Chapter 08 secondary storage

Computer Fundamentals: Pradeep K. Sinha & Priti Sinha

Ref Page Chapter 8: Secondary Storage Devices Slide 1/98


Learning Objectives

In this chapter you will learn about:

§ Secondary storage devices and their need
§ Classification of commonly used secondary storage
devices
§ Difference between sequential and direct access
storage devices
§ Basic principles of operation, types, and uses of
popular secondary storage devices such as magnetic
tape, magnetic disk, and optical disk

(Continued on next slide)

Ref Page 117 Chapter 8: Secondary Storage Devices Slide 2/98


Learning Objectives
(Continued from previous slide..)

§ Commonly used mass storage devices
§ Introduction to other related concepts such as RAID,
Jukebox, storage hierarchy, etc.



Limitations of Primary Storage

§ Limited capacity because the cost per bit of storage
is high
§ Volatile - data stored in it is lost when the electric
power is turned off or interrupted



Secondary Storage

§ Used in a computer system to overcome the limitations
of primary storage
§ Has virtually unlimited capacity because the cost per bit
of storage is very low
§ Has an operating speed far slower than that of the
primary storage
§ Used to store large volumes of data on a permanent
basis
§ Also known as auxiliary memory



Classification of Commonly Used Secondary
Storage Devices
Secondary Storage
Devices

Sequential Access Direct Access Devices
Device

Magnetic Tape

Magnetic Optical Disks Memory Storage
Disks Devices

Flash Memory
Floppy Hard CD-ROM Card
WORM CD-RW DVD Drive
Disk Disks
(CD-R)

Zip Disk Disk Pack Winchester Disk



Sequential-access Storage Devices

§ Arrival at the desired storage location may be preceded
by sequencing through other locations
§ Data can only be retrieved in the same sequence in which
it is stored
§ Access time varies according to the storage location of
the information being accessed
§ Suitable for sequential processing applications where
most, if not all, of the data records need to be processed
one after another
§ Magnetic tape is a typical example of such a storage
device



Direct-access Storage Devices

§ Devices where any storage location may be selected
and accessed at random
§ Permits access to individual information in a more
direct or immediate manner
§ Approximately equal access time is required for
accessing information from any storage location
§ Suitable for direct processing applications such as on-
line ticket booking systems, on-line banking systems
§ Magnetic, optical, and magneto-optical disks are
typical examples of such a storage device



Magnetic Tape Basics

§ Commonly used sequential-access secondary storage
device
§ Physically, the tape medium is a plastic ribbon, which
is usually ½ inch or ¼ inch wide and 50 to 2400 feet
long
§ Plastic ribbon is coated with a magnetizable recording
material such as iron-oxide or chromium dioxide
§ Data are recorded on the tape in the form of tiny
invisible magnetized and non-magnetized spots
(representing 1s and 0s) on its coated surface
§ Tape ribbon is stored in reels or a small cartridge or
cassette



Magnetic Tape - Storage Organization
(Example 1)
A frame Characters for
Track/Channel
numbers corresponding codes

0 12 3 4 5 6 7 8 9 A B C D E F G
Parity bit 7

6 Each vertical
Zone
line represents
5
a binary 1 bit
4

Numeric 3

2

1

Illustrates the concepts of frames, tracks, parity bit, and character-by-character data
storage



Magnetic Tape - Storage Organization (Example 2)

Track/ channel Characters for
numbers A frame for each corresponding codes
character

0 1 2 3 4 5 6 7 8 9 A B Each vertical
8’s digit line represents
9
a binary
Track 2’s digit 8
1 bit
representation Added zone 7
Added zone 6
Zone 5
Parity bit 4
Zone 3
Unit’s digit 2
4’s digit 1

Illustrates the concepts of frames, tracks, parity bit, and character-by-character data
storage




Tape motion

IBG R1 IBG R2 IBG R3 IBG R4 IBG R5 IBG R6

(a) An unblocked tape. There is an IBG after each record.

Tape motion

IBG R1 R2 IBG R3 R4 IBG R5 R6 IBG R7 R8 IBG

(b) A tape which uses a blocking factor of two. There is an IBG after every two records.

