Chapter4- Managing input and output

Unit-2-Chapter-1

Secondary Storage

contentsTypes of Secondary Storage DevicesMagnetic TapeMagnetic DiskTypes of Magnetic DisksOptical DiskTypes of Optical DisksMagneto-Optical Storage Devices Universal Serial BusMemory StickMass Storage Devices

Types of secondary storage devicesSecondary storage facilitates storing the data and instructions permanently.There are to methods of accessing data from secondary storage devicesSequential accessDirect access

Sequential and direct access

Sequential accessMeans the computer system must search the storage device from the beginning until the desired data is found.Eg: magnetic tape where data is stored and processed sequentially.Suppose there is information regarding employees of organization, to look into information of employee number 100, the computer will have to start with employee number 1 and then go to 2,3 and so on till 100.This method is simple , but searching a data takes a lot of time.

Direct accessAlso known as random access.Computer can go directly to the location where the data that the user wants , are stored.Eg: magnetic disk, optical diskData is stored as sequentially numbered blocks.One can access clock 12, the 72 then 2 so on, in any fashion.Used in systems that is used for airline reservation, computer based assistance systemsNo fixed patterns of request for data

Classification of secondary storage devices

Magnetic tapeAppears as same that of music cassettes.It is plastic tape with magnetic coating on it.Data is stored in the form of tiny segments of magnetized and demagnetized portions on the surface of the material.The magnetized portion of the tape refers to the bit value 1 and demagnetized portion refers 0

They are available in various sizesMajor difference is speed at which the tape is moved past the read/write head and the tapes record density.The amount of data is the number of binary digits that can be stored on a linear inch of the tape is the recording density of the tape.They are very durable, can be erased , reusableInexpensive, reliable storage for organizing archives medium , taking backups

Not suitable for the data files that need to be revised or updated often , because data is stored in sequential manner, rewinding and advancing the tape is required for updating the data.If the tapes stretches too much, the it will render itself unusable for data storage and may result in data loss.Today primary role of tape drive is for backing up or duplicating the data stored on the hard disk to protect the system against loss of data during power failures or computer malfunctions.

Magnetic tape

Magnetic tape organization

Magnetic tape organizationDivided into vertical columns (frames) and horizontal rows (channels or tracks)Data stored in a string of frames with one character per frame, each frame spans multiple tracks.A single bit is stored in each track, one byte per frame.The remaining track will store the parity bit.When a byte is written to the tape, the number of 1s in byte is counted, the parity bit is then used to make the number of 1s even or odd.

When the tape is read again, the parity bit is checked to see if any bit is lostMagnetic tape drives two reels:Supply reel and take-up reelboth are mounted on the hubs and the tape moves from the supply reel to take-up reelThe magnetic oxide coated side of the tape of passes directly over the read/write head assembly, thus making contact with the heads.

As the tape passes directly under the read/write head, the data can be either read and transferred to the primary memory or read from the primary memory and written on to the tape.

Basic tape drive mechanism

Information format of magnetic tape

Information format of magnetic tapePhysical marking to indicate the location from where reading and writing on the tape is to begin BOT- beginning of the tapeEOT- End of the tapeUsable recording surface length between BOT and EOTBOT/EOT are usually made up of reflective type of short silver strips.These are sensed by the arrangements of lamps. Photodiodes sensors to indicate the location from where to read or write.

In the magnetic tapes the data are stored in terms of blocks where each block consists of grouping of data known as records.There is some reserved blank spaces called inter-block-gap (IBG)When one block contains more than one record, it is separated by a inter-record-gap (IRG)Whenever an IRG is reached, the moving tape is stopped until record is processed.

Advantages of magnetic tapesHold high recording density, low cost per bit storage.Have virtually unlimited storage capacityPortable, compact in size, lightweight and removableEasy to handle and storeVery inexpensive mode of offline data storage and a large amount of data can be stored in small storage space.

Disadvantages of magnetic tapesSequential in nature, not suitable where data is accessed requires a random orderSlow compared to the magnetic disksKept in dust free environmentAway from corrosive gases and chemicals, they can cause tape-reading errors.Uses parity bit, hence if minor error occurs, it is difficult to recover the data.Not flexible, updating the records is difficultThey wear out.

Magnetic diskThey offer high storage capacity and reliabilityHas capability to access the storage data directly.It consists of a plastic/metal circular plate / platter , which is coated with magnetic oxide layerData is represented as magnetized spotsMagnetized represents 1, absence of magnetized spots represents 0.To read the data, the magnetized spots on the disk are converted into electrical impulses, which are then transferred to the processor.

