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Computer Storage

1. Introduction: 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 or linked to the CPU by a USB port.

2. Terminology Related to Computer Storage: 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: The term bit is a portmanteau of binary digit. 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: Byte is the abbreviated form of binary term. The byte is the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit of memory in many computer architectures. A group of 8 bits is called a byte. There can be 256 different combinations possible in Byte (8 bit). So, binary system, in simple-
Bit = “0” and “1”.
1 Byte = 8 bits = 1 character.
1024 Kibibyte (KiB) / Kilobyte (KB)
1024 Mebibyte (MiB) / Megabyte (MB)
1024 Gibibyte (GiB) / Gigabyte (GB) = 1 TiB
1024 Tebibyte (TiB) = 1 PiB
1024 Pebibyte (PiB) = 1 EiB
1024 Exbibyte (EiB) = 1 ZiB
1024 Zebibyte (ZiB) = 1 YiB
1024 Yobibyte (YiB), 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. 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.

3. Types of Computer 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 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).

b) Secondary and Off-Line Storage: Secondary storage or external memory or auxiliary memory or mass storage devices 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.

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.

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: Optical disk, Zip disks, etc..

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. 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 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.

F) Based on Technologies, Devices and Media: Previously 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 electrically (or, later, optically) sensing whether a particular location on the medium was solid or contained a hole. In modern times the following types of devices are widely used.

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.

4. Commonly Used Computer Storage Media: In modern computers, Hard Disks, CDs, DVDs, memory cards, flash memory devices (pen drive) including “USB drives” are commonly used for off-line mass storage purposes. Off-line storage devices used in the past include punched cards, microforms, floppy disc, magnetic tapes and so on. Some of the commonly used computer storage Medias is described below-

a) Hard Disks: A hard disk is a fixed unit placed within the cabinet of the computer system and it cannot 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 generally non removable magnetic media and 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 1 TB is of hard disk storage is minimal for personnel computer; one can also go for even more.

b) CD: CD is an abbreviation for “Compact Disc” 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. A single, small CD-ROM disk can hold more information than 1,000 floppy disks.

c) DVD: DVD commonly known as “Digital Versatile Disc” or “Digital Video Disc”. It 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).

d) Pen Drive: USB Pen Drive is a small keyring-sized device that can be used to easily transfer files between Universal Serial Bus (USB)-compatible systems. USB flash drives are typically removable and rewritable, much smaller than a floppy disk, and most weigh less than an ounce (30 g). Storage capacities can range from 1 GB to 256 GB with steady improvements in size and price per capacity. Some allow 1 million write or erase cycles and have 10-year data retention, connected by USB 1.1 or USB 2.0.

e) 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.

5. Need of Secondary Storage Devices: 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.

6. Conclusion: 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.

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.
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