Tape motion

IBG R1 R2 R3 IBG R4 R5 R6 IBG R7 R8 R9 IBG

(c) A tape which uses a blocking factor of three. There is an IBG after every three records.

Illustrates the concepts of blocking of records, inter-block gap (IBG), and blocking factor




File header
File header File trailer label of next
label label file

Tape motion

Block of Block of
IBG IBG IBG IBG
records records

Illustrates the concepts of multiple blocks of records forming a file that is
separated from other files by a file header label in the beginning and a file trailer
label at the end of the file



Magnetic Tape-Storage Organization (Example 5)

Tape header
label File trailer
label of
BOT File header File trailer
label last file
marker label

unused IBG IB Blocks of records of IBG IBG IB unused
the file separated by
tape G G tape
IBGs

Tape motion
EOT marker

Illustrates the concepts of Beginning of Tape (BoT) and End of Tape
(EoT) markers, and tape header label



Magnetic Tape Storage Capacity

§ Storage capacity of a tape =
Data recording density x Length
§ Data recording density is the amount of data that can be
stored on a given length of tape. It is measured in bytes
per inch (bpi)
§ Tape density varies from 800 bpi in older systems to
77,000 bpi in some of the modern systems
§ Actual storage capacity of a tape may be anywhere from
35% to 70% of its total storage capacity, depending on
the storage organization used



Magnetic Tape – Data Transfer Rate

§ Refers to characters/second that can be transmitted to
the memory from the tape
§ Transfer rate measurement unit is bytes/second (bps)
§ Value depends on the data recording density and the
speed with which the tape travels under the read/write
head
§ A typical value of data transfer rate is 7.7 MB/second



Magnetic Tape – Tape Drive

§ Used for writing/reading of data to/from a magnetic
tape ribbon
§ Different for tape reels, cartridges, and cassettes
§ Has read/write heads for reading/writing of data on
tape
§ A magnetic tape reel/cartridge/cassette has to be first
loaded on a tape drive for reading/writing of data on it
§ When processing is complete, the tape is removed
from the tape drive for off-line storage



Magnetic Tape – Tape Controller

§ Tape drive is connected to and controlled by a tape
controller that interprets the commands for operating the
tape drive

§ A typical set of commands supported by a tape controller
are:
Read reads one block of data

Write writes one block of data

Write tape header label used to update the contents of tape header label

Erase tape erases the data recorded on a tape

Back space one block rewinds the tape to the beginning of previous block




Magnetic Tape – Tape Controller

Forward space one block forwards the tape to the beginning
of next block

Forward space one file forwards the tape to the beginning
of next file

Rewind fully rewinds the tape

Unload releases the tape drive’s grip so
that the tape spool can be
unmountedfrom the tape drive



Types of Magnetic Tape

§ ½-inch tape reel

§ ½-inch tape cartridge

§ ¼-inch streamer tape

§ 4-mm digital audio tape (DAT)



Half-inch Tape Reel

§ Uses ½ inch wide tape ribbon stored on a tape reel
§ Uses parallel representation method of storing data, in
which data are read/written a byte at a time
§ Uses a read/write head assembly that has one
read/write head for each track
§ Commonly used as archival storage for off-line storage
of data and for exchange of data and programs
between organizations
§ Fast getting replaced by tape cartridge, streamer tape,
and digital audio tape they are more compact, cheaper
and easier to handle



Half-inch Tape Reel



Tape Drive of Half-inch Tape Reel
Supply Take-up
reel Magnetic tape reel

Read/write
head assembly

Vacuum
columns

Tape loops
varying in
length



Half-inch Tape Cartridge

§ Uses ½ inch wide tape ribbon sealed in a cartridge
§ Has 36 tracks, as opposed to 9 tracks for most half-inch
tape reels
§ Stores data using parallel representation. Hence, 4 bytes
of data are stored across the width of the tape. This
enables more bytes of data to be stored on the same
length of tape
§ Tape drive reads/writes on the top half of the tape in
one direction and on the bottom half in the other
direction



Half-inch Tape Cartridge



Quarter-inch Streamer Tape

§ Uses ¼ inch wide tape ribbon sealed in a cartridge
§ Uses serial representation of data recording (data bits
are aligned in a row one after another in tracks)
§ Can have from 4 to 30 tracks, depending on the tape
drive
§ Depending on the tape drive, the read/write head
reads/writes data on one/two/four tracks at a time
§ Eliminates the need for the start/stop operation of
traditional tape drives




