Types of External Memory

  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

  t h

  8 Edit ion Cha pt e r 6 p Ex t e rna l M e m ory

  T ype s of Ex t e rna l M e m ory yp y

• Magnet ic Disk

  RAI D — RAI D — Rem ovable

• Opt ical

  — CD- ROM — CD- Recordable ( CD- R) — CD- R/ W — DVD

• Magnet ic Tape g p

  M a gne t ic Disk g

• • 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

  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 ones May 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 below Magnet 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 head Separat e read head, close t o w rit e head —

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

  d a e R R

  Da t a Orga niza t ion a nd Form a t t ing g g

  Concent ric rings or t racks • — G Gaps bet ween t racks b t t k — Reduce gap t o increase capacit y — Sam e num ber of bit s per t rack ( variable S b f bi k ( i bl packing densit y)

  — Const ant angular velocit y Const ant ang la elocit Tracks divided int o sect ors • Minim um block size is one sect or • May have m ore t han one sect or per block y

• p

  Disk V e loc it y y

• • Bit near cent re of rot at ing disk passes fixed point

  slow er t han bit on out side of disk slow er t han bit on out side of disk

• • I ncrease spacing bet w een bit s in different t racks

• Rot at e disk at const ant angular velocit y ( CAV) • Rot at e disk at const ant angular velocit y ( CAV)

  — 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 — More com plex circuit ry

  m ra g g ia D s d

  Finding Se c t ors g

  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

— Marks t racks and sect ors

  t a rm o

  — Cont act ( Floppy) Fixed gap — Fixed gap

  Cha ra c t e rist ic s

• Fixed ( rare) or m ovable head
• bl f d Rem ovable or f

• • Single or double ( usually) sided

• Single or m ult iple plat t er
• Head m echanism • Head m echanism

  — Flying ( Winchest er)

  Fix e d/M ova ble H e a d Disk

• Fixed head

  — O One read writ e head per t rack d it h d t k — Heads m ount ed on fixed ridged arm Movable head •

  — One read w rit e head per side — Mount ed on a m ovable arm

  Re m ova ble or N ot

  Rem ovable disk • — Can be rem oved from drive and replaced wit h C b d f d i d l d it 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 Nonrem ovable disk • N bl di k

  — Perm anent ly m ount ed in t he drive

  M ult iple Pla t t e r p

  One head per side • Heads are j oined and aligned • d d d l d Aligned t racks on each plat t er form • cylinders Dat a is st riped by cylinder at a s st ped by cy de •

  — reduces head m ovem ent — I ncreases speed ( t ransfer rat e) I ncreases speed ( t ransfer rat e)

  rs e

  Floppy Disk ppy

  8” , 5.25” , 3.5” • Sm all capacit y • S ll — Up t o 1.44Mbyt e ( 2.88M never popular)

  Slow • Universal Universal • Cheap • Obsolet e? • Obsolet e? •

  Winc he st e r H a rd Disk (1 ) ( )

  Developed by I BM in Winchest er ( USA) • Sealed unit • S l d One or m ore plat t ers ( disks) • Heads fly on boundary layer of air as disk • sp spins s Very sm all head t o disk gap • Get t ing m ore robust • Get t ing m ore robust •

  Winc he st e r H a rd Disk (2 ) ( )

  Universal • Cheap • Ch Fast est ext ernal st orage • Get t ing larger all t he t im e •

  — 250 Gigabyt e now easily available 250 Gigabyt e now easily available

  Spe e d p

• 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 ency y
• Transfer rat e

  r fe s n ra T

  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 • Dat a dist ribut ed across physical drives
• Can use redundant capacit y t o st ore

  it i f t i parit y inform at ion

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

  RAI D 0

• No redundancy

• • d ll d k Dat a st riped across all disks

• Round Robin st riping
• I ncrease speed

  — Disks seek in parallel p

— A set of dat a is likely t o be st riped across

m ult iple disks

  RAI D 1

• Mirrored Disks
• d d k Dat a is st riped across d

• • 2 copies of each st ripe on separat e disks

• Read from eit her
• Writ e t o bot h • 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

• Expensive

  RAI D 2

• Disks are synchronized

  Very sm all st ripes • ll — 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

  RAI D 3

  Sim ilar t o RAI D 2 • Only one redundant disk, no m at t er how • O l d d d k h 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 info o su g da a a d pa y o Very high t ransfer rat es •

