Lecture 10 Storage devices

8/7/2019 Lecture 10 Storage devices

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LECTURE 10Storage Devices


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Secondary Storage Devices

CENG 351

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Two major types of secondary storage devices:

1. Direct Access Storage Devices (DASDs)

Magnetic DiscsHard disks (high capacity, low cost, fast)Floppy disks (low capacity, lower cost, slow)

Optical DisksCD-ROM = (Compact disc, read-only memory

2. Serial Devices Magnetic tapes (very fast sequential access)


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Types of Storage Device

Sequential Access: In sequential access storage devices, datacan be accessed in sequential manner only.

And the time taken to read from a particular location will bedependant on the location last accessed.

Sequential access devices are usually used for backup purpose

only, where frequent access of information is not required.

Random Access: Random Access storage devices allow retrieval ofcontent from any location in the same amount of time is independent ofcontent's location...


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Secondary Storage Devices8-4


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Magnetic Storage

Recording of dataonto disks or tape bymagnetizing particlesof an oxide basedsurface coating.

A fairly permanent

type of storage thatcan be modified.


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Organization of Disks

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Disk contains concentric tracks.

Tracks are divided into sectors A sector is the smallest addressable unit in a disk.


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Sector, Cluster & Track7


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Looking at a surface

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Surface of disk showing tracks and sectors

sector

tracks


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Cylinders

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Clusters

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Usually File manager, under the operating system,maintains the logical view of a file.

File manager views the file as a series of clusters,each of a number of sectors. The clusters areordered by their logical order.

Files can be seen in the form of logical sectors orblocks, which needs to be mapped to physicalclusters.

File manager uses a file allocation table (FAT) tomap logical sectors of the file to the physical clusters.


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Floppy disk : It can only store up 1.44Mb of data. All disks must be formatted before data can be written tothe disk. Formatting divides the disk up into sections or sectors onto which data files are stored.

Advantage :

Small & light easy to carry

Cheap Useful for transferring small files

Can be used many times

Security tab to stop data from being written over

Disadvantage :

Easy to be damage

Small storage capacity

Many new computer dont have floppy disk drives

Can transport viruses from one machine to another

Slow to access and retrieve data when compared to a hard disk

Data can be erased if the disk comes into contact with a magnetic field


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Floppy Disk

Round piece of flexibleMylar plastic covered with a

thin layer of magnetic oxideand sealed inside aprotective covering.

May be referred to as afloppy

3 disk capacity is 1.44MBor 1,440,000 bytes


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Traditional Floppy Diskette8-14


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Cont..1. Capacity indication

2. Hub

3. Shutter4. Plastic housing

5. Paper ring

6. Magnetic disk

7. Disk sector


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HARD DISK16

Media Platter

Stepper Motor

Spindle Motor

Media Arm

Absorption point

Screw

Head


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Hard Disks8-17

Use thicker, metallic platters for storage

Faster than a floppy diskette

Large capacity

Sensitive instruments


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Hard-Disk Packs8-18

Removable

Massive storage capacity

Common in mainframes

Resembles stack of vinyl records


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Hard-Disk Pack8-19


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Magnetic Disks

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Bits of data (0s and 1s) are stored on circularmagnetic platters called disks.

A disk rotates rapidly (& never stops). A disk head reads and writes bits of data as

they pass under the head.

Often, several platters are organized into a diskpack (or disk drive).


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A Disk Drive

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Disk drive with 4 platters and 8 surfaces and 8 RW heads

surfaces

Spindle Boom

Read/Write heads


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Components of a Disk

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Platters

Spindle

Disk head

Arm movement

Arm assembly

Tracks

Sector

The platters spin (say, 90rps).

The arm assembly is moved in orout to position a head on a desired

track. Tracks under heads make acylinder(imaginary!).

Only one headreads/writes at any one time.

Block size is a multipleof sector size (which is often fixed).


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Accessing Data

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When a program reads a byte from the disk, the

operating system locates the surface, track and sector

containing that byte, and reads the entire sector intoa special area in main memory called buffer.

The bottleneck of a disk access is moving the

read/write arm.

So it makes sense to store a file in tracks that are

below/above each other on different surfaces, rather than

in several tracks on the same surface.


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External Hard Disk25


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Secondary Storage Devices:

Magnetic Tapes

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Magnetic Tape : uses 'serial access' to find a piece of data. It works in much the same way

as a video tape. To find a specific piece of data, you have to start at the beginning of the

tape and continue fast forwarding until you get to the piece of data that you need.

Advantage :

relatively cheap per megabyte of storage

can store large amounts of data - over 100 Gb

can be set up to do the back up overnight or over the week

Disadvantage :

serial access so can be quite slow to access data

need a special piece of equipment to record and read the data on the tape


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Magnetic Tape8-28

External storage

Provides sequential access

Information stored in sequence

Slower than disks which provide direct

access

Magnetic tape streamers or tapecartridges used by both mainframes

and microcomputers


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Reel 1 Reel 2

tape

Read/write head


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Data Blocks and Records

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Each data block is a sequence of contiguousrecords.