Quarter-inch Streamer Tape

§ Can read/write data more efficiently than the
traditional tape drives because there is no start/stop
mechanism
§ Make more efficient utilization of tape storage area
than traditional tape drives because IBGs are not
needed
§ The standard data formats used in these tapes is
known as the QIC standard



Quarter-inch Streamer Tape (Example)

Recording area Recording area
begins here ends here
Tracks
1 1 0 1 1 0 0 1 1 1 0 1 0.
2 ..
0 0 1 0 1 0 0 1 1 1 0 1...
3 Unused Unused
4 portion portion
of the of the
5 tape tape
6

7
8



4mm Digital Audio Tape (DAT)

§ Uses 4mm wide tape ribbon sealed in a cartridge
§ Has very high data recording density
§ Uses a tape drive that uses helical scan technique for
data recording, in which two read heads and two write
heads are built into a small wheel
§ DAT drives use a data recording format called Digital
Data Storage (DDS), which provides three levels of
error-correcting code
§ Typical capacity of DAT cartridges varies from
4 GB to 14 GB



The Helical Scan Techniques
Used in DAT Drives

Write head B
Read head A
Spinning
Read head B
helical
scan

Write head A

Moving tape

Shaft



Advantages of Magnetic Tapes

§ Storage capacity is virtually unlimited because as many
tapes as required can be used for storing very large
data sets
§ Cost per bit of storage is very low for magnetic tapes.
§ Tapes can be erased and reused many times
§ Tape reels and cartridges are compact and light in
weight
§ Easy to handle and store.
§ Very large amount of data can be stored in a small
storage space




Advantages of Magnetic Tapes

§ Compact size and light weight
§ Magnetic tape reels and cartridges are also easily
portable from one place to another
§ Often used for transferring data and programs from
one computer to another that are not linked together



Limitations of Magnetic Tapes

§ Due to their sequential access nature, they are not
suitable for storage of those data that frequently
require to be accessed randomly
§ Must be stored in a dust-free environment because
specks of dust can cause tape-reading errors
§ Must be stored in an environment with properly
controlled temperature and humidity levels
§ Tape ribbon may get twisted due to warping, resulting
in loss of stored data
§ Should be properly labeled so that some useful data
stored on a particular tape is not erased by mistake



Uses of Magnetic Tapes

§ For applications that are based on sequential data
processing
§ Backing up of data for off-line storage
§ Archiving of infrequently used data
§ Transferring of data from one computer to another that
are not linked together
§ As a distribution media for software by vendors



Magnetic Disk - Basics

§ Commonly used direct-access secondary storage device.
§ Physically, a magnetic disk is a thin, circular
plate/platter made of metal or plastic that is usually
coated on both sides with a magnetizable recording
material such as iron-oxide
§ Data are recorded on the disk in the form of tiny
invisible magnetized and non-magnetized spots
(representing 1s and 0s) on the coated surfaces of the
disk
§ The disk is stored in a specially designed protective
envelope or cartridge, or several of them are stacked
together in a sealed, contamination-free container



Magnetic Disk – Storage Organization
Illustrates the Concept of Tracks

200 Track 000
§ A disk’s surface is divided into
Tracks
a number of invisible
concentric circles called tracks

§ The tracks are numbered
consecutively from outermost
to innermost starting from
zero

Track
§ The number of tracks on a
… …
199 disk may be as few as 40 on
small, low-capacity disks, to
several thousand on large,
high-capacity disks



Illustrates the Concept of Sectors

A sector

§ Each track of a disk is
subdivided into sectors

§ There are 8 or more
sectors per track

§ A sector typically contains
512 bytes

§ Disk drives are designed to
read/write only whole
sectors at a time




Illustrates Grouping of Tracks and Use of Different
Number of Sectors in Tracks of Different Groups for
Increased Storage Capacity

