DVD, Ultra Density Optical (UDO),
Professional Disc for DATA (PDD or ProDATA). Among the list Holographic
Versatile Disc (HVD), Phase-change Dual is also can be proposed. Recordable
CD-ROM disks, called WORM (write-once/read-many) are becoming an affordable
alternative to tapes and hard disk, primarily for archival storage purposes. A
single, small CD-ROM disk can hold more information than 1,000 floppy disks.
i)
Magneto-optical Disc 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.
ii) Ultra
Density Optical Disc Storage: An
Ultra Density Optical disc or UDO is a 5.25" ISO cartridge optical disc
encased in a dust-proof caddy which can store up to 30 GB of data. Utilizing a
design based on a magneto-optical disc, but utilizing phase change technology
combined with a blue violet laser, a UDO disc can store substantially more data
than a magneto-optical disc or MO, because of the shorter wavelength (405 nm)
of the blue-violet laser employed. MOs use a 650-nm-wavelength red laser.
Because its beam width is shorter when burning to a disc than a red-laser for
MO, a blue-violet laser allows more information to be stored digitally in the
same amount of space. Current generations of UDO store up to 120 GB, though up
to 500 GB has been speculated as a possibility for UDO.
iii) Optical
Jukebox Storage: Optical
jukebox storage is a robotic storage device that utilizes optical disk device
and can automatically load and unload optical disks and provide terabytes of
near-line information. The devices are often called optical disk libraries,
robotic drives, or auto changers. Jukebox devices may have up to 1,000 slots
for disks, and usually have a picking device that traverses the slots and
drives. The arrangement of the slots and picking devices affects performance,
depending on the space between a disk and the picking device. Seek times and
transfer rates vary depending upon the optical technology. Jukeboxes are used
in high-capacity archive storage environments. HSM is a strategy that moves
little-used or unused files from fast magnetic storage to optical jukebox
devices in a process called migration. If the files are needed, they are
migrated back to magnetic disk.
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