(8.4.6)-Ch-06-External Memory计算机组成原理

计算机组成原理

---双语教学课件WilliamStallings

ComputerOrganization

andArchitecture

6thEditionChapter6ExternalMemoryTypesofExternalMemoryMagneticDiskRAIDRemovableOpticalCD-ROMCD-Recordable(CD-R)CD-R/WDVDMagneticTapeMagneticDiskDisksubstratecoatedwithmagnetizablematerial(ironoxide…rust)SubstrateusedtobealuminiumNowglassImprovedsurfaceuniformityIncreasesreliabilityReductioninsurfacedefectsReducedread/writeerrorsLowerflightheights(Seelater)BetterstiffnessBettershock/damageresistanceReadandWriteMechanismsRecordingandretrievalviaconductivecoilcalledaheadMaybesingleread/writeheadorseparateonesDuringread/write,headisstationary,platterrotatesWriteCurrentthroughcoilproducesmagneticfieldPulsessenttoheadMagneticpatternrecordedonsurfacebelowRead(traditional)MagneticfieldmovingrelativetocoilproducescurrentCoilisthesameforreadandwriteRead(contemporary)Separatereadhead,closetowriteheadPartiallyshieldedmagnetoresistive(MR)sensorElectricalresistancedependsondirectionofmagneticfieldHighfrequencyoperationHigherstoragedensityandspeedInductiveWriteMRReadDataOrganizationandFormattingConcentricringsortracksGapsbetweentracksReducegaptoincreasecapacitySamenumberofbitspertrack(variablepackingdensity)ConstantangularvelocityTracksdividedintosectorsMinimumblocksizeisonesectorMayhavemorethanonesectorperblockDiskDataLayoutDiskVelocityBitnearcentreofrotatingdiskpassesfixedpointslowerthanbitonoutsideofdiskIncreasespacingbetweenbitsindifferenttracksRotatediskatconstantangularvelocity(CAV)GivespieshapedsectorsandconcentrictracksIndividualtracksandsectorsaddressableMoveheadtogiventrackandwaitforgivensectorWasteofspaceonoutertracksLowerdatadensityCanusezonestoincreasecapacityEachzonehasfixedbitspertrackMorecomplexcircuitryDiskLayoutMethodsDiagramFindingSectorsMustbeabletoidentifystartoftrackandsectorFormatdiskAdditionalinformationnotavailabletouserMarkstracksandsectorsGap1IdGap2DataGap3Gap1IdGap2DataGap3TrackSyncByteHeadSectorCRCSyncByteDataCRCST506format(old!)ForegroundreadingFindothersCharacteristicsFixed(rare)ormovableheadRemovableorfixedSingleordouble(usually)sidedSingleormultipleplatterHeadmechanismContact(Floppy)FixedgapFlying(Winchester)Fixed/MovableHeadDiskFixedheadOnereadwriteheadpertrackHeadsmountedonfixedridgedarmMovableheadOnereadwriteheadpersideMountedonamovablearmRemovableorNotRemovablediskCanberemovedfromdriveandreplacedwithanotherdiskProvidesunlimitedstoragecapacityEasydatatransferbetweensystemsNonremovablediskPermanentlymountedinthedriveMultiplePlatterOneheadpersideHeadsarejoinedandalignedAlignedtracksoneachplatterformcylindersDataisstripedbycylinderreducesheadmovementIncreasesspeed(transferrate)MultiplePlattersCylindersFloppyDisk8”,5.25”,3.5”SmallcapacityUpto1.44Mbyte(2.88Mneverpopular)SlowUniversalCheapObsolete?WinchesterHardDisk(1)DevelopedbyIBMinWinchester(USA)SealedunitOneormoreplatters(disks)HeadsflyonboundarylayerofairasdiskspinsVerysmallheadtodiskgapGettingmorerobustWinchesterHardDisk(2)UniversalCheapFastestexternalstorageGettinglargerallthetimeMultipleGigabytenowusualRemovableHardDiskZIPCheapVerycommonOnly100MJAZNotcheap1GL-120(a:drive)Alsoreads3.5”floppyBecomingmorepopular?AllobsoletedbyCD-RandCD-R/W?SpeedSeektimeMovingheadtocorrecttrack(Rotational)latencyWaitingfordatatorotateunderheadAccesstime=Seek+LatencyTransferrateTimingofDiskI/OTransferRAIDRedundantArrayofIndependentDisksRedundantArrayofInexpensiveDisks6levelsincommonuseNotahierarchySetofphysicaldisksviewedassinglelogicaldrivebyO/SDatadistributedacrossphysicaldrivesCanuseredundantcapacitytostoreparityinformationDataMappingForRAID0RAID0NoredundancyDatastripedacrossalldisksRoundRobinstripingIncreasespeedMultipledatarequestsprobablynotonsamediskDisksseekinparallelAsetofdataislikelytobestripedacrossmultipledisksRAID0,1,2RAID1MirroredDisksDataisstripedacrossdisks2copiesofeachstripeonseparatedisksReadfromeitherWritetobothRecoveryissimpleSwapfaultydisk&re-mirrorNodowntimeExpensive(200%capacity)RAID0,1,2RAID2DisksaresynchronizedVerysmallstripesOftensinglebyte/wordErrorcorrectioncalculatedacrosscorrespondingbitsondisksMultipleparitydisksstoreHammingcodeerrorcorrectionincorrespondingpositionsLotsofredundancyExpensive(175%capacity)Notused(detecterror?&correctit)RAID3SimilartoRAID2Onlyoneredundantdisk,nomatterhowlargethearraySimpleparitybitforeachsetofcorrespondingbitsX4(i)=x3(i)⊕x2(i)⊕x1(i)⊕x0(i)DataonfaileddrivecanbereconstructedfromsurvivingdataandparityinfoX4(i)⊕X4(i)⊕x1(i)=x3(i)⊕x2(i)⊕x1(i)⊕x0(i)⊕X4(i)⊕x1(i)VeryhightransferratesDataonfaileddrivecanbereconstructedfromsurvivingdataandparityinfoX4(i)⊕X4(i)⊕x1(i)