A record is the unit of data that a users programdeals with.

The tape drive reads an entire block of records atonce.

Unlike a disk, a tape starts and stops.

When stopped, the read/write head is over aninterblock gap.


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Optical Storage Means of recording data as light and dark spots on

CD or DVD.

Reading is done through a low-power laser light.

Pits

Dark spots

Lands Lighter, non-spotted surface areas


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Optical Disks8-34

Compact

Permanent storage

Laser beams reflect off pits

Two common types

CD

DVD


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Compact Disc8-35

Optical format

From 650 MB to 1 GB capacity

Rotation speeds vary

Types

Read only: CD-ROM

Write once: CD-R Rewriteable: CD-RW

Picture CDs and Photo CDs


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CD-ROM : CD Read only memory. When you buy a CD from a store, you can read it, but you

cant save any thing on it.

CD-R(WORM) : CD Write once read many. You are able to save to this disk one time, so you

can storage an application or your data on it. After you have saved on it once, you can read it

many times but cant save on it again.

CD-RW : CD Read Write. You can save your data over and over again, like a floppy disk.

Advantages :

Small and portable

Very cheap to produce

Most computers can read CDs. If there is no CD drive, a DVD drive can usually read them

Fairly fast to access the data - quicker than a floppy disk or magnetic tape

Disadvantages :

Fairly fragile, easy to snap or scratch

Smaller stora e ca acit than a hard drive or DVD


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Digital Versatile Disc8-37 Digital Versatile Disk or Digital Video Disk (DVD)

Similar to CDs, but can store more data

Types

Read only

Write once

Rewritable


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DVD-ROM : A new type of read only compact disc that can hold a minimum of 4.7GB.

DVD-R : can only record data once then the data becomes permanent on the disc. This disc

cant be recorded onto second time.

DVD-RW : The data on a DVD-RW disc can be erased and recorded over numerous times

without damaging the medium. DVD-RAM : can be recorded and erased repeatedly but are only compatible with devices

manufactured by the companies that support the DVD-RAM format. DVD-RAM discs are

typically housed in cartridges.

Advantages :

Very large storage capacity

DVD players can read CDs

DVDs are now mass produced so they are relatively cheap

Sound and picture quality is excellent, making them ideal for storing films with video and

sound.


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HD DVD-R : the writable disc variant of HD DVD, available with a single-layer capacity of 15 GB or a dual-layercapacity of 30 GB.

HD DVD-RW : the rewritable disc variant of HD DVD with equal storage capacity to an HD DVD-R.

HD DVD-RAM : the proposed successor to DVD-RAM for random access on optical media using phase-change principals.It would hold 20 gigabytes per layer instead of 15 gigabytes for HD DVD-R, due to differences in recording methods

used, yielding a higher density disc. Advantage :

Cheaper than a Blu-ray

Greater movie support than the competition

manufacturers can produce the high definition players for much less than Blu-ray.

a number of prominent manufacturers have pledged support to the player and format.

Disadvantage :

Thought its cheap but theres a lot of great things that didnt have in HD DVD.(e.g. charity, Four months worth ofgroceries for the average bachelor est.)

Region coding still unclear

Films arent cheaper, even if the player is

Xbox 360 version features no HDMI


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A Look to the FutureBlu-Ray Technology8-40 New standard in storage

Blu-Ray

New disks use blue laser light instead of the red laser

light used in traditional CD players

Disks may ultimately hold

Over 30GB on one-sided disks

Over 50GB on two-sided disks


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Blu-ray : medium designed to supersede the standard DDVD format. Its main uses are for storing high-definition video, PlayStation 3 video games, and other data, with up to 25 GB per single layered, and 50GB per dual layered disc. The disc has the same physical dimensions as standard DVDs and CDs.

Advantage :

Huge storage capacity Mandatory Managed Copy

Backwards compatibility

Clear than DVD

Can put it in the DVD player

Disadvantage :

Very expensive

The cost of film

Presently greater number of movie titles are available on DVD's

Future uncertainty (even without HD-DVD)

Long reading times of the discs


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Flas

Memory : Memory sticks are available from 1 Gb up to 8 Gb. They are typically small, lightweight,removable and rewritable. They consist of a small printed circuit board which is encased in plastic or metalcasing. They usually have a removable cap which covers and protects the part of the stick which is insertedinto a USB port.

Advantage :

Hold more data than CD More reliable than floppy disk, because they have no moving part

More compact and portable than floppy disks or CDs/DVDs.

Being developed with fashionable looking outer casings and are almost becoming a 'fashion accessory.

Disadvantage :

Lost easily

The metal part which is inserted into the US

B

port can be snapped off if they are handled roughly More expensive than floppy disks, CDs and DVDs

Documents

Lecture 10 Storage devices