1.4_Mass_(Secondary)_Storageeddddddddddddd.pdf

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1. UNIT 1: DATA STORAGE

Mass (Secondary) Storage
・ Magnetic Media

+ Optical Media

+ Solid-State Media

2. UNIT 1: DATA STORAGE

Mass (Secondary) Storage

+ Due to the volatility and limited size of a computer’s main (primary) memory, most
computers have additional memory devices called , including

magnetic disks, magnetic tapes, CDs, DVDs, flash drives, and solid-state drives.

mass (secondary) storage: computer memory that is non-volatile and persistent, but cannot
be directly accessed by the processor

+ The advantages of mass (secondary) storage over main memory include less volatility,
large storage capacity, low cost, and in many cases, the ability to remove the storage
medium from the machine for archival purposes.

3. UNIT 1: DATA STORAGE

Mass (Secondary) Storage

+ A major disadvantage of some types of mass (secondary) storage media (such as magnetic
and optical media) is that they typically rely on mechanical motion, requiring significantly
more time to store and retrieve data than a machine's main (primary) memory, where all
activities are performed using only electricity.

+ Moreover, storage media with moving parts is more prone to mechanical failure than
another type of mass (secondary) storage media called solid-state media, which does not
require moving parts.

= However, other electronic considerations can limit the speed or longevity of even
solid-state media in comparison to main (primary) memory.

1.4 Mass (Secondary) Storage

4. UNIT 1: DATA STORAGE

Magnetic Media

・ Magnetic media have been the most common type of storage for a long time.

+ The invention of magnetic tape for sound recording pre-dates the invention of the
computer by many years; as a result, magnetic tape was the first storage device.

+ In contrast, the magnetic disk (also called a hard disk) was specifically invented for
computer storage.

a The magnetic disk also used magnetization to write data, and arrived a few years
after magnetic tape was first used for storage.

magnetic tape: a medium for data storage made of plastic film with a magnetic coating
magnetic disk: a medium for data storage made of a thin, spinning disk with a magnetic
coating; divided into tracks and sectors to hold data

ary) Storage

5. UNIT 1: DATA STORAGE

Magnetic Media

+ For either type of magnetic medium,
interaction with it is controlled by a read
head and a write head.

+ Aread head uses the basic law of
physics that a state of magnetization
will affect an electrical property; a write
head uses the reverse law.

= Although they are separate devices,
the two heads are often combined
into a read/write head.

= The two alternative states of
magnetization are interpreted as a0
ora.

1.4 Mass (Secondary) Storage

me resgwrte head nscrbes data by
aligning each ofthe magnetic partes
In one of wo ways

Parts aligned one way
represent OS; the ather way
ropresont 1.

6. UNIT 1: DATA STORAGE

Magnetic Disks

+ For magnetic disks, read/write heads are

placed above and below the disk (also inca a

referred to as a platter) so that as the disk
spins, each head traverses a circle
called a track.
KEY TERM
track: a concentric circle on the surface of a

disk con 19 a continuous string of bits

+ By repositioning the read/write heads,
different concentric tracks can be
accessed.

4 Mass (Secondary) Storage

<⑪ っ

Sector

Track

Platter

7. UNIT 1: DATA STORAGE

esate
eas

Magnetic Disks

+ Frequently, a magnetic disc storage system
consists of several disks mounted on a
common spindle, one on top of the other.

+ In such cases, the read/write heads move
in unison — all together and at the same
speed.

p Each time they are repositioned, a new Sector
set of tracks called a
becomes accessible.

cylinder: the set of concentric tracks on all
surfaces Track

Platter

10. UNIT 1: DATA STORAGE

Magnetic Disks

+ Since a track can contain more information
than we would normally want to manipulate
at one time, each track is divided into
small arcs called

sector: a section of a track on a disk that can
be accessed independently

+ All sectors on a disk contain the same
number of bits.
ㅁ Typical sector capacities are from 512
bytes to a few KiB (kB).