=x3(i)⊕x2(i)⊕x1(i)⊕x0(i)⊕X4(i)⊕x1(i)x1(i)=x3(i)⊕x2(i)⊕x0(i)⊕X4(i)VeryhightransferratesRAID3&4RAID4EachdiskoperatesindependentlyGoodforhighI/OrequestrateLargestripesBitbybitparitycalculatedacrossstripesoneachdiskX4(i)=x3(i)⊕x2(i)⊕x1(i)⊕x0(i)ParitystoredonparitydiskRAID4whentheupdateoccurs,theparitymustbecalculatedX4’(i)=x3(i)⊕x2(i)⊕x1’(i)⊕x0(i)=x3(i)⊕x2(i)⊕x1’(i)⊕x0(i)⊕x1(i)⊕x1(i)

=x4(i)

⊕x1’(i)⊕x1(i)Itmeansreadingx1(data)&x4(parity),writingx1’(data)&x4’(parity).Bottleneck:if2updatesoccur,2paritywritingdon’trealizeontheparitydisk.RAID3&4RAID5LikeRAID4ParitystripedacrossalldisksRoundrobinallocationforparitystripeAvoidsRAID4bottleneckatparitydiskCommonlyusedinnetworkserversN.B.DOESNOTMEAN5DISKS!!!!!RAID5&6RAID6TwoparitycalculationsStoredinseparateblocksondifferentdisksUserrequirementofNdisksneedsN+2HighdataavailabilityThreedisksneedtofailfordatalossSignificantwritepenaltyRAID5&6OpticalStorageCD-ROMOriginallyforaudio650Mbytesgivingover70minutesaudioPolycarbonatecoatedwithhighlyreflectivecoat,usuallyaluminiumDatastoredaspitsReadbyreflectinglaserConstantpackingdensityConstantlinearvelocityRAID5&6CDOperationCD-ROMDriveSpeedsAudioissinglespeedConstantliniervelocity1.2ms-1Track(spiral)is5.27kmlongGives4391seconds=73.2minutesOtherspeedsarequotedasmultiplese.g.24xQuotedfigureismaximumdrivecanachieveCD-ROMFormatMode0=blankdatafieldMode1=2048bytedata+errorcorrectionMode2=2336bytedataRandomAccesson

CD-ROMDifficultMoveheadtoroughpositionSetcorrectspeedReadaddressAdjusttorequiredlocation(Yawn!)CD-ROMfor&againstLargecapacity(?)EasytomassproduceRemovableRobustExpensiveforsmallrunsSlowReadonlyOtherOpticalStorageCD-Recordable(CD-R)WORMNowaffordableCompatiblewithCD-ROMdrivesCD-RWErasableGettingcheaperMostlyCD-ROMdrivecompatiblePhasechangeMaterialhastwodifferentreflectivitiesindifferentphasestatesDVD-what’sinaname?DigitalVideoDiskUsedtoindicateaplayerformoviesOnlyplaysvi