操作系统有责任高效地使用硬件对于磁盘设备课件

Chapter14:Mass-StorageSystemsDiskStructureDiskSchedulingDiskManagementSwap-SpaceManagementRAIDStructureDiskAttachmentStable-StorageImplementationTertiaryStorageDevicesOperatingSystemIssuesPerformanceIssues

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7/27/2023DiskStructureDiskdrivesareaddressedaslarge1-dimensionalarraysoflogicalblocks,wherethelogicalblockisthesmallestunitoftransfer.

The1-dimensionalarrayoflogicalblocksismappedintothesectorsofthedisksequentially.Sector0isthefirstsectorofthefirsttrackontheoutermostcylinder.Mappingproceedsinorderthroughthattrack,thentherestofthetracksinthatcylinder,andthenthroughtherestofthecylindersfromoutermosttoinnermost.

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7/27/2023DiskSchedulingTheoperatingsystemisresponsibleforusinghardwareefficiently—forthediskdrives,thismeanshavingafastaccesstimeanddiskbandwidth.

操作系统有责任高效地使用硬件——对于磁盘设备，这意味着很短的访问时间和磁盘带宽。Accesstimehastwomajorcomponents访问时间包括两个主要部分Seektimeisthetimeforthediskaretomovetheheadstothecylindercontainingthedesiredsector.寻道时间是指把磁头移到所需柱面的时间。Rotationallatencyistheadditionaltimewaitingforthedisktorotatethedesiredsectortothediskhead.旋转延迟是指等待磁盘上所需要的扇区旋转到磁头下面的时间。

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7/27/2023DiskSchedulingMinimizeseektime最小寻道时间Seektimeseekdistance寻道时间寻道距离Diskbandwidthisthetotalnumberofbytestransferred,dividedbythetotaltimebetweenthefirstrequestforserviceandthecompletionofthelasttransfer.

磁盘带宽，是用传输的总位数，除以第一个服务请求与最后传输完成之间的总时间。

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7/27/2023DiskScheduling(Cont.)SeveralalgorithmsexisttoscheduletheservicingofdiskI/Orequests.Weillustratethemwitharequestqueue(0-199).

98,183,37,122,14,124,65,67 Headpointer53

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7/27/2023FCFSIllustrationshowstotalheadmovementof640cylinders.

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7/27/2023SSTFSelectstherequestwiththeminimumseektimefromthecurrentheadposition.SSTFschedulingisaformofSJFscheduling;maycausestarvationofsomerequests.Illustrationshowstotalheadmovementof236cylinders.

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7/27/2023SSTF(Cont.)

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7/27/2023SCANThediskarmstartsatoneendofthedisk,andmovestowardtheotherend,servicingrequestsuntilitgetstotheotherendofthedisk,wheretheheadmovementisreversedandservicingcontinues.Sometimescalledtheelevatoralgorithm(电梯算法).Illustrationshowstotalheadmovementof208cylinders.

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7/27/2023SCAN(Cont.)

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7/27/2023C-SCANProvidesamoreuniformwaittimethanSCAN.Theheadmovesfromoneendofthedisktotheother.servicingrequestsasitgoes.Whenitreachestheotherend,however,itimmediatelyreturnstothebeginningofthedisk,withoutservicinganyrequestsonthereturntrip.Treatsthecylindersasacircularlistthatwrapsaroundfromthelastcylindertothefirstone.

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7/27/2023C-SCAN(Cont.)

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7/27/2023C-LOOKVersionofC-SCANArmonlygoesasfarasthelastrequestineachdirection,thenreversesdirectionimmediately,withoutfirstgoingallthewaytotheendofthedisk.

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7/27/2023C-LOOK(Cont.)

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7/27/2023SelectingaDisk-SchedulingAlgorithmSSTFiscommonandhasanaturalappealSCANandC-SCANperformbetterforsystemsthatplaceaheavyloadonthedisk.Performancedependsonthenumberandtypesofrequests.Requestsfordiskservicecanbeinfluencedbythefile-allocationmethod.Thedisk-schedulingalgorithmshouldbewrittenasaseparatemoduleoftheoperatingsystem,allowingittobereplacedwithadifferentalgorithmifnecessary.EitherSSTForLOOKisareasonablechoiceforthedefaultalgorithm.

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7/27/2023DiskManagementLow-levelformatting,orphysicalformatting—Dividingadiskintosectorsthatthediskcontrollercanreadandwrite.Touseadisktoholdfiles,theoperatingsystemstillneedstorecorditsowndatastructuresonthedisk.Partitionthediskintooneormoregroupsofcylinders.Logicalformattingor“makingafilesystem”.Bootblockinitializessystem.ThebootstrapisstoredinROM.Bootstraploaderprogram.Methodssuchassectorsparingusedtohandlebadblocks.

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7/27/2023MS-DOSDiskLayout

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7/27/2023Swap-SpaceManagementSwap-space—Virtualmemoryusesdiskspaceasanextensionofmainmemory.Swap-spacecanbecarvedoutofthenormalfilesystem,or,morecommonly,itcanbeinaseparatediskpartition.Swap-spacemanagement4.3BSDallocatesswapspacewhenprocessstarts;holdstextsegment(theprogram)anddatasegment.Kernelusesswapmapstotrackswap-spaceuse.Solaris2allocatesswapspaceonlywhenapageisforcedoutofphysicalmemory,notwhenthevirtualmemorypageisfirstcreated.