  RAI D 4

• Each disk operat es independent ly

  Good for high I / O request rat e • G d f h h / O Large st ripes • Bit by bit parit y calculat ed across st ripes • o on each disk eac d s Parit y st ored on parit y disk •

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

  RAI D 5

• Like RAI D 4
• d ll d k Parit y st riped across all d

• • 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 • Com m only used in net work servers

  RAI D 6

• Two parit y calculat ions

  St ored in separat e blocks on different • S d bl k d ff disks User requirem ent of N disks needs N+ 2 • High dat a availabilit y g dat a a a ab t y •

  — Three disks need t o fail for dat a loss — Significant w rit e penalt y Significant w rit e penalt y

  ID A R

  Opt ic a l St ora ge CD-ROM p g

  Originally for audio • 650Mbyt es giving over 70 m inut es audio • 6 0 b d Polycarbonat e coat ed wit h highly • reflect ive coat , usually alum inium Dat a st ored as pit s at a st o ed as p t s • Read by reflect ing laser • Const ant packing densit y • Const ant packing densit y • Const ant linear velocit y •

  CD-ROM Drive Spe e ds p

• Audio is single speed

  C t t li i l it — Const ant linier velocit y — 1.2 m s ^{- 1}

  T k ( i l) i 5 27k l — Track ( spiral) is 5.27km long — Gives 4391 seconds = 73.2 m inut es

• • Ot her speeds are quot ed as m ult iples

• e.g. 24x g

• • Quot ed figure is m axim um drive can

  achieve achieve

  CD-ROM Form a t

• Mode 0= blank dat a field

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

  Ra ndom Ac c e ss on CD-ROM

• Difficult
• h d h Move head t o rough posit>Set correct speed
• Read address
• Adj ust t o required locat ion • Adj ust t o required locat ion
• ( Yaw n! )

  CD-ROM for & a ga inst g

• Large capacit y ( ?)
• d Easy t o m ass pro
• Rem ovable
• Robust

  • Expensive for sm all runs

  Sl • Slow

• Read only

  Ot he r Opt ic a l St ora ge p g

  CD- Recordable ( CD- R) • — WORM WORM — Now affordable — C Com pat ible wit h CD- ROM drives ibl i h CD ROM d i

  CD- RW • — Erasable — Get t ing cheaper — Most ly CD- ROM drive com pat ible — Phase change

  Mat erial has t wo different reflect ivit ies in different –

  DV D - w ha t ’s in a na m e ?

• Digit al Video Disk

  — Used t o indicat e a player for m ovies U d t i di t l f i Only plays video disks –

• Digit al Versat ile Disk Di it l V t il Di k

  — Used t o indicat e a com put er drive Will read com put er disks and play video disks – Will d di k d l id di k

  Dogs Verit able Dinner • Officially - not hing! ! ! •

  DV D - t e c hnology gy

• Mult i- layer
• h h ( G l ) Very high capacit y ( 4.7G per la
• 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 • Players only play correct region film s

• Can be “ fixed”

  DV D – Writ a ble

  Loads of t rouble wit h st andards • First generat ion DVD drives m ay not read • d d first generat ion DVD- W disks

  First generat ion DVD drives m ay not read • CD- RW disks Wait for it t o set t le down before buying! •

  H igh De finit ion Opt ic a l Disk s g p

• Designed for high definit ion videos
• Much higher capacit y t han DVD • Much higher capacit y t han DVD

  — Short er w avelengt h laser

• – Blue- violet range – 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 s g , , p

  —

  25GB on single layer

  s c ti s ri te c ra a h

  M a gne t ic T a pe g 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 — Open source alt ernat ive t o propriet ary t ape syst em s

  Line a r T a pe -Ope n (LT O) T a pe Drive s p p ( ) p _{LTO-1 LTO-2 LTO-3 LTO-4 LTO-5 LTO-6 Compressed capacity 200 GB 400 GB 800 GB 1600 GB}

Release date 2000 2003 2005 2007 TBA TBA

_{3.2 TB 6.4 TB} Compressed transfer _{rate (MB/s)} ^{40 80 160 240 360 540} _{Tape tracks 384 512 704 896} Linear density 4880 7398 9638 13300 _{(bits/mm) Tape width (cm)} Tape length 609 m 609 m 680 m 820 m _{1.27 1.27 1.27 1.27}

  I nt e rne t Re sourc e s

  Opt ical St orage Technology Associat ion • — Good source of inform at ion about opt ical G d f i f t i b t t i l 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

  Search on RAI D • S h RAI D