§ Innermost group of tracks
has 8 sectors/track

§ Next groups of tracks has
9 sectors/track

§ Outermost group of tracks
has 10 sectors/track



Magnetic Disk – Disk Address or
Address of a Record on a Disk

§ Disk address represents the physical location of the
record on the disk
§ It is comprised of the sector number, track number,
and surface number (when double-sided disks are
used)
§ This scheme is called the CHS addressing or Cylinder-
Head-Sector addressing. The same is also referred to
as disk geometry



(Illustrates the Concept of Cylinder)
Central shaft
Upper surface
not used
Read/Write head
Surface - 0 Direction of
Surface - 1 movement of
access arms
Surface - 2 assembly
Cylinder
Surface - 3

Access arms
Surface - 4
assembly
Surface - 5

Lower surface
not used

No. of disk platters = 4, No. of usable surfaces = 6. A set of corresponding
tracks on all the 6 surfaces is called a cylinder.



Magnetic Disk – Storage Capacity

Storage capacity of a disk system = Number of recording surfaces
× Number of tracks per surface
× Number of sectors per track
× Number of bytes per sector



Magnetic Disk Pack – Access Mechanism

One read/write
head per surface Central shaft

Direction of
movement of
access arms
assembly

Access arms
assembly
Vertical cross section of a disk system. There is one read/write head per
recording surface



Magnetic Disk – Access Time

§ Disk access time is the interval between the instant a
computer makes a request for transfer of data from a
disk system to the primary storage and the instant this
operation is completed

§ Disk access time depends on the following three
parameters:

– Seek Time: It is the time required to position the
read/write head over the desired track, as soon as
a read/write command is received by the disk unit

– Latency: It is the time required to spin the desired
sector under the read/write head, once the
read/write head is positioned on the desired track



Magnetic Disk – Access Time

– Transfer Rate: It is the rate at which data are
read/written to the disk, once the read/write head
is positioned over the desired sector

§ As the transfer rate is negligible as compared to seek
time and latency,

Average access time

= Average seek time + Average latency



Disk Formatting

§ Process of preparing a new disk by the computer
system in which the disk is to be used.
§ For this, a new (unformatted) disk is inserted in the disk
drive of the computer system and the disk formatting
command is initiated
§ Low-level disk formatting
§ Disk drive’s read/write head lays down a magnetic
pattern on the disk’s surface
§ Enables the disk drive to organize and store the
data in the data organization defined for the disk
drive of the computer




Disk Formatting

§ OS-level disk formatting
§ Creates the File Allocation Table (FAT) that is a
table with the sector and track locations of data
§ Leaves sufficient space for FAT to grow
§ Scans and marks bad sectors
§ One of the basic tasks handled by the computer’s
operating system
§ Enables the use of disks manufactured by third party
vendors into one’s own computer system



Magnetic Disk – Disk Drive

§ Unit used for reading/writing of data on/from a
magnetic disk

§ Contains all the mechanical, electrical and
electronic components for holding one or more
disks and for reading or writing of information on
to it




Magnetic Disk – Disk Drive

§ Although disk drives vary greatly in their shape, size
and disk formatting pattern, they can be broadly
classified into two types:

– Those with interchangeable magnetic disks,
which allow the loading and unloading of
magnetic disks as and when they are needed for
reading/writing of data on to them

– Those with fixed magnetic disks, which come
along with a set of permanently fixed disks. The
disks are not removable from their disk drives



Magnetic Disk – Disk Controller

§ Disk drive is connected to and controlled by a disk
controller, which interprets the commands for
operating the disk drive
§ Typically supports only read and write commands,
which need disk address (surface number,
cylinder/track number, and sector number) as
parameters
§ Connected to and controls more than one disk drive, in
which case the disk drive number is also needed as a
parameters of read and write commands



Types of Magnetic Disks

Magnetic Disks

Floppy Disks Hard Disks

Zip/Bernoulli Disks Disk Packs Winchester
Disks



Floppy Disks

§ Round, flat piece of flexible plastic disks coated with
magnetic oxide
§ So called because they are made of flexible plastic
plates which can bend
§ Also known as floppies or diskettes
§ Plastic disk is encased in a square plastic or vinyl jacket
cover that gives handling protection to the disk surface




Floppy Disks

§ The two types of floppy disks in use today are:

§ 5¼-inch diskette, whose diameter is 5¼-inch.
It is encased in a square, flexible vinyl jacket

