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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,buthowgoodaretheyAnaccuratesimulationmodelshouldemphasizetheperformancecriticalareas.ThispaperhasbeenpublishedinIEEEComputer27317–29,March1994.ItsupersedesHPLabstechnicalreportsHPL–93–68rev1andHPL–OSR–93–29.Copyright©1994IEEE.Internalorpersonaluseofthismaterialispermitted.However,permissiontoreprint/republishthismaterialforadvertisingorpromotionalpurposesorforcreatingnewcollectiveworksforresaleorredistributionmustbeobtainedfromtheIEEE.Toreceivemoreinformationonobtainingpermission,sendablankemailmessagetoinfo.pub.permissionieee.org.NotethisfilewasobtainedbyscanningandperformingOCRontheIEEEpublishedcopy.Asaresult,itmaycontaintypographicorothererrorsthatarenotinthepublishedversion.Minorclarificationsandupdateshavebeenmadetothebibliography.1Modernmicroprocessortechnologyisadvancingatanincrediblerate,andspeedupsof40to60percentcompoundedannuallyhavebecomethenorm.Althoughdiskstoragedensitiesarealsoimprovingimpressively60to80percentcompoundedannually,performanceimprovementshavebeenoccurringatonlyabout7to10percentcompoundedannuallyoverthelastdecade.Asaresult,disksystemperformanceisfastbecomingadominantfactorinoverallsystembehavior.Naturally,researcherswanttoimproveoverallI/Operformance,ofwhichalargecomponentistheperformanceofthediskdriveitself.Thisresearchofteninvolvesusinganalyticalorsimulationmodelstocomparealternativeapproaches,andthequalityofthesemodelsdeterminesthequalityoftheconclusionsindeed,thewrongmodelingassumptionscanleadtoerroneousconclusions.Nevertheless,littleworkhasbeendonetodevelopordescribeaccuratediskdrivemodels.Thismayexplainthecommonplaceuseofsimple,relativelyinaccuratemodels.Webelievethereismuchroomforimprovement.Thisarticledemonstratesanddescribesacalibrated,highqualitydiskdrivemodelinwhichtheoverallerrorfactoris14timessmallerthanthatofasimplefirstordermodel.Wedescribethevariousdiskdriveperformancecomponentsseparately,thenshowhowtheirinclusionimprovesthesimulationmodel.Thisenablesaninformedtradeoffbetweeneffortandaccuracy.Inaddition,weprovidedetailedcharacteristicsfortwodiskdrives,aswellasabriefdescriptionofasimulationenvironmentthatusesthediskdrivemodel.CharacteristicsofmoderndiskdrivesTomodeldiskdrives,wemustunderstandhowtheybehave.Thus,webeginwithanoverviewofthecurrentstateoftheartinnonremovablemagneticdiskdriveswithembeddedSCSISmallComputerSystemsInterconnectcontrollers,sincethesearewidelyavailable.Diskdrivescontainamechanismandacontroller.Themechanismismadeupoftherecordingcomponentstherotatingdisksandtheheadsthataccessthemandthepositioningcomponentsanarmassemblythatmovestheheadsintothecorrectpositiontogetherwithatrackfollowingsystemthatkeepsitinplace.Thediskcontrollercontainsamicroprocessor,somebuffermemory,andaninterfacetotheSCSIbus.Thecontrollermanagesthestorageandretrievalofdatatoandfromthemechanismandperformsmappingsbetweenincominglogicaladdressesandthephysicaldisksectorsthatstoretheinformation.Below,welookmorecloselyateachoftheseelements,emphasizingfeaturesthatneedtobeconsideredwhencreatingadiskdrivemodel.Itwillbecomeclearthatnotallthesefeaturesareequallyimportanttoamodelsaccuracy.Therecordingcomponents.Moderndisksrangeinsizefrom1.3to8inchesindiameter2.5,3.5,and5.25inchesarethemostcommonsizestoday.Smallerdiskshavelesssurfaceareaandthusstorelessdatathantheirlargercounterpartshowever,theyconsumelesspower,canspinfaster,andhavesmallerseekdistances.Historically,asstoragedensitieshaveincreasedtowhere2–3gigabytescanfitonasingledisk,thenextsmallerdiameterintheserieshasbecomethemostcosteffectiveandhencethepreferredstoragedevice.2Increasedstoragedensityresultsfromtwoimprovements.Thefirstisbetterlinearrecordingdensity,whichisdeterminedbythemaximumrateoffluxchangesthatcanberecordedandreadbackcurrentvaluesarearound50,000bitsperinchandwillapproximatelydoublebytheendofthedecade.Thesecondcomesfrompackingtheseparatetracksofdatamorecloselytogether,whichishowmostoftheimprovementsareoccurring.Currentvaluesareabout2,500tracksperinch,risingtoperhaps20,000TPIbytheendofthedecade.Theproductofthesetwofactorswillprobablysustainagrowthrateabove60percentperyeartotheendofthedecade.Asinglediskcontainsone,two,orasmanyasadozenplatters,asshowninFigure1.Thestackofplattersrotatesinlocksteponacentralspindle.Although3,600rpmwasadefactostandardformanyyears,spindlerotationspeedhasincreasedrecentlytoasmuchas7,200rpm.Themedianrotationspeedisincreasingatacompoundrateofabout12percentperyear.Ahigherspinspeedincreasestransferratesandshortensrotationlatenciesthetimefordatatorotateunderthehead,butpowerconsumptionincreasesandbetterbearingsarerequiredforthespindle.Thespinspeedistypicallyquotedasaccuratewithin0.5to1percentinpractice,thediskspeedsvaryslowlyaroundthenominalrate.Althoughthisisperfectlyreasonableforthedisksoperation,itmakesitnearlyimpossibletomodelthedisksrotationalpositionsome100200revolutionsafterthelastknownoperation.Fortunately,manyI/Ooperationsoccurinbursts,sotheuncertaintyappliesonlytothefirstrequestintheburst.Eachplattersurfacehasanassociateddiskheadresponsibleforrecordingwritingandlatersensingreadingthemagneticfluxvariationsontheplatterssurface.Thediskdrivehasasinglereadwritedatachannelthatcanbeswitchedbetweentheheads.Thischannelisresponsibleforencodinganddecodingthedatastreamintoorfromaseriesofmagneticphasechangesstoredonthedisk.Significantfractionsoftheencodeddatastreamarededicatedtoerrorcorrection.Theapplicationofdigitalsignalprocessingmaysoonincreasechannelspeedsabovetheircurrent100megabitspersecond.Multichanneldiskscansupportmorethanoneread/writeoperationatatime,makinghigherdatatransferratespossible.However,thesedisksarerelativelycostlybecauseoftechnicaldifficultiessuchascontrollingthecrosstalkbetweentheconcurrentlyactivechannelsandkeepingmultipleheadsalignedontheirplatterssimultaneously.Thelatterisbecomingmoredifficultastrackdensitiesincrease.Figure1themechanicalcomponentsofadiskdrive.b.topview.a.sideview.armassemblyarmheadspindlesectortrackarmheadarmpivotplattercylinder3Thepositioningcomponents.Eachdatasurfaceissetuptostoredatainaseriesofconcentriccircles,ortracks.Asinglestackoftracksatacommondistancefromthespindleiscalledacylinder.Todaystypical3.5inchdiskhasabout2,000cylinders.Astrackdensitiesincrease,thenotionofverticalalignmentthatisassociatedwithcylindersbecomeslessandlessrelevantbecausetrackalignmenttolerancesaresimplytoofine.Essentially,then,wemustconsiderthetracksoneachplatterindependently.Toaccessthedatastoredinatrack,thediskheadmustbemovedoverit.Thisisdonebyattachingeachheadtoadiskarmaleverthatispivotednearoneendonarotationbearing.Allthediskarmsareattachedtothesamerotationpivot,sothatmovingoneheadcausestheotherstomoveaswell.Therotationpivotismoreimmunetolinearshocksthantheolderschemeofmountingtheheadonalinearslider.Thepositioningsystemstaskistoensurethattheappropriateheadgetstothedesiredtrackasquicklyaspossibleandremainsthereeveninthefaceofexternalvibration,shocks,anddiskflawsforexample,nonconcentricandnoncirculartracks.Seeking.Thespeedofheadmovement,orseeking,islimitedbythepoweravailableforthepivotmotorhalvingtheseektimerequiresquadruplingthepowerandbythearmsstiffness.Accelerationsof3040garerequiredtoachievegoodseektimes,andtooflexibleanarmcantwistandbringtheheadintocontactwiththeplattersurface.Smallerdiameterdiskshavecorrespondinglyreduceddistancesfortheheadtomove.Thesediskshavesmaller,lighterarmsthatareeasiertostiffenagainstflexingallcontributingtoshorterseektimes.Aseekiscomposedofaspeedup,wherethearmisaccelerateduntilitreacheshalfoftheseekdistanceorafixedmaximumvelocity,acoastforlongseeks,wherethearmmovesatitsmaximumvelocity,aslowdown,wherethearmisbroughttorestclosetothedesiredtrack,andasettle,wherethediskcontrolleradjuststheheadtoaccessthedesiredlocation.Veryshortseekslessthan,say,twotofourcylindersaredominatedbythesettletime1–3milliseconds.Infact,aseekmaynotevenoccurtheheadmayjustresettleintopositiononanewtrack.Shortseekslessthan200–400cylindersspendalmostalloftheirtimeintheconstantaccelerationphase,andtheirtimeisproportionaltothesquarerootoftheseekdistanceplusthesettletime.Longseeksspendmostoftheirtimemovingataconstantspeed,takingtimethatisproportionaltodistanceplusaconstantoverhead.Asdisksbecomesmallerandtrackdensitiesincrease,thefractionofthetotalseektimeattributedtothesettlephaseincreases.Averageseektimesarecommonlyusedasafigureofmeritfordiskdrives,buttheycanbemisleading.Suchaveragesarecalculatedinvariousways,asituationfurthercomplicatedbythefactthatindependentseeksarerareinpractice.Shorterseeksaremuchmorecommon,l,2althoughtheiroverallfrequencyisverymuchafunctionoftheworkloadandtheoperatingsystemdrivingthedisk.Ifdiskrequestsarecompletelyindependentofoneanother,theaverageseekdistancewillbeonethirdofthefullstroke.Thus,somesourcesquotetheonethirdstrokeseektimeastheaverage.Otherssimplyquotethefullstroketimedividedbythree.Anotherwayistosumthetimesneededtoperformoneseekof4eachsizeanddividethissumbythenumberofdifferentseeksizes.PerhapsthebestofthecommonlyusedtechniquesistoweighttheseektimebythenumberofpossibleseeksofeachsizeThus,thereareN–1differentsingletrackseeksthatcanbedoneonadiskwithNcylinders,butonlyonefullstrokeseek.Thisemphasizestheshorterseeks,providingasomewhatbetterapproximationtomeasuredseekdistanceprofiles.Whatmatterstopeoplebuildingmodels,however,istheseektimeversusdistanceprofile.Weencouragemanufacturerstoincludetheseintheirdiskspecifications,sincetheonlyalternativeistodeterminethemexperimentally.Theinformationrequiredtodeterminehowmuchpowertoapplytothepivotmotorandforhowlongonaparticularseekisencodedintabularforminthediskcontroller.Ratherthaneverypossiblevalue,asubsetofthetotalisstored,andinterpolationisusedforintermediateseekdistances.TheresultingfinegrainedseektimeprofilecanlookratherlikeasawtoothThermalexpansion,armpivotbearingstickiness,andotherfactorsoccasionallymakeitnecessarytorecalibratethesetables.Thiscantake500800milliseconds.Recalibrationsaretriggeredbytemperaturechangesandbytimers,sotheyoccurmostfrequentlyjustafterthediskdriveispoweredup.Insteadystateconditions,recalibrationoccursonlyonceevery1530minutes.Obviously,thiscancausedifficultieswithrealtimeorguaranteedbandwidthsystemssuchasmultimediafileservers,sodiskdrivesarenowappearingwithmodifiedcontrollerfirmwarethateitheravoidsthesevisiblerecalibrationscompletelyorallowsthehosttoscheduletheirexecution.Trackfollowing.Finetuningtheheadpositionattheendofaseekandkeepingtheheadonthedesiredtrackisthefunctionofthetrackfollowingsystem.Thissystemusespositioninginformationrecordedonthediskatmanufacturingtimetodeterminewhetherthediskheadiscorrectlyaligned.Thisinformationcanbeembeddedinthetargetsurfaceorrecordedonaseparatededicatedsurface.Theformermaximizescapacity,soitismostfrequentlyusedindiskswithasmallnumberofplatters.Astrackdensityincreases,someformofembeddedpositioningdatabecomesessentialforfinegrainedcontrolperhapscombinedwithadedicatedsurfaceforcoarsepositioningdata.However,theembeddeddatamethodaloneisnotgoodatcopingwithshockandvibrationbecausefeedbackinformationisonlyavailableintermittentlybetweendatasectors.Thetrackfollowingsystemisalsousedtoperformaheadswitch.Whenthecontrollerswitchesitsdatachannelfromonesurfacetothenextinthesamecylinder,thenewheadmayneedrepositioningtoaccommodatesmalldifferencesinthealignmentofthetracksonthedifferentsurfaces.Thetimetakenforsuchaswitch0.51.5msistypicallyonethirdtoonehalfofthetimetakentodoasettleattheendofaseek.Similarly,atrackswitchorcylinderswitchoccurswhenthearmhastobemovedfromthelasttrackofacylindertothefirsttrackofthenext.Thistakesaboutthesametimeastheendofseeksettlingprocess.Sincesettlingtimeincreasesastrackdensityincreases,andthetracksondifferentplattersarebecominglesswellaligned,headswitchingtimesareapproachingthosefortrackswitching.Nowadays,manydiskdrivesuseanaggressive,optimisticapproachtoheadsettlingbeforeareadoperation.Thismeanstheywillattemptareadassoonastheheadisneartherighttrackafterall,ifthedataareunreadablebecausethesettlehasnotquitecompleted,nothinghasbeenlost.Thereisenougherrorcorrectionandidentificationdatainamisreadsectortoensurethatthedataarenotwronglyinterpreted.Ontheotherhand,ifthedataareavailable,itmightjustsaveanentirerevolutionsdelay.Forobviousreasons,5thisapproachisnottakenforasettlethatimmediatelyprecedesawrite.Thedifferenceinthesettletimesforreadsandwritescanbeasmuchas0.75ms.Datalayout.ASCSIdiskappearstoitsclientcomputerasalinearvectorofaddressableblocks,eachtypically2561,024bytesinsize.Theseblocksmustbemappedtophysicalsectorsonthedisk,whicharethefixedsizedatalayoutunitsontheplatters.Separatingthelogicalandphysicalviewsofthediskinthiswaymeansthatthediskcanhidebadsectorsanddosomelowlevelperformanceoptimizations,butitcomplicatesthetaskofhigherlevelsoftwarethatistryingtosecondguessthecontrollerforexample,the4.2BSDUnixfastfilesystem.Zoning.Tracksarelongerattheoutsideofaplatterthanattheinside.Tomaximizestoragecapacity,lineardensityshouldremainnearthemaximumthatthedrivecansupportthus,theamountofdatastoredoneachtrackshouldscalewithitslength.Thisisaccomplishedonmanydisksbyatechniquecalledzoning,whereadjacentdiskcylindersaregroupedintozones.Zonesneartheouteredgehavemoresectorspertrackthanzonesontheinside.Therearetypically3to20zones,andthenumberislikelytodoublebytheendofthedecade.Sincethedatatransferrateisproportionaltotherateatwhichthemediapassesunderthehead,theouterzoneshavehigherdatatransferrates.Forexample,onaHewlettPackardC22403.5inchdiskdrive,thebursttransferratewithnointertrackheadswitchesvariesfrom3.1megabytespersecondattheinnerzoneto5.3MBpsattheoutermostzone.3Trackskewing.Fastersequentialaccessacrosstrackandcylinderboundariesisobtainedbyskewinglogicalsectorzerooneachtrackbyjusttheamountoftimerequiredtocopewiththemostlikelyworstcaseheadortrackswitchtimes.Thismeansthatdatacanbereadorwrittenatnearlyfullmediaspeed.Eachzonehasitsowntrackandcylinderskewfactors.Sparing.Itisprohibitivelyexpensivetomanufactureperfectsurfaces,sodisksinvariablyhavesomeflawedsectorsthatcannotbeused.Flawsarefoundthroughextensivetestingduringmanufacturing,andalistisbuiltandrecordedonthediskforthecontrollersuse.Sothatflawedsectorsarenotused,referencestothemareremappedtootherportionsofthedisk.Thisprocess,knownassparing,isdoneatthegranularityofsinglesectorsorwholetracks.Thesimplesttechniqueistoremapabadsectorortracktoanalternatelocation.Alternatively,slipsparingcanbeused,inwhichthelogicalblockthatwouldmaptothebadsectorandtheonesafteritareslippedbyonesectororbyawholetrack.Manycombinationsoftechniquesarepossible,sodiskdrivedesignersmustmakeacomplextradeoffinvolvingperformance,expectedbadsectorrate,andspaceutilization.AconcreteexampleistheHPC2240diskdrive,whichusesbothformsoftracklevelsparingsliptracksparingatdiskformattimeandsingletrackremappingfordefectsdiscoveredduringoperation.Thediskcontroller.Thediskcontrollermediatesaccesstothemechanism,runsthetrackfollowingsystem,transfersdatabetweenthediskdriveanditsclient,and,inmanycases,managesanembeddedcache.Controllersarebuiltaroundspeciallydesignedmicroprocessors,whichoftenhavedigitalsignalprocessingcapabilityandspecialinterfacesthatletthemcontrolhardwaredirectly.Thetrendistowardmorepowerfulcontrollersforhandlingincreasinglysophisticatedinterfacesandforreducingcostsbyreplacingpreviouslydedicatedelectroniccomponentswithfirmware.InterpretingtheSCSIrequestsandperformingtheappropriatecomputationstakestime.Controllermicroprocessorspeedisincreasingjustaboutfastenoughtostayaheadoftheadditionalfunctionsthe6controllerisbeingaskedtoperform,socontrolleroverheadisslowlydeclining.Itistypicallyintherange0.31.0ms.Businterface.Themostimportantaspectsofadiskdriveshostchannelareitstopology,itstransferrate,anditsoverhead.SCSIiscurrentlydefinedasabus,althoughalternativeversionsarebeingdiscussed,asareencapsulationsofthehigherlevelsoftheSCSIprotocolacrossothertransmissionmedia,suchasFibreChannel.MostdiskdrivesusetheSCSIbusoperationssynchronousmode,whichcanrunatthemaximumbusspeed.Thiswas5MBpswithearlySCSIbusesdifferentialdriversandthefastSCSIspecificationincreasedthisto10MBpsacoupleofyearsago.Disksarenowappearingthatcandrivethebusat20MBpsfast,wide,andthestandardisdefinedupto40MBps.ThemaximumbustransferrateisnegotiatedbetweenthehostcomputerSCSIinterfaceandthediskdrive.ItappearslikelythatsomeserialchannelsuchasFibreChannelwillbecomeamorepopulartransmissionmediumatthehigherspeeds,partlybecauseitwouldhavefewerwiresandrequireasmallerconnector.BecauseSCSIisabus,morethanonedevicecanbeattachedtoit.SCSIinitiallysupporteduptoeightaddresses,afigurerecentlydoubledwiththeuseofwideSCSI.Asthenumberofdevicesonthebusincreases,contentionforthebuscanoccur,leadingtodelaysinexecutingdatatransfers.Thismattersmoreifthediskdrivesaredoinglargetransfersoriftheircontrolleroverheadsarehigh.Inadditiontothetimeattributedtothetransferrate,theSCSIbusinterfacesatthehostanddiskalsorequiretimetoestablishconnectionsanddeciphercommands.OnSCSI,thecostofthelowlevelprotocolforacquiringcontrolofthebusisontheorderofafewmicrosecondsifthebusisidle.TheSCSIprotocolalsoallowsadiskdrivetodisconnectfromthebusandreconnectlateronceithasdatatotransfer.Thiscyclemaytake200µsbutallowsotherdevicestoaccessthebuswhilethedisconnecteddeviceprocessesdata,resultinginahigheroverallthroughput.Inolderchannelarchitectures,therewasnobufferinginthediskdriveitself.Asaresult,ifthediskwasreadytotransferdatatoahostwhoseinterfacewasnotready,thenthediskhadtowaitanentirerevolutionforthesamedatatocomeundertheheadagainbeforeitcouldretrythetransfer.InSCSI,thediskdriveisexpectedtohaveaspeedmatchingbuffertoavoidthisdelay,maskingtheasynchronybetweenthebusandthemechanism.SincemostSCSIdrivestakedataoffthemediamoreslowlythantheycansenditoverthebus,thedrivepartiallyfillsitsbufferbeforeattemptingtocommencethebusdatatransfer.Theamountofdatareadintothebufferbeforethetransferisinitiatediscalledthefenceitssizeisapropertyofthediskcontroller,althoughitcanbespecifiedonmodernSCSIdiskdrivesbyacontrolcommand.Writerequestscancausethedatatransfertothedisksbuffertooverlaptheheadrepositioning,uptothelimitpermittedbythebufferssize.TheseinteractionsareillustratedinFigure2.Cachingofrequests.Thefunctionsofthespeedmatchingbufferinthediskdrivecanbereadilyextendedtoincludesomeformofcachingforbothreadsandwrites.Cachesindiskdrivestendtoberelativelysmallcurrently64kilobytesto1megabytebecauseofspacelimitationsandtherelativelyhighcostofthedualportedstaticRAMneededtokeepupwithboththediskmechanismandthebusinterface.Readahead.Areadthathitsinthecachecanbesatisfiedimmediately,thatis,injustthetimeneededforthecontrollertodetectthehitandsendthedatabackacrossthebus.Thisisusuallymuchquickerthanseekingtothedataandreadingitoffthedisk,somostmodernSCSIdisksprovidesome7formofreadcaching.Themostcommonformisreadaheadactivelyretrievingandcachingdatathatthediskexpectsthehosttorequestmomentarily.Aswewillshow,readcachingturnsouttobeveryimportantwhenitcomestomodelingadiskdrive,butitisoneoftheleastwellspecifiedareasofdisksystembehavior.Forexample,areadthatpartiallyhitsinthecachemaybepartiallyservicedbythecachewithonlythenoncachedportionbeingreadfromdisk,oritmaysimplybypassthecachealtogether.Verylargereadrequestsmayalwaysbypassthecache.Onceablockhasbeenreadfromthecache,somecontrollersdiscarditotherskeepitincaseasubsequentreadisdirectedtothesameblock.Someearlydiskdriveswithcachesdidonarrivalreadaheadtominimizerotationlatencyforwholetracktransfersassoonastheheadarrivedattherelevanttrack,thedrivestartedreadingintoitscache.Attheendofonerevolution,thefulltracksworthofdatahadbeenread,andthiscouldthenbesenttothehostwithoutwaitingforthedataafterthelogicalstartpointtobereread.Thisissometimesratherunfortunatelycalledazerolatencyreadandisalsowhydiskcachememoryisoftencalledatrackbuffer.Astracksgetlongerbutrequestsizesdonot,onarrivalcachingbringslessbenefitforexample,with8Kbyteaccessestoadiskwith32Kbytetracks,themaximumbenefitisonly25percentofarotationtime.Onarrivalcachinghasbeenlargelysupplantedbysimplereadaheadin0whichthediskcontinuestoreadwherethelasthostrequestleftoff.Thisprovestobeoptimalforsequentialreadsandallowsthemtoproceedatthefulldiskbandwidth.Withoutreadahead,twobacktobackreadswouldbedelayedbyalmostafullrevolutionbecausethediskandhostprocessingtimeforinitiatingthesecondreadrequestwouldbelargerthantheintersectorgap.EvenherethereisapolicychoiceShouldthereadaheadbeaggressive,crossingtrackandcylinderboundaries,orshoulditstopwhentheendofthetrackisreachedAggressivereadaheadisoptimalforsequentialaccess,butitdegradesrandomaccessesbecauseheadandtrackswitchestypicallycannotbeabortedonceinitiated,soanunrelatedrequestthatarriveswhiletheswitchisinprogresscanbedelayed.Figure2overlapofbusphasesandmechanismactivity.Thelowleveldetailsofbusarbitrationandselectionhavebeenelidedforsimplicity.datatransferoffmechanismheadswitchseekhostsendscommandcontrollerdisconnectsfrombusstartsseekSCSIbusdatatransferstohoststatusmessagetohostrotationlatencycontrollerdecodesitdatatransfertomechanismheadswitchseekhostsendscommandcontrollerstartsseekSCSIbusdatatransferfromhoststatusmessagetohostrotationlatencycontrollerdecodesitReadWriteSCSIbusdiskmechanismSCSIbusdiskmechanism

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