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38-An Introduction to Disk Drive Modeling.pdf

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38-An Introduction to Disk Drive Modeling.pdf

ANINTRODUCTIONTODISKDRIVEMODELINGCHRISRUEMMLERANDJOHNWILKESHEWLETTPACKARDLABORATORIES,PALOALTO,CAMUCHRESEARCHINI/OSYSTEMSISBASEDONDISKDRIVESIMULATIONMODELS,BUTHOWGOODARETHEYANACCURATESIMULATIONMODELSHOULDEMPHASIZETHEPERFORMANCECRITICALAREASTHISPAPERHASBEENPUBLISHEDINIEEECOMPUTER27317–29,MARCH1994ITSUPERSEDESHPLABSTECHNICALREPORTSHPL–93–68REV1ANDHPL–OSR–93–29COPYRIGHT1994IEEEINTERNALORPERSONALUSEOFTHISMATERIALISPERMITTEDHOWEVER,PERMISSIONTOREPRINT/REPUBLISHTHISMATERIALFORADVERTISINGORPROMOTIONALPURPOSESORFORCREATINGNEWCOLLECTIVEWORKSFORRESALEORREDISTRIBUTIONMUSTBEOBTAINEDFROMTHEIEEETORECEIVEMOREINFORMATIONONOBTAININGPERMISSION,SENDABLANKEMAILMESSAGETOINFOPUBPERMISSIONIEEEORGNOTETHISFILEWASOBTAINEDBYSCANNINGANDPERFORMINGOCRONTHEIEEEPUBLISHEDCOPYASARESULT,ITMAYCONTAINTYPOGRAPHICOROTHERERRORSTHATARENOTINTHEPUBLISHEDVERSIONMINORCLARIFICATIONSANDUPDATESHAVEBEENMADETOTHEBIBLIOGRAPHY1MODERNMICROPROCESSORTECHNOLOGYISADVANCINGATANINCREDIBLERATE,ANDSPEEDUPSOF40TO60PERCENTCOMPOUNDEDANNUALLYHAVEBECOMETHENORMALTHOUGHDISKSTORAGEDENSITIESAREALSOIMPROVINGIMPRESSIVELY60TO80PERCENTCOMPOUNDEDANNUALLY,PERFORMANCEIMPROVEMENTSHAVEBEENOCCURRINGATONLYABOUT7TO10PERCENTCOMPOUNDEDANNUALLYOVERTHELASTDECADEASARESULT,DISKSYSTEMPERFORMANCEISFASTBECOMINGADOMINANTFACTORINOVERALLSYSTEMBEHAVIORNATURALLY,RESEARCHERSWANTTOIMPROVEOVERALLI/OPERFORMANCE,OFWHICHALARGECOMPONENTISTHEPERFORMANCEOFTHEDISKDRIVEITSELFTHISRESEARCHOFTENINVOLVESUSINGANALYTICALORSIMULATIONMODELSTOCOMPAREALTERNATIVEAPPROACHES,ANDTHEQUALITYOFTHESEMODELSDETERMINESTHEQUALITYOFTHECONCLUSIONS;INDEED,THEWRONGMODELINGASSUMPTIONSCANLEADTOERRONEOUSCONCLUSIONSNEVERTHELESS,LITTLEWORKHASBEENDONETODEVELOPORDESCRIBEACCURATEDISKDRIVEMODELSTHISMAYEXPLAINTHECOMMONPLACEUSEOFSIMPLE,RELATIVELYINACCURATEMODELSWEBELIEVETHEREISMUCHROOMFORIMPROVEMENTTHISARTICLEDEMONSTRATESANDDESCRIBESACALIBRATED,HIGHQUALITYDISKDRIVEMODELINWHICHTHEOVERALLERRORFACTORIS14TIMESSMALLERTHANTHATOFASIMPLEFIRSTORDERMODELWEDESCRIBETHEVARIOUSDISKDRIVEPERFORMANCECOMPONENTSSEPARATELY,THENSHOWHOWTHEIRINCLUSIONIMPROVESTHESIMULATIONMODELTHISENABLESANINFORMEDTRADEOFFBETWEENEFFORTANDACCURACYINADDITION,WEPROVIDEDETAILEDCHARACTERISTICSFORTWODISKDRIVES,ASWELLASABRIEFDESCRIPTIONOFASIMULATIONENVIRONMENTTHATUSESTHEDISKDRIVEMODELCHARACTERISTICSOFMODERNDISKDRIVESTOMODELDISKDRIVES,WEMUSTUNDERSTANDHOWTHEYBEHAVETHUS,WEBEGINWITHANOVERVIEWOFTHECURRENTSTATEOFTHEARTINNONREMOVABLEMAGNETICDISKDRIVESWITHEMBEDDEDSCSISMALLCOMPUTERSYSTEMSINTERCONNECTCONTROLLERS,SINCETHESEAREWIDELYAVAILABLEDISKDRIVESCONTAINAMECHANISMANDACONTROLLERTHEMECHANISMISMADEUPOFTHERECORDINGCOMPONENTSTHEROTATINGDISKSANDTHEHEADSTHATACCESSTHEMANDTHEPOSITIONINGCOMPONENTSANARMASSEMBLYTHATMOVESTHEHEADSINTOTHECORRECTPOSITIONTOGETHERWITHATRACKFOLLOWINGSYSTEMTHATKEEPSITINPLACETHEDISKCONTROLLERCONTAINSAMICROPROCESSOR,SOMEBUFFERMEMORY,ANDANINTERFACETOTHESCSIBUSTHECONTROLLERMANAGESTHESTORAGEANDRETRIEVALOFDATATOANDFROMTHEMECHANISMANDPERFORMSMAPPINGSBETWEENINCOMINGLOGICALADDRESSESANDTHEPHYSICALDISKSECTORSTHATSTORETHEINFORMATIONBELOW,WELOOKMORECLOSELYATEACHOFTHESEELEMENTS,EMPHASIZINGFEATURESTHATNEEDTOBECONSIDEREDWHENCREATINGADISKDRIVEMODELITWILLBECOMECLEARTHATNOTALLTHESEFEATURESAREEQUALLYIMPORTANTTOAMODEL’SACCURACYTHERECORDINGCOMPONENTSMODERNDISKSRANGEINSIZEFROM13TO8INCHESINDIAMETER;25,35,AND525INCHESARETHEMOSTCOMMONSIZESTODAYSMALLERDISKSHAVELESSSURFACEAREAANDTHUSSTORELESSDATATHANTHEIRLARGERCOUNTERPARTS;HOWEVER,THEYCONSUMELESSPOWER,CANSPINFASTER,ANDHAVESMALLERSEEKDISTANCESHISTORICALLY,ASSTORAGEDENSITIESHAVEINCREASEDTOWHERE2–3GIGABYTESCANFITONASINGLEDISK,THENEXTSMALLERDIAMETERINTHESERIESHASBECOMETHEMOSTCOSTEFFECTIVEANDHENCETHEPREFERREDSTORAGEDEVICE2INCREASEDSTORAGEDENSITYRESULTSFROMTWOIMPROVEMENTSTHEFIRSTISBETTERLINEARRECORDINGDENSITY,WHICHISDETERMINEDBYTHEMAXIMUMRATEOFFLUXCHANGESTHATCANBERECORDEDANDREADBACK;CURRENTVALUESAREAROUND50,000BITSPERINCHANDWILLAPPROXIMATELYDOUBLEBYTHEENDOFTHEDECADETHESECONDCOMESFROMPACKINGTHESEPARATETRACKSOFDATAMORECLOSELYTOGETHER,WHICHISHOWMOSTOFTHEIMPROVEMENTSAREOCCURRINGCURRENTVALUESAREABOUT2,500TRACKSPERINCH,RISINGTOPERHAPS20,000TPIBYTHEENDOFTHEDECADETHEPRODUCTOFTHESETWOFACTORSWILLPROBABLYSUSTAINAGROWTHRATEABOVE60PERCENTPERYEARTOTHEENDOFTHEDECADEASINGLEDISKCONTAINSONE,TWO,ORASMANYASADOZENPLATTERS,ASSHOWNINFIGURE1THESTACKOFPLATTERSROTATESINLOCKSTEPONACENTRALSPINDLEALTHOUGH3,600RPMWASADEFACTOSTANDARDFORMANYYEARS,SPINDLEROTATIONSPEEDHASINCREASEDRECENTLYTOASMUCHAS7,200RPMTHEMEDIANROTATIONSPEEDISINCREASINGATACOMPOUNDRATEOFABOUT12PERCENTPERYEARAHIGHERSPINSPEEDINCREASESTRANSFERRATESANDSHORTENSROTATIONLATENCIESTHETIMEFORDATATOROTATEUNDERTHEHEAD,BUTPOWERCONSUMPTIONINCREASESANDBETTERBEARINGSAREREQUIREDFORTHESPINDLETHESPINSPEEDISTYPICALLYQUOTEDASACCURATEWITHIN05TO1PERCENT;INPRACTICE,THEDISKSPEEDSVARYSLOWLYAROUNDTHENOMINALRATEALTHOUGHTHISISPERFECTLYREASONABLEFORTHEDISK’SOPERATION,ITMAKESITNEARLYIMPOSSIBLETOMODELTHEDISK’SROTATIONALPOSITIONSOME100200REVOLUTIONSAFTERTHELASTKNOWNOPERATIONFORTUNATELY,MANYI/OOPERATIONSOCCURINBURSTS,SOTHEUNCERTAINTYAPPLIESONLYTOTHEFIRSTREQUESTINTHEBURSTEACHPLATTERSURFACEHASANASSOCIATEDDISKHEADRESPONSIBLEFORRECORDINGWRITINGANDLATERSENSINGREADINGTHEMAGNETICFLUXVARIATIONSONTHEPLATTER’SSURFACETHEDISKDRIVEHASASINGLEREADWRITEDATACHANNELTHATCANBESWITCHEDBETWEENTHEHEADSTHISCHANNELISRESPONSIBLEFORENCODINGANDDECODINGTHEDATASTREAMINTOORFROMASERIESOFMAGNETICPHASECHANGESSTOREDONTHEDISKSIGNIFICANTFRACTIONSOFTHEENCODEDDATASTREAMAREDEDICATEDTOERRORCORRECTIONTHEAPPLICATIONOFDIGITALSIGNALPROCESSINGMAYSOONINCREASECHANNELSPEEDSABOVETHEIRCURRENT100MEGABITSPERSECONDMULTICHANNELDISKSCANSUPPORTMORETHANONEREAD/WRITEOPERATIONATATIME,MAKINGHIGHERDATATRANSFERRATESPOSSIBLEHOWEVER,THESEDISKSARERELATIVELYCOSTLYBECAUSEOFTECHNICALDIFFICULTIESSUCHASCONTROLLINGTHECROSSTALKBETWEENTHECONCURRENTLYACTIVECHANNELSANDKEEPINGMULTIPLEHEADSALIGNEDONTHEIRPLATTERSSIMULTANEOUSLYTHELATTERISBECOMINGMOREDIFFICULTASTRACKDENSITIESINCREASEFIGURE1THEMECHANICALCOMPONENTSOFADISKDRIVEBTOPVIEWASIDEVIEWARMASSEMBLYARMHEADSPINDLESECTORTRACKARMHEADARMPIVOTPLATTERCYLINDER3THEPOSITIONINGCOMPONENTSEACHDATASURFACEISSETUPTOSTOREDATAINASERIESOFCONCENTRICCIRCLES,ORTRACKSASINGLESTACKOFTRACKSATACOMMONDISTANCEFROMTHESPINDLEISCALLEDACYLINDERTODAY’STYPICAL35INCHDISKHASABOUT2,000CYLINDERSASTRACKDENSITIESINCREASE,THENOTIONOFVERTICALALIGNMENTTHATISASSOCIATEDWITHCYLINDERSBECOMESLESSANDLESSRELEVANTBECAUSETRACKALIGNMENTTOLERANCESARESIMPLYTOOFINEESSENTIALLY,THEN,WEMUSTCONSIDERTHETRACKSONEACHPLATTERINDEPENDENTLYTOACCESSTHEDATASTOREDINATRACK,THEDISKHEADMUSTBEMOVEDOVERITTHISISDONEBYATTACHINGEACHHEADTOADISKARMALEVERTHATISPIVOTEDNEARONEENDONAROTATIONBEARINGALLTHEDISKARMSAREATTACHEDTOTHESAMEROTATIONPIVOT,SOTHATMOVINGONEHEADCAUSESTHEOTHERSTOMOVEASWELLTHEROTATIONPIVOTISMOREIMMUNETOLINEARSHOCKSTHANTHEOLDERSCHEMEOFMOUNTINGTHEHEADONALINEARSLIDERTHEPOSITIONINGSYSTEM’STASKISTOENSURETHATTHEAPPROPRIATEHEADGETSTOTHEDESIREDTRACKASQUICKLYASPOSSIBLEANDREMAINSTHEREEVENINTHEFACEOFEXTERNALVIBRATION,SHOCKS,ANDDISKFLAWSFOREXAMPLE,NONCONCENTRICANDNONCIRCULARTRACKSSEEKINGTHESPEEDOFHEADMOVEMENT,ORSEEKING,ISLIMITEDBYTHEPOWERAVAILABLEFORTHEPIVOTMOTORHALVINGTHESEEKTIMEREQUIRESQUADRUPLINGTHEPOWERANDBYTHEARM’SSTIFFNESSACCELERATIONSOF3040GAREREQUIREDTOACHIEVEGOODSEEKTIMES,ANDTOOFLEXIBLEANARMCANTWISTANDBRINGTHEHEADINTOCONTACTWITHTHEPLATTERSURFACESMALLERDIAMETERDISKSHAVECORRESPONDINGLYREDUCEDDISTANCESFORTHEHEADTOMOVETHESEDISKSHAVESMALLER,LIGHTERARMSTHATAREEASIERTOSTIFFENAGAINSTFLEXINGALLCONTRIBUTINGTOSHORTERSEEKTIMESASEEKISCOMPOSEDOFASPEEDUP,WHERETHEARMISACCELERATEDUNTILITREACHESHALFOFTHESEEKDISTANCEORAFIXEDMAXIMUMVELOCITY,ACOASTFORLONGSEEKS,WHERETHEARMMOVESATITSMAXIMUMVELOCITY,ASLOWDOWN,WHERETHEARMISBROUGHTTORESTCLOSETOTHEDESIREDTRACK,ANDASETTLE,WHERETHEDISKCONTROLLERADJUSTSTHEHEADTOACCESSTHEDESIREDLOCATIONVERYSHORTSEEKSLESSTHAN,SAY,TWOTOFOURCYLINDERSAREDOMINATEDBYTHESETTLETIME1–3MILLISECONDSINFACT,ASEEKMAYNOTEVENOCCUR;THEHEADMAYJUSTRESETTLEINTOPOSITIONONANEWTRACKSHORTSEEKSLESSTHAN200–400CYLINDERSSPENDALMOSTALLOFTHEIRTIMEINTHECONSTANTACCELERATIONPHASE,ANDTHEIRTIMEISPROPORTIONALTOTHESQUAREROOTOFTHESEEKDISTANCEPLUSTHESETTLETIMELONGSEEKSSPENDMOSTOFTHEIRTIMEMOVINGATACONSTANTSPEED,TAKINGTIMETHATISPROPORTIONALTODISTANCEPLUSACONSTANTOVERHEADASDISKSBECOMESMALLERANDTRACKDENSITIESINCREASE,THEFRACTIONOFTHETOTALSEEKTIMEATTRIBUTEDTOTHESETTLEPHASEINCREASES“AVERAGE”SEEKTIMESARECOMMONLYUSEDASAFIGUREOFMERITFORDISKDRIVES,BUTTHEYCANBEMISLEADINGSUCHAVERAGESARECALCULATEDINVARIOUSWAYS,ASITUATIONFURTHERCOMPLICATEDBYTHEFACTTHATINDEPENDENTSEEKSARERAREINPRACTICESHORTERSEEKSAREMUCHMORECOMMON,L,2ALTHOUGHTHEIROVERALLFREQUENCYISVERYMUCHAFUNCTIONOFTHEWORKLOADANDTHEOPERATINGSYSTEMDRIVINGTHEDISKIFDISKREQUESTSARECOMPLETELYINDEPENDENTOFONEANOTHER,THEAVERAGESEEKDISTANCEWILLBEONETHIRDOFTHEFULLSTROKETHUS,SOMESOURCESQUOTETHEONETHIRDSTROKESEEKTIMEASTHE“AVERAGE”OTHERSSIMPLYQUOTETHEFULLSTROKETIMEDIVIDEDBYTHREEANOTHERWAYISTOSUMTHETIMESNEEDEDTOPERFORMONESEEKOF4EACHSIZEANDDIVIDETHISSUMBYTHENUMBEROFDIFFERENTSEEKSIZESPERHAPSTHEBESTOFTHECOMMONLYUSEDTECHNIQUESISTOWEIGHTTHESEEKTIMEBYTHENUMBEROFPOSSIBLESEEKSOFEACHSIZETHUS,THEREAREN–1DIFFERENTSINGLETRACKSEEKSTHATCANBEDONEONADISKWITHNCYLINDERS,BUTONLYONEFULLSTROKESEEKTHISEMPHASIZESTHESHORTERSEEKS,PROVIDINGASOMEWHATBETTERAPPROXIMATIONTOMEASUREDSEEKDISTANCEPROFILESWHATMATTERSTOPEOPLEBUILDINGMODELS,HOWEVER,ISTHESEEKTIMEVERSUSDISTANCEPROFILEWEENCOURAGEMANUFACTURERSTOINCLUDETHESEINTHEIRDISKSPECIFICATIONS,SINCETHEONLYALTERNATIVEISTODETERMINETHEMEXPERIMENTALLYTHEINFORMATIONREQUIREDTODETERMINEHOWMUCHPOWERTOAPPLYTOTHEPIVOTMOTORANDFORHOWLONGONAPARTICULARSEEKISENCODEDINTABULARFORMINTHEDISKCONTROLLERRATHERTHANEVERYPOSSIBLEVALUE,ASUBSETOFTHETOTALISSTORED,ANDINTERPOLATIONISUSEDFORINTERMEDIATESEEKDISTANCESTHERESULTINGFINEGRAINEDSEEKTIMEPROFILECANLOOKRATHERLIKEASAWTOOTHTHERMALEXPANSION,ARMPIVOTBEARINGSTICKINESS,ANDOTHERFACTORSOCCASIONALLYMAKEITNECESSARYTORECALIBRATETHESETABLESTHISCANTAKE500800MILLISECONDSRECALIBRATIONSARETRIGGEREDBYTEMPERATURECHANGESANDBYTIMERS,SOTHEYOCCURMOSTFREQUENTLYJUSTAFTERTHEDISKDRIVEISPOWEREDUPINSTEADYSTATECONDITIONS,RECALIBRATIONOCCURSONLYONCEEVERY1530MINUTESOB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