§ 3½-inch diskette, whose diameter is 3½-inch.
It is encased in a square, hard plastic jacket

§ Most popular and inexpensive secondary storage
medium used in small computers



A 5¼-inch Floppy Disk
User’s label for
Manufacturer’s
identification purpose
label

Write protect notch

Drive spindle
hole in disk
5¼ - inch

Drive access opening
in jacket

Aperture in the jacket
through which the
read/write head makes
direct contact with the
disk surface

5¼ - inch

A 5¼-inch floppy disk enclosed within jacket. The drive mechanism clamps
on to a portion of the disk exposed by the drive access opening in the jacket



A 3½-inch Floppy Disk
Opening for inserting the
Light indicator disk inside the drive
(blinks when
read/write
operation takes Floppy disk
place) (a) Front view of a floppy disk eject button
drive.

Sliding metal
piece cover

3½” - inch
User’s label for
identification
Write-protect
plastic tab
3½” - inch
(b) A 3½ - inch floppy disk.



Storage Capacities of Various Types of Floppy
Disks

Size No. of Approximate
(Diameter No. of No. of No. of Capacity
surfaces tracks sectors/track bytes/sector in bytes capacity
in inches)

5¼ 2 40 9 512 3,68,640 360 KB

5¼ 2 80 15 512 12,28,800 1.2 MB

3½ 2 40 18 512 7,37,280 720 KB

3½ 2 80 18 512 14,74,560 1.4 MB

3½ 2 80 36 512 29,49,120 2.88 MB



Hard Disks

§ Round, flat piece of rigid metal (frequently aluminium)
disks coated with magnetic oxide
§ Come in many sizes, ranging from 1 to 14-inch
diameter.
§ Depending on how they are packaged, hard disks are of
three types:
§ Zip/Bernoulli disks
§ Disk packs
§ Winchester disks
§ Primary on-line secondary storage device for most
computer systems today



Zip/Bernoulli Disks

§ Uses a single hard disk platter encased in a plastic
cartridge
§ Disk drives may be portable or fixed type
§ Fixed type is part of the computer system, permanently
connected to it
§ Portable type can be carried to a computer system,
connected to it for the duration of use, and then can be
disconnected and taken away when the work is done
§ Zip disks can be easily inserted/removed from a zip drive
just as we insert/remove floppy disks in a floppy disk
drive



Disk Packs

§ Uses multiple (two or more) hard disk platters
mounted on a single central shaft
§ Disk drives have a separate read/write head for each
usable disk surface (the upper surface of the top-most
disk and the lower surface of the bottom most disk is
not used)
§ Disks are of removable/interchangeable type in the
sense that they have to be mounted on the disk drive
before they can be used, and can be removed and
kept off-line when not in use



Winchester Disks

§ Uses multiple (two or more) hard disk platters
mounted on a single central shaft

§ Hard disk platters and the disk drive are sealed
together in a contamination-free container and cannot
be separated from each other




Winchester Disks

§ For the same number of disks, Winchester disks have
larger storage capacity than disk packs because:

– All the surfaces of all disks are used for data
recording

They employ much greater precision of data recording,
resulting in greater data recording density

§ Named after the .30-30 Winchester rifle because the
early Winchester disk systems had two 30-MB disks
sealed together with the disk drive



Advantages of Magnetic Disks

§ More suitable than magnetic tapes for a wider range of
applications because they support direct access of data
§ Random access property enables them to be used
simultaneously by multiple users as a shared device. A
tape is not suitable for such type of usage due to its
sequential-access property
§ Suitable for both on-line and off-line storage of data





§ Except for the fixed type Winchester disks, the storage
capacity of other magnetic disks is virtually unlimited
as many disks can be used for storing very large data
sets
§ Due to their low cost and high data recording densities,
the cost per bit of storage is low for magnetic disks.
§ An additional cost benefit is that magnetic disks can be
erased and reused many times
§ Floppy disks and zip disks are compact and light in
weight. Hence they are easy to handle and store.
§ Very large amount of data can be stored in a small
storage space





§ Due to their compact size and light weight, floppy disks
and zip disks are also easily portable from one place to
another
§ They are often used for transferring data and programs
from one computer to another, which are not linked
together
§ Any information desired from a disk storage can be
accessed in a few milliseconds because it is a direct
access storage device





