Tải bản đầy đủ - 0 (trang)
Chapter 9.  Hard Disk Storage

Chapter 9.  Hard Disk Storage

Tải bản đầy đủ - 0trang

HardDriveHistoryandAdvancements

In1957,CyrilNorthcoteParkinsonpublishedhisfamous

compilationofessaystitledParkinson'sLaw,whichstartsoff

withthestatement,"Workexpandssoastofillthetime

availableforitscompletion."AcorollaryofParkinson'smost

famous"law"canbeappliedtoharddrives:Dataexpandssoas

tofillthespaceavailableforitsstorage.This,ofcourse,means

thatnomatterhowbigadriveyouget,youwillfindawaytofill

it.Ihavelivedbythatdictumsincepurchasingmyfirsthard

diskdriveover20yearsago.

AlthoughIamwellawareoftheexponentialgrowthof

everythingassociatedwithcomputers,Iamstillamazedathow

largeandfastmoderndriveshavebecome.Thefirstharddrive

Ipurchasedin1983wasa10MB(that's10megabytes,not

gigabytes).TheMiniscribemodel2012wasa5.25-inch(platter)

drivethatwasabout8"x5.75"x3.25"(203mmx146mmx83mm)

inoverallsizeandweighed5.5lb.(2.5kg).That'sheavierthan

someoftoday'slaptopcomputers!Bycomparison,oneofthe

biggestdrivesavailabletodate,the500GBHitachi7K500SATA

driveusessmaller3.5-inchplatters,isabout5.75"x4"x1"

(146mmx102mmx25mm)inoverallsize,weighsonly1.54lb.

(0.70kg),andstoresawhopping500GB,whichis50,000times

morestorageinapackagethatisaboutone-sixththesizeand

one-fourththeweightofmyoldMiniscribe.Byanother

comparison,a160GB2.5-inchSeagateMomentus5400.3160

driveusesevensmaller2.5-inchplatters,isabout

3.94"x2.76"x0.37"(100mmx70mmx9.5mm)inoverallsize,

weighsonly0.22lb.(99g),andstores160GB,whichis16,000

timesmorestorageinapackagethatisabout37timessmaller

and1/25ththeweightofmyfirstdrive.

Obviouslythelargestoragecapacitiesfoundonmoderndrives

areuselessunlessyoucanalsotransferthedatatoandfrom

thediskquickly.TheharddiskasfoundintheoriginalIBMXTin



1983hadaconstantdatatransferratefromthemediaofabout

100KBps.Today,mostcommonlyuseddrivesfeaturetheSerial

Atainterfaceofferingvariablemediadatatransferratesofupto

66MBps(averageratesarelower,uptoabout50MBps).Much

liketheincreaseindrivecapacitythespeedoftheinterfacehas

alsocomealongwaysincetheMFMandRLLinterfacesthat

werecommonplaceinthe'80s.Asalways,theinterfacesare

muchfasterthantheactualdrives.TheParallelATA,SerialATA,

andSCSIinterfacesarecommonplacenowadaysofferdata

transferratesofupto133MBpsforParallelATA,150and

300MBpsforSerialATAand320MBpsbandwidthforUltra-320

SCSI.Alloftheseinterfacesaremuchfasterthanthedrives

theysupport,meaningthatthetruetransferrateyouwillseeis

almostentirelylimitedbythedriveandnottheinterfaceyou

choose.Themoderninterfaceshavebandwidthtosparefor

futuredevelopmentsandadvancesinharddisktechnology.

Insummary,itisclearthattheseareprettylargestepsinjust

over20yearstime!



Note

ThebookParkinson'sLaw(ISBN1568490151)isstill

inprintandisconsideredoneoftheessentialtomes

ofbusinessormanagementstudytoday.



Togiveyouanideaofhowfarharddriveshavecomeinthe

past20years,I'veoutlinedsomeofthemoreprofoundchanges

inharddiskstorage:

Maximumstoragecapacitieshaveincreasedfromthe5MB

and10MB5.25-inchfull-heightdrivesavailablein1982to

500GBin2005for3.5-inchhalf-heightdrives(Hitachi



7K500,500GBSATA),160GBfornotebooksystemwith2.5inchdrives(SeagateMomentus5400.3160),and60GBfor

1.8-inchdrives(ToshibaMK-6006GAH,60GB).Harddrives

smallerthan40GBarerareindesktoporevenlaptop

systems.

Data-transferratestoandfromthemedia(sustained

transferrates)haveincreasedfromabout100KBpsforthe

originalIBMXTin1983toanaverageof50MBpsforsome

ofthefastestdrivestoday(WesternDigitalRaptor

WD74GD)ormorethan80MBpsforthefastestSCSIdrive

(SeagateCheetah15K.4).

Averageseektimes(howlongittakestomovetheheadsto

aparticularcylinder)havedecreasedfrommorethan85ms

(milliseconds)forthe10MBdrivesusedbyIBMinthe1983

vintagePC-XTto3.3msforsomeofthefastestdrivestoday

(SeagateCheetah15K.4).