Writing data onto the disk is accomplished by converting the electrical impulses received by the processor into magnetized spots on the disk.The data on the disk can be erased and reused virtually infinitely.There is a protective case or cartridge to shield the disk to protect from dust and other external interference.

Magnetic disk

Storage organization of a magnetic diskIts divided into imaginary tracks and sectors.Tracks are concentric circle where data are stored, are numbered from the outermost ring to inner most ring starting with zero.Sectors refer to the number fixed-size areas that can be accessed by one of the disk drive read/write heads, in one rotation of the disk, without the head having to change the position.An intersection of a track and a disk sector is known as track sector.

Generally a disk has eight or more disks sectors per track.Disks are marked on both the surfaces, hence called double-sided disks.Each sector is assigned a disk address before a disk drive can access a piece of data.The disk address comprises a sector number, track number and surface numberThe tracks sectors are grouped into collection known as cluster. Cluster is also referred as minimum amount of disk space used by a single file.

Disk pack and cylinder

Many disks are maintained and used together to create a large disk-storage system.All the disks are stacked top of each other with a common spindle, which rotates them.There is a gap between each platters, so that read/write head can perform the operation.All the read/write of each disks are attached to a single disk arm assembly.All the read/write heads are on equal diameter track on the different platters at one time.Tracks of equal diameter on different platters form a cylinder.

Accessing data from magnetic diskData in a magnetic disk is recorded on the surface of the circular tracks with the help of read/write head, which is mounted on the access arm assembly.These heads can be in multiple numbers to access the adjacent tracks simultaneously and making a disk faster.The access arm assembly can be positioned in both inward and outward directions so that read/ write can move on the horizontal surface of the diskIn case of multiple disks, each disk surface has its own read/write head that works in harmony with other read/write heads to record the data.

The process of accessing data comprises of the following three steps.SeekThe read/write heads are positioned on the specific track on disk platter. The time taken in doing so is known as seek time.It is the average time required to move the heads to the desired track on the diskMay range between 2and 15 ms , commonly 9 ms.

RotateOnce the positioned on the desired tracks, the head of the specific platter is activated.Since the disk is rotated constantly, the head has to wait for the required sector or cluster to come under it.This waiting time is known as rotational delay time or latency of the disk.The rotational latency of disk with 7200 rpm is 4.17ms.

Data transferThe data is transferred to or from the disk to the primary memory.The rate at which the data are read per from or written to the disk is known as data transfer rate.Measured as kilobyte per seconds.If the data transfer rate per rotation will be 125x512=64000 bytesHence transfer of data in a second is 64000/60 = 6,400,000 bytes or 6.4MB

Types of magnetic disksAll the disks come in the form of round platters.Available in different sizes, shapes and designs.Some are attached to read/write head assembly whereas some are available in the form of removable disks.Broadly divided as Floppy diskHard diskZip disk

Floppy disk

Floppy disk drive

A floppy disk is round , flat piece of Mylar plastic coated with ferric oxide and encased in a protective plastic cover (disk jacket).It is a removable disk and is read and written by a floppy disk drive including reading , writing data onto the disk and rotating the disk.The disk drive read/write head alters the magnetic orientation of the particles where orientation in one direction represents 1 and orientation in other direction 0

Floppy disk

Read/ write operation of floppy diskFloppy disk has to inserted into the floppy disk driveDrive is made up of a box with a slot into which user inserts the diskWhen the user inserts the disk, the drive grabs the disk and spins inside its plastic jacketThe drive has multiple levers that get attached to the disk.One lever opens the metal plate or shutter, to expose the data access area.Other levers and gears move two read/write heads until they almost touch the diskette on both the side.

The drive circuit board receives instructions for reading/writing the data from/to the disk through FDD controller.If the data are to be written onto the disk, the circuit board first verifies that no light is visible through a small window in the floppy diskIf the photo-sensor on the opposite side of the FDD detects a beam id light, floppy drive detects to be write protected and does not allow the recording of the data

The circuit board translates the instructions into signals that control the movement of the disk and the read/write heads.A motor located beneath the disk spins a shaft that engages notch on the hub of the disk, causing the disk to spin.When the heads are in correct position, electrical impulses create a magnetic field in one of heads to write data to either the top or bottom of the diskOn reading the data, the electrical signals are sent to the computer from the corresponding magnetic field generated by the metallic particle on the disk.