§ Data transfer rate for a magnetic disk system is
normally higher than a tape system
§ Magnetic disks are less vulnerable to data corruption
due to careless handling or unfavorable temperature
and humidity conditions than magnetic tapes



Limitations of Magnetic Disks

§ Although used for both random processing and
sequential processing of data, for applications of the
latter type, it may be less efficient than magnetic
tapes
§ More difficult to maintain the security of information
stored on shared, on-line secondary storage devices,
as compared to magnetic tapes or other types of
magnetic disks





§ For Winchester disks, a disk crash or drive failure often
results in loss of entire stored data. It is not easy to
recover the lost data. Suitable backup procedures are
suggested for data stored on Winchester disks
§ Some types of magnetic disks, such as disk packs and
Winchester disks, are not so easily portable like
magnetic tapes
§ On a cost-per-bit basis, the cost of magnetic disks is
low, but the cost of magnetic tapes is even lower





§ Must be stored in a dust-free environment
§ Floppy disks, zip disks and disk packs should be
labeled properly to prevent erasure of useful data by
mistake



Uses of Magnetic Disks

§ For applications that are based on random data
processing
§ As a shared on-line secondary storage device.
Winchester disks and disk packs are often used for
this purpose
§ As a backup device for off-line storage of data. Floppy
disks, zip disks, and disk packs are often used for this
purpose




Uses of Magnetic Disks

§ Archiving of data not used frequently, but may be
used once in a while. Floppy disks, zip disks, and
disk packs are often used for this purpose
§ Transferring of data and programs from one
computer to another that are not linked together.
Floppy disks and zip disks are often used for this
purpose
§ Distribution of software by vendors. Originally sold
software or software updates are often distributed by
vendors on floppy disks and zip disks



Optical Disk – Basics

§ Consists of a circular disk, which is coated with a thin
metal or some other material that is highly reflective
§ Laser beam technology is used for recording/reading
of data on the disk
§ Also known as laser disk / optical laser disk, due to
the use of laser beam technology
§ Proved to be a promising random access medium for
high capacity secondary storage because it can store
extremely large amounts of data in a limited space



Optical Disk – Storage Organization

§ Has one long spiral track, which starts at the outer edge
and spirals inward to the center
§ Track is divided into equal size sectors

(a) Track pattern on an optical disk (b) Track pattern on a magnetic disk

Difference in track patterns on optical and magnetic disks.



Optical Disk – Storage Capacity

Storage capacity of an optical disk

= Number of sectors
× Number of bytes per sector

The most popular optical disk uses a disk of 5.25 inch
diameter with storage capacity of around 650 Megabytes



Optical Disk – Access Mechanism

Laser Laser
beam beam
source source

Prism Prism

Sensor Sensor
Laser beam gets Laser beam gets
scattered by a pit reflect by a land
(represents 0) (represents1)

Land Land
Pit Land
Pit Pit



Optical Disk – Access Time

§ With optical disks, each sector has the same length
regardless of whether it is located near or away from
the disk’s center
§ Rotation speed of the disk must vary inversely with
the radius. Hence, optical disk drives use a constant
linear velocity (CLV) encoding scheme
§ Leads to slower data access time (larger access time)
for optical disks than magnetic disks
§ Access times for optical disks are typically in the
range of 100 to 300 milliseconds and that of hard
disks are in the range of 10 to 30 milliseconds



Optical Disk Drive

§ Uses laser beam technology for reading/writing of data
§ Has no mechanical read/write access arm
§ Uses a constant linear velocity (CLV) encoding scheme,
in which the rotational speed of the disk varies inversely
with the radius



Optical Disk Drive

Volume control button used
Light
when the drive is used to Optical disk
indicator
play a music CD

A headphone socket Is placed on
enables the user to top of this
plug-in head-phones groove
and listen to recorded Tray eject Direction of
sound when the drive button movement of
is used to play music the tray
CDs. Optical disk tray



Types of Optical Disks

The types of optical disks in use today are:

CD-ROM

§ Stands for Compact Disk-Read Only Memory
§ Packaged as shiny, silver color metal disk of 5¼
inch (12cm) diameter, having a storage capacity of
about 650 Megabytes
§ Disks come pre-recorded and the information
stored on them cannot be altered
§ Pre-stamped (pre-recorded) by their suppliers, by a
process called mastering





§ Provide an excellent medium to distribute large
amounts of data in electronic dorm at low cost.
§ A single CD-ROM disk can hold a complete
encyclopedia, or a dictionary, or a world atlas, or
biographies of great people, etc
§ Used for distribution of electronic version of
conference proceedings, journals, magazines,
books, and multimedia applications such as video
games
§ Used by software vendors for distribution of
software to their customers




WORM Disk / CD-Recordable (CD-R)

§ Stands for Write Once Read Many. Data can be written
only once on them, but can be read many times
§ Same as CD-ROM and has same storage capacity
§ Allow users to create their own CD-ROM disks by using
a CD-recordable (CD-R) drive that can be attached to
a computer as a regular peripheral device
§ Data to be recorded can be written on its surface in
multiple recording sessions





§ Sessions after the first one are always additive
and cannot alter the etched/burned information of
earlier sessions
§ Information recorded on them can be read by any
ordinary CD-ROM drive
§ They are used for data archiving and for making a
permanent record of data. For example, many
banks use them for storing their daily transactions




CD-Read/Write (CD-RW)

§ Same as CD-R and has same storage capacity
§ Allow users to create their own CD-ROM disks by
using a CD-recordable (CD-R) drive that can be
attached to a computer as a regular peripheral
device
§ Data to be recorded can be written on its surface in
multiple recording sessions
§ Made of metallic alloy layer whose chemical
properties are changed during burn and erase
§ Can be erased and written afresh




Digital Video / Versatile Disk (DVD)

§ Looks same as CD-ROM but has capacity of 4.7 GB or
8.5 GB
§ Designed primarily to store and distribute movies
§ Can be used for storage of large data
§ Allows storage of video in 4:3 or 16:9 aspect-ratios in
MPEG-2 video format using NTSC or PAL resolution
§ Audio is usually Dolby® Digital (AC-3) or Digital
Theater System (DTS) and can be either monaural or
5.1 Surround Sound



Advantages of Optical Disks

§ The cost-per-bit of storage for optical disks is very low
because of their low cost and enormous storage density.
§ The use of a single spiral track makes optical disks an ideal
storage medium for reading large blocks of sequential data,
such as music.
§ Optical disk drives do not have any mechanical read/write
heads to rub against or crash into the disk surface. This
makes optical disks a more reliable storage medium than
magnetic tapes or magnetic disks.
§ Optical disks have a data storage life in excess of 30 years.
This makes them a better storage medium for data
archiving as compared to magnetic tapes or magnetic disks.



Advantages of Optical Disks

§ As data once stored on an optical disk becomes
permanent, danger of stored data getting
inadvertently erased/overwritten is removed
§ Due to their compact size and light weight, optical
disks are easy to handle, store, and port from one
place to another
§ Music CDs can be played on a computer having a CD-
ROM drive along with a sound board and speakers.
This allows computer systems to be also used as
music systems



Limitations of Optical Disks

§ It is largely read-only (permanent) storage medium.
Data once recorded, cannot be erased and hence the
optical disks cannot be reused
§ The data access speed for optical disks is slower than
magnetic disks
§ Optical disks require a complicated drive mechanism



Uses of Optical Disks

§ For distributing large amounts of data at low cost
§ For distribution of electronic version of conference
proceedings, journals, magazines, books, product
catalogs, etc
§ For distribution of new or upgraded versions of
software products by software vendors




Uses of Optical Disks

§ For storage and distribution of a wide variety of
multimedia applications
§ For archiving of data, which are not used frequently,
but which may be used once in a while
§ WORM disks are often used by end-user companies to
make permanent storage of their own proprietary
information



Memory Storage Devices

Flash Drive (Pen Drive)

§ Relatively new secondary storage device based on
flash memory, enabling easy transport of data from
one computer to another
§ Compact device of the size of a pen, comes in
various shapes and stylish designs and may have
different added features
§ Plug-and-play device that simply plugs into a USB
(Universal Serial Bus) port of a computer, treated as
removable drive
§ Available storage capacities are 8MB, 16MB, 64MB,
128MB, 256MB, 512MB, 1GB, 2GB, 4GB, and 8GB