Platter

Sector

Track

11. UNIT 1: DATA STORAGE

Magnetic Disks

+ In the simplest magnetic disks, each track
contains the same number of sectors.
= Thus, the bits within a sector on a track
near the outer edge of the disk are less
compactly stored than those on the
tracks near the center.

+ In contrast, high-capacity magnetic disks
utilize a technique called
to vary the number of sectors in each track.
KEY TERM

zoned-bit recording: a method for disk
storage where the sectors per track vary

Platter Without Zones

ary) Storage

Sector

Track

Platter With Zones

12. UNIT 1: DATA STORAGE

Magnetic Disks

+ In zoned-bit recording, tracks near the outer
edge contain significantly more sectors than
those near the center.

a Tracks are split into areas called zones.

2 All tracks within a zone have the same
number of sectors, but each zone has
more sectors per track than the zone
inside of it.

+ In this manner, efficient use of the entire disk
surface is achieved.

Sector

Track

Platter With Zones

13. UNIT 1: DATA STORAGE

Magnetic Disks

+ The capacity of a magnetic disk storage system depends on the number of platters
used and the density in which the tracks and sectors are placed.

= Low-capacity systems may consist of a single platter, especially where size of the
hard disk drive (HDD) must remain compact.

p High-capacity systems, capable of holding many tebibytes (terabytes), may consist
of three to six platters mounted on a common spindle.

・ Furthermore, data may be stored on both the upper and lower surfaces of each platter.

14. UNIT 1: DATA STORAGE

Performance of Magnetic Disks

+ Several measurements are used to evaluate the performance of a magnetic disk
storage System: seek time, rotational latency, access time, ano transfer rate

KEY TERM

2. Disk rotation until the desired

seek time: the time required to move the read/write 3. Disk rotation unt sector Rosana latency (lose)

heads from one track to another ne (¢ 5) 1. Head movement
rotational latency: the time required for the specified
sector to spin to the read/write head once the head has

from current position
to desired cylinder.
EX Seek time 0-105ms)

been positioned over the desired track

access time: the time required for the desired data to
be accessed; the sum of seek time and rotational delay
transfer rate: the rate at which data can be transferred
to or from the disk

15. UNIT 1: DATA STORAGE

Performance of Magnetic Disks

+ A factor limiting the access time and transfer rate is the speed at which a magnetic disk
system rotates.

+ To facilitate fast rotation speeds, the read/write heads in these systems do not touch the
disk but instead “float” just off the surface.
ㅁ The spacing is so close that even a single particle of dust could become jammed
between the head and disk surface, destroying both (a phenomenon known as a head
crash).

+ Thus, magnetic disk systems are typically housed in cases that are sealed at the factory.

a With this construction, disk systems are able to rotate at speeds of several hundred
times per second, achieving transfer rates that are measured in MiB (MB) per
second.

1.4 Mass (Secondary) Storage

17. UNIT 1: DATA STORAGE

Performance of Magnetic Disks

+ Since magnetic disk systems require physical motion for their operation, their operational
speed is limited when compared to speeds within electronic circuitry.

ㅁ Delay times within an electronic circuit are measured in units of nanoseconds
(billionths of a second) or less.

p In contrast, seek times, latency times, and access times of magnetic disk systems are
measured in milliseconds (thousandths of a second).

+ Thus, the time required to retrieve information from a magnetic disk system can seem like
an eternity to an electronic circuit awaiting a result.

1.4 Mass (Secondary) Storage

18. UNIT 1: DATA STORAGE

Magnetic Tape

・ Magnetic tape, although developed
before the magnetic disk, is now less
widely used — its most common use
today is to back up (make a copy of) the
data on a magnetic disk in case the disk
is later damaged.

+ Tapes come in several varieties, from
small tape cartridges to large reel-to-
reel models.

4 Mass (Secondary) Storage

Tape reel

Tape motion

Take-up ree!

19. UNIT 1: DATA STORAGE

Magnetic Tape

+ Magnetic tape drives have extremely
long seek times, just as their
predecessors audio and video cassettes
suffer from long rewind and fast-
forward times.

ㅁ To access data in the middle of the
tape, all the data before the desired
piece must be accessed.

