Types of External Memory

(1)

Willia m St a llings Willia m St a llings

Com put e r Orga niza t ion a nd Arc hit e c t ure

a nd Arc hit e c t ure 8t h _{Edit ion}

Cha pt e r 6p

(2)

T ype s of Ex t e rna l M e m oryyp y • Magnet ic Disk

RAI D

—RAI D

—Rem ovable

• Opt ical

—CD- ROM

—CD- Recordable ( CD- R)

—CD- R/ W

—DVD

(3)

M a gne t ic Diskg

• Disk subst rat e coat ed wit h m agnet izable m at erial ( iron oxide rust )

m at erial ( iron oxide…rust )

• Subst rat e used t o be alum inium

• Now glass

—I m proved surface uniform it y

– I ncreases reliabilit y

—Reduct ion in surface defect s

– Reduced read/ writ e errors

—Low er flight height s ( See lat er)

—Bet t er st iffness

(4)

Re a d a nd Writ e M e c ha nism s

• Recording & ret rieval via conduct ive coil called a head

• May be single read/ writ e head or separat e onesMay be single read/ writ e head or separat e ones

• During read/ writ e, head is st at ionary, plat t er rot at es

• Writ e

— Current t hrough coil produces m agnet ic field

— Pulses sent t o head

— Magnet ic pat t ern recorded on surface belowMagnet ic pat t ern recorded on surface below

• Read ( t radit ional)

— Magnet ic field m oving relat ive t o coil produces current

— Coil is t he sam e for read and writ e

• Read ( cont em porary)

— Separat e read head, close t o w rit e headSeparat e read head, close t o w rit e head

— Part ially shielded m agnet o resist ive ( MR) sensor

— Elect rical resist ance depends on direct ion of m agnet ic field

Hi h f i

— High frequency operat ion

(5)

(6)

Da t a Orga niza t ion a nd Form a t t ingg g • Concent ric rings or t racks

G b t t k

—Gaps bet ween t racks

—Reduce gap t o increase capacit y

S b f bi k ( i bl

—Sam e num ber of bit s per t rack ( variable packing densit y)

Const ant ang la elocit

—Const ant angular velocit y

• Tracks divided int o sect ors

• Minim um block size is one sect or

(7)

(8)

Disk V e loc it yy

—Gives pie shaped sect ors and concent ric t racks

—I ndividual t racks and sect ors addressable

—Move head t o given t rack and wait for given sect or

—Wast e of space on out er t racks

– Low er dat a densit y

• Can use zones t o increase capacit y

Each zone has fixed bit s per t rack

—Each zone has fixed bit s per t rack

(9)

(10)

Finding Se c t orsg

• Must be able t o ident ify st art of t rack and sect or

sect or

• Form at disk

—Addit ional inform at ion not available t o user

(11)

(12)

Cha ra c t e rist ic s

• Fixed ( rare) or m ovable head

bl f d

• Rem ovable or fixed

• Single or double ( usually) sided

• Single or m ult iple plat t er

• Head m echanism

—Cont act ( Floppy) Fixed gap

—Fixed gap

(13)

Fix e d/M ova ble H e a d Disk • Fixed head

O d it h d t k

—One read writ e head per t rack

—Heads m ount ed on fixed ridged arm

• Movable head

—One read writ e head per side

(14)

Re m ova ble or N ot • Rem ovable disk

C b d f d i d l d it h

—Can be rem oved from drive and replaced wit h anot her disk

Provides unlim it ed st orage capacit y

—Provides unlim it ed st orage capacit y

—Easy dat a t ransfer bet w een syst em s

N bl di k

• Nonrem ovable disk

(15)

M ult iple Pla t t e rp

• One head per side

d d d l d

• Heads are j oined and aligned

• Aligned t racks on each plat t er form cylinders

• Dat a is st riped by cylinderat a s st ped by cy de

—reduces head m ovem ent

(16)

M

u

lt

ip

le

P

la

tt

e

rs

(17)

(18)

Floppy Diskppy

• 8” , 5.25” , 3.5”

S ll

• Sm all capacit y

—Up t o 1.44Mbyt e ( 2.88M never popular)

• Slow

• UniversalUniversal

• Cheap

• Obsolet e?

(19)

Winc he st e r H a rd Disk (1 )( )

• Developed by I BM in Winchest er ( USA)

S l d

• Sealed unit

• One or m ore plat t ers ( disks)

• Heads fly on boundary layer of air as disk spins

sp s

• Very sm all head t o disk gap

• Get t ing m ore robust

(20)

Winc he st e r H a rd Disk (2 )( ) • Universal

Ch

• Cheap

• Fast est ext ernal st orage

• Get t ing larger all t he t im e

(21)

Spe e dp

• Seek t im e

M i h d t t t k

—Moving head t o correct t rack

• ( Rot at ional) lat ency

—Wait ing for dat a t o rot at e under head

• Access t im e = Seek + Lat encyy

(22)

(23)

RAI D

• Redundant Array of I ndependent Disks

d d f k

• Redundant Array of I nexpensive Disks

• 6 levels in com m on use

• Not a hierarchy

• Set of physical disks viewed as single

• Set of physical disks viewed as single logical drive by O/ S

• Dat a dist ribut ed across physical drives

• Can use redundant capacit y t o st ore

it i f t i

(24)

RAI D 0

• No redundancy

d ll d k

• Dat a st riped across all disks

• Round Robin st riping

• I ncrease speed

—Mult iple dat a request s probably not on sam e Mult iple dat a request s probably not on sam e disk

—Disks seek in parallelp

(25)