Memory Storage Devices

Memory Card (SD/MMC)

§ Similar to Flash Drive but in card shape
§ Plug-and-play device that simply plugs into a port
of a computer, treated as removable drive
§ Useful in electronic devices like Camera, music
player
§ Available storage capacities are 8MB, 16MB, 64MB,
128MB, 256MB, 512MB, 1GB, 2GB, 4GB, and 8GB



Mass Storage Devices

§ As the name implies, these are storage systems
having several trillions of bytes of data storage
capacity
§ They use multiple units of a storage media as a single
secondary storage device
§ The three commonly used types are:
1. Disk array, which uses a set of magnetic disks
2. Automated tape library, which uses a set of
magnetic tapes
3. CD-ROM Jukebox, which uses a set of CD-ROMs
§ They are relatively slow having average access times
in seconds



Disk Array

§ Set of hard disks and hard disk drives with a
controller mounted in a single box, forming a single
large storage unit
§ It is commonly known as a RAID (Redundant Array
of Inexpensive Disks)
§ As a secondary storage device, provides enhanced
storage capacity, enhanced performance, and
enhanced reliability



Disk Array

§ Enhanced storage capacity is achieved by using
multiple disks
§ Enhanced performance is achieved by using parallel
data transfer technique from multiple disks
§ Enhanced reliability is achieved by using techniques
such as mirroring or striping
§ In mirroring, the system makes exact copies of files
on two hard disks
§ In striping, a file is partitioned into smaller parts and
different parts of the file are stored on different disks



A RAID Unit

Multiple
disks

RAID Controller

Computer



Automated Tape Library

§ Set of magnetic tapes and magnetic tape drives with
a controller mounted in a single box, forming a
single large storage unit
§ Large tape library can accommodate up to several
hundred high capacity magnetic tapes bringing the
storage capacity of the storage unit to several
terabytes
§ Typically used for data archiving and as on-line data
backup devices for automated backup in large
computer centers



CD-ROM Jukebox

§ Set of CD-ROMs and CD-ROM drives with a
controller mounted in a single box, forming a single
large storage unit
§ Large CD-ROM jukebox can accommodate up to
several hundred CD-ROM disks bringing the storage
capacity of the storage unit to several terabytes
§ Used for archiving read-only data in such
applications as on-line museums, on-line digital
libraries, on-line encyclopedia, etc



Storage Hierarchy
As a single type of storage is not superior in speed of access, capacity, and
cost, most computer systems make use of a hierarchy of storage
technologies as shown below.

Smaller capacity, faster Cache Larger capacity,
access time, and higher memory slower access time,
cost per bit stored Main and lower cost per
memory bit stored
On-line, direct-access and
sequential-access secondary
storage device such as hard disk
Off-line, direct-access and sequential-access
secondary storage devices such as magnetic
tape, floppy disk, zip disk, WORM disk, etc.
Mass storage devices such as tape library, CD juke box, etc.



Key Words/Phrases

§ Automated tape library § Floppy disk
§ Auxiliary memory § Hard disk
§ Block § Inter-block gap (IBG)
§ Blocking § Inter-record gap (IRG)
§ Blocking factory § Land
§ CD-ROM § Latency
§ CD-ROM jukebox § Magnetic disk
§ Check bit § Magnetic tape
§ Cylinder § Magnetic tape drive
§ Data transfer rate § Mass storage devices
§ Direct access device § Master file
§ Disk array § Odd parity
§ Disk controller § Off-line storage
§ Disk drive § On-line storage
§ Disk formatting § Optical disk
§ Disk pack § Parallel representation
§ DVD § Parity bit
§ Even parity § Pit
§ File Allocation Tube (FAT)




Key Words/Phrases

§ QIC Standard
§ Record
§ Redundant Array of Inexpensive Disks (RAID)
§ Secondary storage
§ Sector
§ Seek time
§ Sequential access device
§ Storage hierarchy
§ Tape controller
§ Track
§ Transaction file
§ Winchester disk
§ WORM disk
§ Zip disk


Chapter 08 secondary storage

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Chapter 08 secondary storage