Chapter12MassStorageStructureOperatingSystemConceptsChapter12:Mass-StorageSystemsDiskStructureDiskAttachmentDiskSchedulingDiskManagementSwap-SpaceManagementRAIDStructureStable-StorageImplementationTertiaryStorageDevicesOperatingSystemIssuesPerformanceIssuesOperatingSystemConceptsOperatingSystemConceptsDiskHardware(Cont.)DiskparametersfortheoriginalIBMPCfloppydiskandaWesternDigitalWD18300harddiskOperatingSystemConceptsDiskStructureDiskdrivesareaddressedaslarge1-dimensionalarraysoflogicalblocks,wherethelogicalblockisthesmallestunitoftransfer.

The1-dimensionalarrayoflogicalblocksismappedintothesectorsofthedisksequentially.Sector0isthefirstsectorofthefirsttrackontheoutermostcylinder.Mappingproceedsinorderthroughthattrack,thentherestofthetracksinthatcylinder,andthenthroughtherestofthecylindersfromoutermosttoinnermost.OperatingSystemConceptsDiskAttachmentDisksmaybeattachedoneoftwoways:HostattachedviaanI/OportNetworkattachedviaanetworkconnectionOperatingSystemConceptsNetwork-AttachedStorageOperatingSystemConceptsStorage-AreaNetworkOperatingSystemConceptsOperatingSystemConceptsDiskSchedulingTheoperatingsystemisresponsibleforusinghardwareefficiently—forthediskdrives,thismeanshavingafast

accesstimeanddiskbandwidth.AccesstimehastwomajorcomponentsSeektimeisthetimeforthediskaretomovetheheadstothecylindercontainingthedesiredsector.Rotationallatencyistheadditionaltimewaitingforthedisktorotatethedesiredsectortothediskhead.OperatingSystemConceptsDiskScheduling(Cont.)MinimizeseektimeSeektimeseekdistanceDiskbandwidthisthetotalnumberofbytestransferred,dividedbythetotaltimebetweenthefirstrequestforserviceandthecompletionofthelasttransfer.OperatingSystemConceptsDiskScheduling(Cont.)SeveralalgorithmsexisttoscheduletheservicingofdiskI/Orequests.Weillustratethemwitharequestqueue(0-199).

98,183,37,122,14,124,65,67 Headpointer53OperatingSystemConceptsFCFSIllustrationshowstotalheadmovementof640cylinders.OperatingSystemConceptsSSTF(ShortestSeekTimeFirst)Selectstherequestwiththeminimumseektimefromthecurrentheadposition.SSTFschedulingisaformofSJFscheduling;maycausestarvationofsomerequests.Illustrationshowstotalheadmovementof236cylinders.OperatingSystemConceptsSSTF(Cont.)OperatingSystemConceptsSCANThediskarmstartsatoneendofthedisk,andmovestowardtheotherend,servicingrequestsuntilitgetstotheotherendofthedisk,wheretheheadmovementisreversedandservicingcontinues.Sometimescalledtheelevatoralgorithm.Illustrationshowstotalheadmovementof208cylinders.OperatingSystemConceptsSCAN(Cont.)OperatingSystemConceptsC-SCANProvidesamoreuniformwaittimethanSCAN.Theheadmovesfromoneendofthedisktotheother.servicingrequestsasitgoes.Whenitreachestheotherend,however,itimmediatelyreturnstothebeginningofthedisk,withoutservicinganyrequestsonthereturntrip.Treatsthecylindersasacircularlistthatwrapsaroundfromthelastcylindertothefirstone.OperatingSystemConceptsC-SCAN(Cont.)OperatingSystemConceptsC-LOOKVersionofC-SCANArmonlygoesasfarasthelastrequestineachdirection,thenreversesdirectionimmediately,withoutfirstgoingallthewaytotheendofthedisk.OperatingSystemConceptsC-LOOK(Cont.)OperatingSystemConceptsSelectingaDisk-SchedulingAlgorithmSSTFiscommonandhasanaturalappealSCANandC-SCANperformbetterforsystemsthatplaceaheavyloadonthedisk.Performancedependsonthenumberandtypesofrequests.Requestsfordiskservicecanbeinfluencedbythefile-allocationmethod.ContiguousallocationandlinkedallocationOperatingSystemConceptsSelectingaDisk-SchedulingAlgorithm(Cont.)Thedisk-schedulingalgorithmshouldbewrittenasaseparatemoduleoftheoperatingsystem,allowingittobereplacedwithadifferentalgorithmifnecessary.EitherSSTForLOOKisareasonablechoiceforthedefaultalgorithm.OperatingSystemConceptsDiskFormattingLow-levelformatting,orphysicalformatting—Dividingadiskintosectorsthatthediskcontrollercanreadandwrite.Touseadisktoholdfiles,theoperatingsystemstillneedstorecorditsowndatastructuresonthedisk.Partitionthediskintooneormoregroupsofcylinders.Logicalformattingor“makingafilesystem”.OperatingSystemConceptsDiskFormatting(1)AdisksectorOperatingSystemConceptsErrorHandlingAdisktrackwithabadsectorSubstitutingaspareforthebadsectorShiftingallthesectorstobypassthebadoneBootingfromaDiskinWindows2000OperatingSystemConceptsRAIDStructureRAID–multiplediskdrivesprovidesreliabilityviaredundancy.RAIDisarrangedintosixdifferentlevels.OperatingSystemConceptsRAID(cont)Severalimprovementsindisk-usetechniquesinvolvetheuseofmultipledisksworkingcooperatively.Diskstripingusesagroupofdisksasonestorageunit.RAIDschemesimproveperformanceandimprovethereliabilityofthestoragesystembystoringredundantdata.Mirroringorshadowingkeepsduplicateofeachdisk.Blockinterleavedparityusesmuch