+ However, low cost and high data
capacities still make magnetic tape
useful for applications where data is
primarily read or written linearly, such
as in archival data backups.

ary) Storage

Tape reel

Tape motion

Take-up ree!

20. 20

UNIT 1: DATA STORAGE

Optical Media

+ Another type of mass (secondary) storage
stores and reads data optically; that is,

aH laser beams — an example is the

optical disc: a medium for data storage read
from and written to using a laser beam

* Optical discs are thin circular discs made
out of polycarbonate substrate (a type of
very strong plastic) that are topped with
layers of other materials used to store
data and protect the disc.

1.4 Mass (Secondary) Storage

Disk motion

laquer layer
aluminum layer

polycarbonate
layer (clear plastic)

Data recorded on a single track,
consisting of individual sectors,
that spirals toward the outer edge

21. 21

UNIT 1: DATA STORAGE

Optical Media

Data can be stored on one or both
sides of an optical disc, and some types
of discs use multiple recording layers
on each side of the disk to increase
storage capacity.

An optical disc contains a single spiral
track (instead of multiple tracks like
magnetic disks), and the track is divided
into sectors to keep data organized.
p This single track begins at the
center of the disc and spirals out to
the edge of the disc.

1.4 Mass (Secondary) Storage

Disk motion

laquer layer
aluminum layer

polycarbonate
layer (clear plastic)

Data recorded on a single track,
consisting of individual sectors,
that spirals toward the outer edge

22. 22

UNIT 1: DATA STORAGE

Optical Media

+ Advantages of optical discs include
large capacity for their size and
durability — they are generally more
durable than magnetic media.

+ However, the discs should be handled
carefully to protect their recorded
surfaces from scratches, fingerprints,
and other marks that can interfere with
reading and writing data.

1.4 Mass (Secondary) Storage

Disk motion

laquer layer
aluminum layer

polycarbonate
layer (clear plastic)

Data recorded on a single track,
consisting of individual sectors,
that spirals toward the outer edge

23. 23

UNIT 1: DATA STORAGE

Representing Data on Optical Discs

+ Data is written to an optical disc in one
of two ways:
a With read-only optical discs (like
movie, music, and software CDs or
DVDs), the surface of the disc is

molded or stamped with a pattern.

a With recordable or rewritable optical
discs, the reflectivity of the disc is
changed using a laser.

+ In either case, the disc is read with a

laser and the computer interprets the
reflection of the laser off the disc
surface as Os and 1s.

laquer layer

‘aluminum layer

polycarbonate
layer (clear plastic)

motion

0000000000100000000001 0000000

‘aluminum
data layer II
LAND

aN

\

090 y

24. 24

UNIT 1: DATA STORAGE

Representing Data on Optical Discs

・ To accomplish this with molded or
stamped optical discs, tiny impressions
are created on the disc’s surface — these
impressions are called pits, and the areas
that are unchanged are called /ands.

+ When the disc is read, the amount of
laser light reflected back from the disc
changes when the laser reaches a
transition between a pit and a land.

a When the laser detects a transition, it
is interpreted as a 1; no transition for
a specific period of time indicates a
0.

motion

laquer layer

‘aluminum layer

polycarbonate

layer (clear plastic)

0000000000100000000001 0000000

‘aluminum
data layer II
LAND

aN

vase 4

\

25. 25

UNIT 1: DATA STORAGE

Representing Data on Optical Discs

・ For recordable or rewritable discs, the

recording laser beam changes the
reflectivity of areas of the disc —
nonreflective areas are pits, and
reflective areas are lands.

As with molded or stamped discs, the

transition between a pit and a land
represents a 1 and no transition for a

specific distance along the track
represents a 0.