In1982and1983,a10MBdriveandcontrollercostmore

than$2,000($200permegabyte),whichwouldbemore

thandoublethatintoday'sdollars.Today,thecostof

desktopharddrives(withintegratedcontrollers)has

droppedto0.05centpermegabyteorless,orabout100GB

for$50.Laptopdriveshavefallento0.1centsper

megabyteorless,orabout100GBfor$100!



ArealDensity

Arealdensityisoftenusedasatechnologygrowth-rate

indicatorfortheharddiskdriveindustry.Arealdensityis

definedastheproductofthelinearbitsperinch(bpi),

measuredalongthelengthofthetracksaroundthedisk,

multipliedbythenumberoftracksperinch(tpi),measured

radiallyonthedisk(seeFigure9.1).Theresultsareexpressed

inunitsofmegabitsorgigabitspersquareinch(Mbit/sq.in.or

Gbit/sq.in.)andareusedasameasureofefficiencyindriverecordingtechnology.Currenthigh-end2.5-inchdrivesrecord

atarealdensitiesover100Gbit/sq.in.Thisdensitywillallowfor

3.5-inchdriveswithcapacitiesof1000GBor1TBinthenext

fewyears.



Figure9.1.Arealdensity,combiningtracksper

inchandbitsperinch.



[Viewfullsizeimage]



Drivesrecorddataintracks,whicharecircularbandsofdataon

thedisk.Eachtrackisdividedintosectors.Figure9.2showsan

actualfloppydisksprayedwithmagneticdeveloper(powdered

iron)suchthatanimageoftheactualtracksandsectorscanbe



clearlyseen.Thediskshownisa5.25-inch360KBfloppy,which

has40tracksperside,andeachtrackisdividedintonine

sectors.Notethateachsectorisdelineatedbygapsinthe

recording,whichprecedeandfollowthetrackandsector

headers(whereIDandaddressinformationresides).Youcan

clearlyseethetriplegapprecedingthefirstsector,which

includesthetrackandsectorheaders.Then,followingina

counterclockwisedirection,youseeeachsubsequentsector,

precededbygapsdelineatingtheheaderforthatsector.Datais

writtenintheareabetweentheheaders.



Figure9.2.A360KBfloppydiskmediasprayed

withmagneticdeveloper(powderediron)

showingtheactualtrackandsectorimages.



[Viewfullsizeimage]



Noticethatsector9islongerthantherest;thisistoenable

rotationalspeeddifferencesbetweendrivessothatallthedata

canbewrittenbeforerunningintothestartofthetrack.Also

noticethatagoodportionofthedisksurfaceisn'tusedbecause

itissimplyimpracticaltohavetheheadstravelinandoutthat

far,andthedifferenceinlengthbetweenthesectorsonthe

innerandoutertracksbecomesmoreofaproblem.

Arealdensityhasbeenrisingsteadilysincethefirstmagnetic

storagedrive(IBMRAMAC)wasintroducedin1956,initiallyata

growthrateofabout25%peryear(doublingeveryfouryears),

andsincetheearly1990satagrowthrateofabout60%per

year(doublingevery1.5years).Thedevelopmentand

introductionofmagneto-resistiveheadsin1991andgiant



magneto-resistiveheadsin1997propelledtheincreaseinthe

arealdensitygrowthrate.Inthe47+yearssincetheRAMAC

drivewasintroduced,thearealdensityofmagneticstoragehas

increasedmorethanfivemillionfold.

Currently,drivemanufacturershaveachievedarealdensities

greaterthan100Gbit/sq.in.,whichisconsiderednearthepoint

atwhichthesuperparamagneticeffecttakesplace.Thisisan

effectinwhichthemagneticdomainsbecomesosmallthatthey

areintrinsicallyunstableatroomtemperature.Techniquessuch

asextremelyhighcoercivitymediaandperpendicular(vertical

polarity)recordingareprojectedtoenablemagneticstorage

densitiesof200Gbit/sq.in.ormore.Verticalpolarityrecording,

orperpendicularrecordingasitisoftencalled,alignsthebits

vertically,perpendiculartothedisk,whichallowsadditional

roomonadisktopackmoredata,thusenablinghigher

recordingdensities.

Severaldrivemanufacturershavealreadyannounced2.5-and

3.5-inchdrivesbasedonthistechnology.Thistechnologywill

seeitsdebutin2.5-inchdrivesforlaptopsand1-inchdrivesfor

portableelectronics,suchasdigitalcamerasandPDAsfirst,

beforefindingitswayinto3.5-inchdrivesfordesktopPCsand

servers.

Figure9.3showshowarealdensityhasincreasedfromwhen

magneticstoragewasfirstdeveloped(1956RAMAC)through

thepresenttime.



Figure9.3.Evolutionofarealdensityinmagnetic

diskstorage.



