计算机操作系统原理13

[e-mail]201013.1OperatingSystemConceptsChapter13:Mass-StorageSystemsDiskStructureDiskSchedulingDiskManagementSwap-SpaceManagementTertiaryStorageDevicesOperatingSystemIssues[e-mail]201013.2OperatingSystemConcepts13.1DiskStructureDiskdrivesareaddressedaslarge1-dimensionalarraysoflogicalblocks,wherethelogicalblockisthesmallestunitoftransfer.Thesizeoflogicalblockis512bytes,whichcanbechooseadifferentsizewhendoinglow-levelformatting.The1-dimensionalarrayoflogicalblocksismappedintothesectorsofthedisksequentially.Sector0isthefirstsectorofthefirsttrackontheoutermostcylinder.Mappingproceedsinorderthroughthattrack,thentherestofthetracksinthatcylinder,a[e-mail]201013.3OperatingSystemConcepts13.1DiskStructure(2)Thenumberofsectorspertrackhasbeenincreasingasdisktechnologyimproves,andtheouterzoneofadiskusuallyhasseveralhundredsectorspertrack.Thenumberofcylindersperdiskhasbeenincreasing;[e-mail]201013.4OperatingSystemConcepts13.1DiskStructure(2)[e-mail]201013.5OperatingSystemConcepts13.2DiskSchedulingOperatingsystemisresponsibleforusinghardwareefficiently—forthediskdrives,thismeanshavingafastaccesstimeanddiskbandwidth.Accesstime--hastwomajorcomponentsSeektime--thetimeforthediskarmtomovetheheadstothecylindercontainingthedesiredsector.Rotationallatency--theadditionaltimewaitingforthedisktorotatethedesiredsectortothediskhead.Diskbandwidth--thetotalnumberofbytestransferred,dividedbythetotaltimebetween[e-mail]201013.6OperatingSystemConcepts13.2DiskScheduling(1)ByschedulingtheserviceofdiskI/Orequestsinagoodorder,wecanminimizeseektime.SeektimeseekdistanceSeveralalgorithmsexisttoscheduletheservicingofdiskI/Orequests.Weillustratethemwitharequestqueue(0-199).98,183,37,122,14,124,65,[e-mail]201013.7OperatingSystemConcepts13.2.1FCFSSchedulingFCFS–thesimplestoneandisintrinsicallyfair,butgenerallydoesnotprovidethefastestservice.[e-mail]201013.8OperatingSystemConcepts13.2.2SSTFSchedulingSSTF(shortest-seek-time-first)--Selectstherequestwiththeminimumseektimefromthecurrentheadposition.SSTFschedulingisaformofSJFscheduling;maycausestarvationofsomerequests.SSTFalgorithmisasubstantialimprovementovertheFCFS,butitisnotoptimal.illustrationshowsatotalheadmovementofonly236cylinders(208ispossible?)–littlemorethanone-t[e-mail]201013.9OperatingSystemConcepts13.2.2SSTFScheduling(1)[e-mail]201013.10OperatingSystemConcepts13.2.3SCANSchedulingSCAN--diskarmstartsatoneendofthedisk,andmovestowardtheotherend,servicingrequestsalongthewayuntilitgetstotheotherendofthedisk,wheretheheadmovementisreversedandservicingcontinues.Sometimescalledelevatoralgorithm,sincediskarmbehavesjustlikeanelevatorinabuilding,firstservingalltherequestsgoingup,andthenreversingtoservicerequeststheotherway.WithSCAN,needtoknowthedirectionofheadmovement,inadditiontothehead’scurrentposition.[e-mail]201013.11OperatingSystemConcepts13.2.3SCANScheduling(1)[e-mail]201013.12OperatingSystemConcepts13.2.4C-SCANSchedulingC-SCAN(CircularSCAN)isavariantofSCANdesignedtoprovideamoreuniformwaittimethanSCAN.C-SCAN--headmovesfromoneendofthedisktotheother,servicingrequestsasitgoes.Whenreachingtheotherend,however,itreturnstothebeginningofthediskimmediately,withoutservicinganyrequestsonthereturntrip.Treatsthecylindersasacircularlistthatwrapsaroundfromthelastcylindertothefirstone.uniformn.制服adj.统一的,相同的,一致的,始终如一的,均衡的vt.使成一样,使穿制服[e-mail]201013.13OperatingSystemConcepts13.2.4C-SCANScheduling(1)[e-mail]201013.14OperatingSystemConcepts13.2.5C-LOOKSchedulingC-LOOKisaversionofC-SCAN(alsoLookisversionofSCAN),becauseitlookforarequestbeforecontinuingtomoveinagivendirection.C-LOOK--Armonlygoesasfarasthelastrequestineachdirection,thenreversesdirectionimmediately,[e-mail]201013.15OperatingSystemConcepts13.2.5C-LOOKScheduling(1)[e-mail]201013.16OperatingSystemConcepts13.2.6SelectingaDisk-SchedulingAlgorithmSSTFiscommonandhasanaturalappeal.SCANandC-SCANperformbetterforsystemsthatplaceaheavyloadonthedisk--lesslikelyhaveastarvationproblem.Performancedependsheavilyonthenumberandtypesofrequestswithanyschedulingalgorithms.Requestsfordiskservicecanbeinfluencedbythefile-allocationmethod(contiguous,linkedorindexed).Thedisk-schedulingalgorithmshouldbewrittenasaseparatemoduleoftheoperatingsystem,allowingittobereplacedwithadifferentalgorithmifnecessary.[e-mail]201013.17OperatingSystemConcepts13.3DiskManagementOperatingsystemisrespon