Since floppy disk head touches the diskette, both media and head wear out quickly .To reduce wear and tear, personal computers retract the heads and stop rotation when a drive is not reading or writing the data.Next write/ read command is given, there is a delay of about half a second while the motor gathers maximum speed.

Hard diskAlso called as hard drive or fixed driveprimary storage unit of the computerIt consists of a stack of disk platters that are made up of aluminum alloy or glass substrate coated with a magnetic material and protective layersThey are tightly sealed to prevent any dust particles, which causes head crash from getting insideA hard disk can be internal (fixed) or external (removable) which can hold huge amount of data.

Hard disk

Laptop hard disk

Desktop hard disk

The capacity is the amount of information that a hard disk can store , measured in bytes.A typical computer comes with 80-320 GB of hard diskHard disk is measured in terms of access timeA hard disk with lower access time is faster than a hard disk with a higher access time is slower

Hard disk

Read/write operation of a hard diskA hard disk uses round , flat disks made up of a glass or metals which are coated on both sides with a special material designed to store information in the form of magnetic patterns.Each platter has its information recorded in tracks which is further broken into smaller sectors.The platters rotate at a high speed, driven by a special spindle motor connected to spindle.Special read/write devices are mounted onto sliders and used either record data onto the disk or read data from disk

The sliders are mounted onto the arms, all of which are mechanically connected to single assembly and positioned over the surface of the disk by the device called actuator.Each platter has two heads , one of the top of the platter and one on the bottom, so disk with six platters would have six surfaces and six heads.Data is recorded onto the magnetic surface of the disk in exactly same way as the floppy diskThe surface is treated as array of dot positions with each domain of magnetic polarization set to binary 1 or 0.

To read data from disk, the operating system first reads File Allocation Table (FAT) in windows operating system , at the beginning of the partition.This tells the operating system in which sector and track to find the data and then with this information, the head can read the requested data.The heads in most of the hard disk is slightly above the surface, the distance between the head and disk surface is much less compared to the thickness of human hair.

When the head accidently touch the media because of an electrical malfunction, the surface becomes scratched.Any data stored where the head has touched disk is lost, this is called head crash.

Zip disk

Zip disk drive

Zip diskIts removable storage device and it has capacity to store about 100-250 MB of dataMeasured at 18x13x4 cm and weighs about half kgIts rubber feet to stabilize the unit in either vertical or horizontal positionThe disk is made up of plastic material on which magnetic oxide particles are coated.There are two indicator lights, green for power and amber for disk access.It has eject button on ON/OFF switch.

Is similar in diameter to that of 3 inch floppy but requires a separate drive to read or write the data onto the zip disk.There are different versions of zip disk which can hold 100,250 and 750 MB of dataA zip disk can store about 70 times more than the floppy diskIt can be used to store heavy graphics , music or presentation files,

Advantages of magnetic diskThey follow direct access mode for reading or writing data onto the data files.Used for both online and offline storage of dataEasily movable from one place to another because of their small sizeData transfer rate is higher than magnetic tapes.Due to low cost and high recording densities, cost per magnetic disk is minimumThe storage capacity is unlimited, as many no. of disk required can be added.Less prone to corruption of data

Disadvantages of magnetic diskMust be stored in dust free environment in order to protect them from crashing down.Not ideal for devices that requires access required is of sequential nature rather than direct or random natureExpensive than magnetic tapesMore susceptible to breach of security and access gain to sensitive online disk files from remote terminals.

Optical diskAn optical disk is flat ,circular , plastic disk coated with a material on which bits is stored in the form of lightly reflective areas and significantly less reflective areas.From which the stored data may be read and illuminated with a narrow-beam source such as laser-diode.These disks are capable of storing a huge amount of data in limited amount of space.Consists of a rotating disk coated with a thin layer of metal that acts as a reflective surface and a laser bean , which is used to read/write head for recording the data.

Comes in various sizes: Compact disk (CD)Digital Versatile Disk (DVD)Blue-Ray disk (BD)Each requires a different type of disk drive and a disk.CD with 700MB capacity and 12 cm diameter is popular means of optical storage.Storage capacity of an optical disk is huge, the cost per bit storage is very low.

Storage organization of an optical diskOptical disks consists of a single long track in spiral .This track starts from the outer edge and spirals inward to the centre of the diskThe spiral shape of the tack makes the optical disk suitable for the storing large block of sequential data onto it such as musicIn case of magnetic disk it is easy to locate magnetic disk tracks since they are located at fixed distance from the centre.These tracks are further divided into small sectors of the same length, irrespective to the location on the disk surface

With more data recording density , the optical drives have more complicated mechanism as compared to the magnetic disk drives.Rotation speed varies inversely with the radius of the disk.The disk moves slowly when data is read near the edges and moves fast when close to the centre.Read/write operation is performed through a laser beam hence no access arm is required that is used in case of magnetic disk.

In magnetic disk tracks are arranged in the form of concentric circles, in optical disk the tracks are in spiral fashion.Accessing data from concentric circle is slower than a spiral trackGenerally the access time for optical disk is from 10 to 40 milliseconds.

Access mechanism In optical diskLaser beam technology is used for reading or writing the data on the disk surface of the optical disk, uses two laser beam sources of different intensities.The greater intensity laser beam is used to write on the recording surface by turning it ON and OFF at varying rate so that tiny pits are burnt into the metal coating of the diskThe lesser intensity laser beam is used to read the stored data, which is strongly reflected by the coated surface called land and weakly reflected by the burnt surface called pit.The change in pattern is detected by a photo sensor converted into digital signals.

Compact disk

Types of optical disk-Compact diskMost popular and cheapest optical disk in the marketUsed for storing music in the form of audio, can record upto 80min of uninterrupted playing time.Is shiny, silver color metal disk of 12 cm diameter,Made up o three layers : a polycarbonate base through which light can passA layer of aluminumProtective layer of acrylic on the top of it.

The pits are normally 0.5 microns wide, 0.83 to 3 microns long, o,15 microns deep.A CD has one track that spirals from the centre to outside the edge.This single track is divided into sectors of equal length and density, files are stored on these particular contiguous sectorsDifferent form of CDsCD-ROM (compact disk-read only memory)CD-R (compact disk-recordable)CD-RW ( compact disk rewritable)

A CD-ROM disk comes with prerecorded data by manufactured and can be read but cannot be altered.CD-R allows to record your own data, once written to it later it cannot be altered.CD-RW is a disk which is rewritable version of the CD-R that means it allows writing, erasing and rewriting of data several times.The data written onto the CD formats can be read only with the help of a CD-driveHowever, CD-R and CD-RW disks need one special peripheral known as CD writer or CD burner.

Reading data from CD-ROMA CD drive reads information from the CDs spiral track of pits and lands starting from the centre of the disk and moving to the outer edgeA light is beamed from semiconductor laser through the bottom of polycarbonate layer and the aluminum coating monitors the light being reflected. Cd is read through the bottom of the disk, each pit appears as an elevated bump to the reading light beam.Light striking the land areas is reflected normally and detected by photodiode.

As the disk rotates at speed between 200 and 500 rpm, the light bounces off the pits causing the frequency of the light to change.The reflected light passes through the prism and then onto a photo sensor.Light reflected from a pit is 180 degree out of phase with the light from the lands, differences in intensity is measured by the photoelectric cells, which converts into a corresponding electrical pulse.

Writing a data to a CDThe entire surface of disk is reflective, the laser can shine through the dye and the reflect off the gold layer.For CD-R disk to work , there must be way for a laser to create a non-reflective area on disk.CD-R disk, has an extra layer that the laser can modify.This extra layer is a greenish dye.When the data is written , the writing laser heats up the dye layer and changes its transparency.This change in dye creates the equivalent of non-reflective bump.

The decomposition of the dye in the pit area through the heat of laser is irreversible(permanent)Once CD-R is written, it cannot be erased o rewrittenHowever, CD and CD-R drives can be read the modified dye as a bump later on.CD-RW is rewritable because it uses phase changing material on its recording layer, usually alloy of silver, tellurium , indium and antimony metals.Phase-changing metal its state when heated to a high temperature and can be converted back to its original state when the heated at a temperature below its melting point.

CD-RW disk, the recording layer has a polycrystaline structure .While writing to disk , the laser heats up the selected areas to very high temperature which melts the crystals into a non-crystalline amorphous phaseThese areas have lower reflectance than the remaining crystalline areas.

This difference in reflectance helps in reading the recorded data as in case of CD-R diskTo erase the data, process is called annealing, during this process the area on the layer that has been changed to amorphous phase is converted back to original crystalline state by heating it to a temperature slightly below the melting point of the phase changing material.