[Viewfullsizeimage]



Toincreasearealdensitywhilemaintainingthesameexternal

driveformfactors,drivemanufacturershavedevelopedmedia

andheadtechnologiestosupportthesehigherarealdensities,

suchasceramic/glassplatters,giantmagneto-resistive(GMR)

heads,pseudo-contactrecording,andpartialresponse

maximumlikelihood(PRML)electronics.Theprimarychallenge

inachievinghigherdensitiesismanufacturingdriveheadsand

diskstooperateatclosertolerances.Improvementsin

tolerancesandtheuseofmoreplattersinagivenformfactor

continuetofuelimprovementsindrivecapacity,butdrive

makerscontinuetoseekevengreatercapacityincreases,both

byimprovingcurrenttechnologiesandbydevelopingnewones.

Tofitmoredataonaplatterofagivensize,thetracksmustbe

placedclosertogether,andtheheadsmustbecapableof

achievinggreaterprecisionintheirplacementsoverthetracks.

Thisalsomeansthatasharddiskcapacitiesincrease,heads

mustfloateverclosertothedisksurfaceduringoperation.The

gapbetweentheheadanddiskisascloseas10nanometers

(0.01microns)insomedrives,whichisapproximatelythe



thicknessofacellmembrane.Bycomparison,ahumanhairis

typically80micronsindiameter,whichis8,000timesthicker

thanthegapbetweentheheadanddiskinsomedrives.The

prospectofactualcontactornearcontactrecordingisbeing

consideredforfuturedrivestofurtherincreasedensity.



FormFactors

ThecornerstoneofthePCindustryhasalwaysbeen

standardization.Withdiskdrives,thisisevidentinthephysical

andelectricalformfactorsthatcomprisemoderndrives.By

usingindustry-standardformfactors,youcanpurchasea

systemorchassisfromonemanufacturerandyetphysically

andelectricallyinstalladrivefromadifferentmanufacturer.

Formfactorstandardsensurethatavailabledriveswillfitinthe

bay,thescrewholeswilllineup,andthestandardcablesand

connectionswillplugin.Withoutindustrystandardsforform

factors,therewouldbenocompatibilitybetweendifferent

chassis,motherboards,cables,anddrives.

Youmightwonderhowtheseformfactorsareestablished.In

somecases,itissimplythatonemanufacturermakesapopular

productofaparticularshapeandconnectionprotocol,and

otherssimplycopyorclonethoseparameters,makingother

productsthatarephysicallyand/orelectricallycompatible.In

othercases,variouscommitteesorgroupsthatdictatecertain

industrystandardshavebeenformed.Thenitisuptothe

companiesthatmakeapplicableproductstocreatethemto

conformtothesestandards.

Overtheyears,diskdriveshavebeenintroducedinseveral

industry-standardformfactors,normallyidentifiedbythe

approximatesizeoftheplatterscontainedinsidethedrive.

Table9.1liststhedifferentdiskdriveformfactorsthathave

beenusedinPCsandportables.

Table9.1.HardDiskFormFactors



Table9.1.HardDiskFormFactors

5.25-inchHalf-Height

Drives:



5.25-inchDrives:



Height



3.25in.



82.6mm Height



1.63in.



41.3mm



Width



5.75in.



146.0mm Width



5.75in.



146.0mm



Depth



8.00in.



203.2mm Depth



8.00in.



203.2mm



Volume



149.5ci 2449.9

cc



74.8ci



1224.9

cc



3.5-inchHalf-Height

Drives:



Volume



3.5-inch1/3-Height

Drives:



Height



1.63in.



41.3mm Height



1.00in.



25.4mm



Width



4.00in.



101.6mm Width



4.00in.



101.6mm



Depth



5.75in.



146.0mm Depth



5.75in.



146.0mm



Volume



37.4ci



612.5cc Volume



23.0ci



376.9cc











































































2.5-inchDrives:

Height1



19.0mm 0.75in.



Height2



17.0mm 0.67in.



Height3



12.7mm 0.50in.



2.5-inchDrives:

Height4



12.5mm 0.49in.



Height5



9.5mm



0.37in.



Height6



8.5mm



0.33in.



Width



70.0mm 2.76in.



Depth



100.0mm 3.94in.



Vol1



133.0cc 8.1ci



Vol2



119.0cc 7.3ci



Vol3



88.9cc



5.4ci



Vol4



87.5cc



5.3ci



Vol5



66.5cc



4.1ci



Vol6



59.5cc



3.6ci



1.8-inchDrives:



































































































1.8-inchPCCardDrives:



Height1



9.5mm



0.37in.



Height1



8.0mm



0.31in.



Height2



7.0mm



0.28in.



Height2



5.0mm



0.20in.



Width



70.0mm 2.76in.



Width



54.0mm 2.13in.



Depth



60.0mm 2.36in.



Depth



78.5mm 3.09in.



Vol1



39.9cc



2.4ci



Vol1



33.9cc



2.1ci



Vol2



29.4cc



1.8ci



Vol2



21.2cc



1.3ci







































1-inchMicroDrives:

Height



5.0mm



0.20in.



Width



42.8mm 1.69in.



Depth



36.4mm 1.43in.



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Chapter 9.  Hard Disk Storage

Tải bản đầy đủ ngay(0 tr)

×
x