Nass

motion

laquer layer

‘aluminum layer

polycarbonate

layer (clear plastic)

0000000000100000000001 0000000

‘aluminum
data layer II
LAND

aN

vase 4

\

27. UNIT 1: DATA STORAGE

Types of Optical Discs

ーー | cD DVD HDDVD Blu-ray
+ Different types of optical discs use different 下 ee
types of laser beams. Pe Yin o a

: ho ::. ee,
& Compact discs (CD) use infrared lasers. la dl 8:
which allow data to be stored more am m WE
compactly on the same size disc. zur: =: ==: =
en ee ーー
a Blu-ray discs (BD) use blue-violet lasers,
which can store data even more compactly.

KEY TERM 700 MB ic delivery; custom CDs
19 music, photos, etc.

25m

compact disc (CD): a low capacity optical disc ovo 4.7 GB movie and software delivery; custom
digital versatile disc (DVD): a medium capacity

DVD-DL (double layer) 8.5GB DVDs containing videos, photos, etc.

80 2568
optical disc BD-DL (double layer) 50GB movie and video game delivery
BOXL 1

Blu-ray disc (BD): a high capacity optical disc 0098.

29. 29

UNIT 1: DATA STORAGE

Solid-State Media

+ À common property of mass (secondary) storage systems based on magnetic or optic
technology is that physical motion, such as spinning disks, moving read/write heads,
and aiming laser beams, is required to store and retrieve data.

a This means that data storage and retrieval is slow compared to the speed of
electronic circuitry.

+ Solid-state media has the potential of alleviating this drawback.

solid-state media: non-volatile computer storage with no moving parts

ary) Storage

30. 30

UNIT 1: DATA STORAGE

Solid-State Media

+ The basis for most solid-state media is something called flash memory, where bits are
stored by sending electronic signals directly to the storage medium, causing electrons
to be trapped in tiny chambers of silicon dioxide.

ㅁ Since these chambers are able to hold their captive electrons for many years without
external power, this technology is excellent for portable, non-volatile storage.

Floating

Control Control

Oxide

Drain Source

Erased Programmed

1.4 Mass (Secondary) Storage

31. 31

UNIT 1: DATA STORAGE

Solid-State Media

+ Although data stored in flash memory systems can be accessed in small, byte-sized
units as in RAM, current technology dictates that stored data be erased in large blocks
(big chunks erased in a “flash”, hence the name “flash memory”).

a Moreover, repeated erasing slowly damages the silicon dioxide chambers,
meaning that current flash memory technology is not suitable for general main
(primary) memory applications where its contents might be altered many times a
second.

« However, in applications where alterations to the memory contents can be controlled
at a reasonable level (such as in digital cameras and smartphones), flash memory has
become the mass (secondary) storage technology of choice.

32. 32

UNIT 1: DATA STORAGE

USB Flash Drives

. USB flash drives consist of flash

memory media integrated into a self-
contained unit that connects to a
computer or other device with a
standard USB port and is powered via
that port.

USB flash drive: a small storage device that
plugs into a USB port and contains flash
memory media

1.4 Mass (

CUSTOM LANYARD DAVE

(Secondary) Storage

CONVENTIONAL DRIVE

0000

NATION
ING GUIDE

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CUSTOM WALLET DRIVE

33. 33

UNIT 1: DATA STORAGE

USB Flash Drives

+ The high capacity of these portable units
as well as the fact that they are easily
connected to and disconnected from a
computer make them ideal for portable

data storage.

+ However, the vulnerability of their tiny
storage chambers means that they are
not as reliable as optical disks for truly >

long-term applications.

CUSTOM LANYARD DAVE

1.4 Mass (Secondary) Storage

CONVENTIONAL DRIVE

"004

NATION
ANNING GUIDE

CUSTOM WALLET DRIVE

34. 34

UNIT 1: DATA STORAGE

Solid-State Drives (SSDs)

+ Larger flash memory devices called solid-
state drives (SSDs) are explicitly designed

to take the place of magnetic hard disks.

solid-state drive (SSD): a hard drive that
uses flash memory media instead of metal
magnetic hard disks

+ SSDs are more resistant to vibrations and
physical shock than magnetic hard disks.
p Further, the lack of moving parts means
quieter operation and lower access
times.

By Transcend

Data is stored in flash memory
chips located inside the drive;
unlike magnetic drives, there
are no moving parts,