RAI D 1

• Mirrored Disks

d d k

• Dat a is st riped across disks

• 2 copies of each st ripe on separat e disks

• Read from eit her

• Writ e t o bot h

• Recovery is sim ple

S f lt di k & i

—Swap fault y disk & re- m irror

—No dow n t im e

(26)

RAI D 2

• Disks are synchronized

ll

• Very sm all st ripes

—Oft en single byt e/ w ord

• Error correct ion calculat ed across corresponding bit s on disks

• Mult iple parit y disks st ore Ham m ing code error correct ion in corresponding posit ions error correct ion in corresponding posit ions

• Lot s of redundancy Expensive

—Expensive

(27)

RAI D 3

• Sim ilar t o RAI D 2

O l d d d k h

• Only one redundant disk, no m at t er how large t he array

• Sim ple parit y bit for each set of corresponding bit s

• Dat a on failed drive can be reconst ruct ed from surviving dat a and parit y infoo su g da a a d pa y o

(28)

RAI D 4

• Each disk operat es independent ly

G d f h h / O

• Good for high I / O request rat e

• Large st ripes

• Bit by bit parit y calculat ed across st ripes on each disk

o eac d s

(29)

RAI D 5

• Like RAI D 4

d ll d k

• Parit y st riped across all disks

• Round robin allocat ion for parit y st ripe

• Avoids RAI D 4 bot t leneck at parit y disk

• Com m only used in net work servers

N B DOES NOT MEAN 5 DI SKS! ! ! ! !

(30)

RAI D 6

• Two parit y calculat ions

S d bl k d ff

• St ored in separat e blocks on different disks

• User requirem ent of N disks needs N+ 2

• High dat a availabilit yg dat a a a ab t y

—Three disks need t o fail for dat a loss

(31)

R

A

ID

0

,

1

,

2

,

(32)

R

A

ID

3

&

(33)

R

A

ID

5

&

(34)

(35)

Opt ic a l St ora ge CD-ROMp g • Originally for audio

6 0 b 0 d

• 650Mbyt es giving over 70 m inut es audio

• Polycarbonat e coat ed wit h highly reflect ive coat , usually alum inium

• Dat a st ored as pit sat a st o ed as p t s

• Read by reflect ing laser

• Const ant packing densit y

(36)

C

D

O

p

e

ra

ti

o

n

(37)

CD-ROM Drive Spe e dsp • Audio is single speed

C t t li i l it

—Const ant linier velocit y

(38)

CD-ROM Form a t

• Mode 0= blank dat a field

• Mode 1= 2048 byt e dat a+ error correct ion

(39)

Ra ndom Ac c e ss on CD-ROM • Difficult

h d h

• Move head t o rough posit ion

• Set correct speed

• Read address

• Adj ust t o required locat ion

(40)

CD-ROM for & a ga instg • Large capacit y ( ?)

d

• Easy t o m ass produce

• Rem ovable

• Robust

• Expensive for sm all runs Sl

• Slow

(41)

Ot he r Opt ic a l St ora gep g • CD- Recordable ( CD- R)

WORM

—WORM

—Now affordable

C ibl i h CD ROM d i

(42)

DV D - w ha t ’s in a na m e ? • Digit al Video Disk

U d t i di t l f i

—Used t o indicat e a player for m ovies

– Only plays video disks

Di it l V t il Di k

• Digit al Versat ile Disk

—Used t o indicat e a com put er drive

Will d di k d l id di k

– Will read com put er disks and play video disks

• Dogs Verit able Dinner

(43)

DV D - t e c hnologygy • Mult i- layer

h h ( G l )

• Very high capacit y ( 4.7G per layer)

• Full lengt h m ovie on single disk

—Using MPEG com pression

• Finally st andardized ( honest ! )Finally st andardized ( honest ! )

• Movies carry regional coding

• Players only play correct region film s

(44)

DV D – Writ a ble

• Loads of t rouble w it h st andards

d d

• First generat ion DVD drives m ay not read first generat ion DVD- W disks

• First generat ion DVD drives m ay not read CD- RW disks

(45)

C

D

a

n

d

D

V

(46)

H igh De finit ion Opt ic a l Disk sg p

• Designed for high definit ion videos

• Much higher capacit y t han DVD

—Short er wavelengt h laser

– Blue- violet range

—Sm aller pit s

• HD- DVD

• HD DVD

—15GB single side single layer

• Blue- ray

• Blue ray

—Dat a layer closer t o laser

– Tight er focus, less dist ort ion, sm aller pit sg , , p

—25GB on single layer

(47)

(48)

M a gne t ic T a peg p • Serial access

Sl

• Slow

• Very cheap

• Backup and archive

• Linear Tape- Open ( LTO) Tape Drives

• Linear Tape Open ( LTO) Tape Drives

—Developed lat e 1990s

Open source alt ernat ive t o propriet ary t ape

(49)

Line a r T a pe -Ope n (LT O) T a pe Drive sp p ( ) p

LTO-1 LTO-2 LTO-3 LTO-4 LTO-5 LTO-6

Release date 2000 2003 2005 2007 TBA TBA

Compressed capacity 200 GB 400 GB 800 GB 1600 GB 3.2 TB 6.4 TB

Compressed transfer rate (MB/s)

40 80 160 240 360 540

Linear density (bits/mm)

4880 7398 9638 13300

Tape tracks 384 512 704 896

Tape length 609 m 609 m 680 m 820 m

Tape width (cm) 1.27 1.27 1.27 1.27

(50)

I nt e rne t Re sourc e s

• Opt ical St orage Technology Associat ion

G d f i f t i b t t i l

—Good source of inform at ion about opt ical st orage t echnology and vendors

Ext ensive list of relevant links

—Ext ensive list of relevant links

• DLTt ape

—Good collect ion of t echnical inform at ion and links t o vendors

S h RAI D