DVDAlso known as Digital video diskHas seven-fold increase in data capacity over the CD.The tracks in DVD are placed closer together, allows more tracks per disk.The pits which store data is stored are also lot smaller.The minimum pit length of single layer DVD is 0.4microns as compared to 0.834 micronsLaser used in DVD to create pits was of the range 635 to 650 nm.

Comparing track pitch and pit length

Types of DVD

Dvd specification allows the information to be scanned from more than one layer of DVD simply b changing the focus of the read laser.The second layer can be read from inside of the disk out as well as from the outside inThe second layer of each disk data is recorded backwards ,or in reverse spiral track.To facilitate the focusing of the laser on the small pits, manufactures uses a thin plastic substrate thereby reducing the depth of plastic layer that laser has to travel to reach the pits.

Blu-ray diskIs an optical storage device, which is used record and play back high definition video and audio, images and other kind of data.Developed by Blu-ray Disk Association (BDA)It uses a blue violet laser having shorter wavelength (405nm) than red laser(650nm)Due to shorter wavelength, the laser can be focused more precisely on the small spot thereby resulting in storage capacity 10 times that of DVD.

Available in different forms:BD-ROMComes with pre-recorded content that can only be readBD-RIt is WORM type disk, where you can record data only once nut read many times.BD-RWIt is rewritableData can be erased and recorded a number of times in a same diskBD-RERewritable disk used for high definition audio/vedio and television recording

Comes in two different sizes: 12 cm and 8 cmEach disk can have single layer or double layers, depending on which the data capacity differs.Storage capacity with single layer disk is 25GBWith dual layer it is 50GBStorage of mini disk with single layer is 7.8 GB and dual layer is 15.6 GBOne can edit the programs recorded on the disk or change their order.The user can create a playlists of the programs that is stored on disk , so that can be randomly accessed.Allows to write one program while simultaneously watching another program.

Advantages of optical diskPosses large capacity to store information in the form of multimedia, graphics and video files.They can store more amount of data in less amount of space as compared to magnetic tapes and floppy or zip disks.Life span of data on optical disk is more about 10-20 years as compared to magnetic disksMore recording densityOptical disks are not affected by magnetic field.Its hard to break or melt, physically strong.Easily portable due to lightweight and small size

Disadvantages of optical diskCostlier when compared to other devices like floppy disk or zip diskNot easy to copy as floppy disk, one needs software and hardware for writing the diskThey posses low data access speed when compared to magnetic diskThe drive mechanism is more complicated when compared to magnetic and floppy disks.

Magneto-optical storage devicesIt includes basic principles of both magnetic and optical storage systems.The system writes magnetically and reads optically.It is a plastic or glass disk coated with a compound (ternary alloy of terium ferric cobalt) with special properties.Such materials are easily oxidized, dielectric barrier layers are used to protect the MO layer from oxidation.The barriers together with the reflector coating acts as an optical signal enhancement.

The thin film structure that gives the format its unlimited re-writability is based on a MO alloy layer enveloped by a barrier layer on each side.Available in two formats : 5 inch and 3 inchUnder the pressure of inexpensive and relatively fast CD-R and CD-RW , MO drives seem to be loosing ground.

Basics of MO reading

Traditional magnetic recording systems uses currents induced in the magnetic heads by the changing magnetic fluxes on the disk surfaces to read the dataMagneto-optical systems use polarized light to read the data from the disk.The changes in polarized light occurs due to presence of magnetic field on the surface of the disk, this is called Kerr effect, where the polarity of the reflected light is altered depending on the orientation of the magnetic particles.If the beam of the polarized light is shined on the surface , the light polarization of the reflected beam will change slightly , if it is reflected from a magnetized surface

If the magnetization is reversed , the change in polarization is reversed too.The magnetized areas. That is pits cannot be seen in regular light, but only in polarized lightThe change in direction of magnetization could be associated with numbers 0 or 1 making this technique useful for binary storage.

Basics of MO recordingAround 200 C, every magnetic material loses magnetization due to a complete disordering of their magnetic domains, as a result lose the data stored on them.Materials coercivety, is the measure of the material resistance to magnetization by the applied magnetic field decreases as the temperature approaches the curie point, and reaches zero when the temperature is exceeded.Materials coercivity drops at higher temperature allows thermally assisted magnetic recording with relatively weal magnetic fields.

The recording layer is heated by the laser to a point where its magnetic orientation is dissipated.As this spot on the disk cools, the new magnetic orientation-corresponding to the new information is set by the magnetic head to correspond to 0 and 1 of digitals signalsTo rewrite the data on MO disk, it is necessary to erase all previous signals before new data can be recorded.Two lasers are used to record the data (one to erase and one to record) or single laser must first erase the data in first rotation and then record the data the second time.

