会员注册 | 登录 | 微信快捷登录 QQ登录 微博登录 | 帮助中心 人人文库renrendoc.com美如初恋!
站内搜索 百度文库

热门搜索: 直缝焊接机 矿井提升机 循环球式转向器图纸 机器人手爪发展史 管道机器人dwg 动平衡试验台设计

38-An Introduction to Disk Drive Modeling.pdf38-An Introduction to Disk Drive Modeling.pdf -- 5 元

宽屏显示 收藏 分享

页面加载中... ... 广告 0 秒后退出

资源预览需要最新版本的Flash Player支持。
您尚未安装或版本过低,建议您

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
编号:201401051948266814    大小:290.00KB    格式:PDF    上传时间:2014-01-05
  【编辑】
5
关 键 词:
工业、机械、能源、设计、建模、模具、工学
温馨提示:
1: 本站所有资源如无特殊说明,都需要本地电脑安装OFFICE2007和PDF阅读器。图纸软件为CAD,CAXA,PROE,UG,SolidWorks等.压缩文件请下载最新的WinRAR软件解压。
2: 本站的文档不包含任何第三方提供的附件图纸等,如果需要附件,请联系上传者。文件的所有权益归上传用户所有。
3.本站RAR压缩包中若带图纸,网页内容里面会有图纸预览,若没有图纸预览就没有图纸。
4. 未经权益所有人同意不得将文件中的内容挪作商业或盈利用途。
5. 人人文库网仅提供交流平台,并不能对任何下载内容负责。
6. 下载文件中如有侵权或不适当内容,请与我们联系,我们立即纠正。
7. 本站不保证下载资源的准确性、安全性和完整性, 同时也不承担用户因使用这些下载资源对自己和他人造成任何形式的伤害或损失。
  人人文库网所有资源均是用户自行上传分享,仅供网友学习交流,未经上传用户书面授权,请勿作他用。
0条评论

还可以输入200字符

暂无评论,赶快抢占沙发吧。

当前资源信息

4.0
 
(2人评价)
浏览:13次
baixue100上传于2014-01-05

官方联系方式

客服手机:17625900360   
2:不支持迅雷下载,请使用浏览器下载   
3:不支持QQ浏览器下载,请用其他浏览器   
4:下载后的文档和图纸-无水印   
5:文档经过压缩,下载后原文更清晰   

相关资源

相关资源

相关搜索

工业、机械、能源、设计、建模、模具、工学  
关于我们 - 网站声明 - 网站地图 - 友情链接 - 网站客服客服 - 联系我们
co[email protected] 2015-2017 人人文库网网站版权所有
苏ICP备12009002号-5