Universal serial busDeveloped by IntelSet of connectivity specifications that establishes communication between personal computers and devices such as mouse, keyboard, pen drive , external hard disk etcEvery computer and laptop comes with one or more USB ports.All USB devices come with a USB connector that is plugged into the USB port on the computer.As you plug it to the computer, it is detected and required software is configured automatically, no need to restart the computer.

USB

pen driveIs a removable storage device that is frequently used now a days to transfer data such as audio , video and files from one computer to another.it consists of small printed circuit board , which is fitted inside a plastic , metal or rubber casing to protect it.The USB connector is present at one end of the pen drive is protected by either removable cap or pulling it back in the casingRanges upto 1 GB to 32GBFast, robust , reliable, portable, consumes very less power, no battery required.

External hard disk

External hard diskIt is used outside the computer case and connected to system through USB interfaces in the PC.Storage capacity ranges upto 20GB to several TBsCompatible with the operating systems that support interface standards such as USB MSC (mass storage class) and IEEE 1394It is reliable and has high capacity storage mediaPortable and plug and play feature.As soon as the hard disk is connected to the system via USB, it is ready for transferring of data.Used for back up of the data

Memory stickAlso known as memory card.Designed to be used in portable electronic devices such as mobile phones , digital cameras etcLaunched by SONY and immediately gained popularity due to its support for fast data transfer speed and large storage capacityStarted with 128MB , now in market 32GB memory cards are availableEven a smallest memory card of 4MB can store about 80 images which is much more than a standard 3 inch floppy disk. It can be removed from the portable device and accessed by a personal computer using memory stick-capable memory card reader.

Memory stick and SD cards

Types of memory stickMemory stick PROMemory stick DUOMemory stick PRO DUOMemory stick Pro-HG DUOMemory stick Micro (M2)Memory stick PROIntroduced in 2003- by SONY and SCANDISKProvides theoretical storage capacity of upto 32GBIt provides 4 bit parallel interface with theoretical transfer rate of upto 480 Mb/sUsed in megapixel cameras and camcorders.

Memory stick DUOUsed for small, pocket-sized devices such as mobiles phones , music players , digital cameras etc.Equipped with magicgate technology that is used to encrypt data stored on card.Small in size but costs moreMemory stick PRO DUOSupersedes the memory transfer rate of memory stick duoProvides a larger memory space upto 32GB and high speed data transfer to/ from the card.

Memory card PRO-HG DUOIt has 8-bit parallel interfaceClock frequency of 60 MHZProvides high transfer speed of data than memory stick PRO DUOMemory stick Micro (M2):Light , compact storage media Dimension 15 mmx12.5 mmx1.2 mmUsed for mobile devicesOffers capacity upto 16MB to 32GB Transfer rate of 160 Mb/s

Mass storage devicesIn order to have large amount of storage, a different kind of storage system is used.Multiple units of similar kinds of storage media are associated together to form a chain of mass storage devices.These may include multiple magnetic tape reels or cartridges , multiple arrays of magnetic disks or multiple CD-ROMs as storage device.Three types:Redundant Array of Inexpensive Disk (RAID)Automated Tape libraryCD-ROM jukebox

RAIDIdea is to combine multiple hard disks into an array if disk drives to obtain high performance, large capacity and reliability.Disk array can be made fault tolerant by using several techniques such as RAID0,RAID1, RAID6

Automated tape libraryComprises numerous set of magnetic tapes along with their drive and controllers mounted in a single unit.The unit comprises one or more tape drives to perform read/write operations on the tape in the tape libraryIn tape environment, these tape can be simultaneously read or written, thus resulting in the speed transfer of data.Reliability is increased, if one disk fails, then unit can continue to operate with other drives.These tape libraries can store up to several terabytes of data in it, so they can be used for offline storage

Automated tape library

CD-ROM jukeboxIt comprises numerous sets of CD-ROM disks along with their drives and controller mounted in a single unit.The unit comprises one or more CD-ROM drives to perform read/write operations on the CD-ROM in the jukebox,In multiple CD-ROM drive environment , these CD-ROM can be simultaneously read or write resulting in speedy transfer of dataMultiple drives lead to reliability of storage unit because if one of the drive fails , then the unit can continue to operate with the other CD-ROM drives.

CD-ROM Jukebox

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