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7/27/20234.3BSDText-SegmentSwapMap

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7/27/20234.3BSDData-SegmentSwapMap

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7/27/2023RAIDStructureRAID–multiplediskdrivesprovidesreliabilityviaredundancy.RAIDisarrangedintosixdifferentlevels.

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7/27/2023RAID(cont)Severalimprovementsindisk-usetechniquesinvolvetheuseofmultipledisksworkingcooperatively.

Diskstripingusesagroupofdisksasonestorageunit.

RAIDschemesimproveperformanceandimprovethereliabilityofthestoragesystembystoringredundantdata.Mirroringorshadowingkeepsduplicateofeachdisk.Blockinterleavedparityusesmuchlessredundancy.

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7/27/2023RAIDLevels

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7/27/2023RAID(0+1)and(1+0)

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7/27/2023DiskAttachmentDisksmaybeattachedoneoftwoways:HostattachedviaanI/OportNetworkattachedviaanetworkconnection

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7/27/2023Network-AttachedStorage

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7/27/2023Storage-AreaNetwork

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7/27/2023Stable-StorageImplementationWrite-aheadlogschemerequiresstablestorage.

Toimplementstablestorage:Replicateinformationonmorethanonenonvolatilestoragemediawithindependentfailuremodes.Updateinformationinacontrolledmannertoensurethatwecanrecoverthestabledataafteranyfailureduringdatatransferorrecovery.

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7/27/2023TertiaryStorageDevicesLowcostisthedefiningcharacteristicoftertiarystorage.

Generally,tertiarystorageisbuiltusingremovablemedia

CommonexamplesofremovablemediaarefloppydisksandCD-ROMs;othertypesareavailable.

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7/27/2023RemovableDisksFloppydisk—thinflexiblediskcoatedwithmagneticmaterial,enclosedinaprotectiveplasticcase.

Mostfloppiesholdabout1MB;similartechnologyisusedforremovabledisksthatholdmorethan1GB.Removablemagneticdiskscanbenearlyasfastasharddisks,buttheyareatagreaterriskofdamagefromexposure.

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7/27/2023RemovableDisks(Cont.)Amagneto-opticdiskrecordsdataonarigidplattercoatedwithmagneticmaterial.Laserheatisusedtoamplifyalarge,weakmagneticfieldtorecordabit.Laserlightisalsousedtoreaddata(Kerreffect).Themagneto-opticheadfliesmuchfartherfromthedisksurfacethanamagneticdiskhead,andthemagneticmaterialiscoveredwithaprotectivelayerofplasticorglass;resistanttoheadcrashes.

Opticaldisksdonotusemagnetism;theyemployspecialmaterialsthatarealteredbylaserlight.

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7/27/2023WORMDisksThedataonread-writediskscanbemodifiedoverandover.WORM(“WriteOnce,ReadManyTimes”)diskscanbewrittenonlyonce.Thinaluminumfilmsandwichedbetweentwoglassorplasticplatters.Towriteabit,thedriveusesalaserlighttoburnasmallholethroughthealuminum;informationcanbedestroyedbynotaltered.Verydurableandreliable.ReadOnlydisks,suchadCD-ROMandDVD,comfromthefactorywiththedatapre-recorded.

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7/27/2023TapesComparedtoadisk,atapeislessexpensiveandholdsmoredata,butrandomaccessismuchslower.Tapeisaneconomicalmediumforpurposesthatdonotrequirefastrandomaccess,e.g.,backupcopiesofdiskdata,holdinghugevolumesofdata.Largetapeinstallationstypicallyuserobotictapechangersthatmovetapesbetweentapedrivesandstorageslotsinatapelibrary.stacker–librarythatholdsafewtapessilo–librarythatholdsthousandsoftapesAdisk-residentfilecanbearchivedtotapeforlowcoststorage;thecomputercanstageitbackintodiskstorageforactiveuse.

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7/27/2023OperatingSystemIssuesMajorOSjobsaretomanagephysicaldevicesandtopresentavirtualmachineabstractiontoapplications

Forharddisks,theOSprovidestwoabstraction:Rawdevice–anarrayofdatablocks.Filesystem–theOSqueuesandschedulestheinterleavedrequestsfromseveralapplications.

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7/27/2023ApplicationInterfaceMostOSshandleremovabledisksalmostexactlylikefixeddisks—anewcartridgeisformattedandanemptyfilesystemisgeneratedonthedisk.Tapesarepresentedasarawstoragemedium,i.e.,andapplicationdoesnotnotopenafileonthetape,itopensthewholetapedriveasarawdevice.Usuallythetapedriveisreservedfortheexclusiveuseofthatapplication.SincetheOSdoesnotprovidefilesystemservices,theapplicationmustdecidehowtousethearrayofblocks.Sinceeveryapplicationmakesupitsownrulesforhowtoorganizeatape,atapefullofdatacangenerallyonlybeusedbytheprogramthatcreatedit.

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7/27/2023TapeDrivesThebasicoperationsforatapedrivedifferfromthoseofadiskdrive.locatepositionsthetapetoaspecificlogicalblock,notanentiretrack(correspondstoseek).Thereadpositionoperationreturnsthelogicalblocknumberwherethetapeheadis.Thespaceoperationenablesrelativemotion.Tapedrivesare“append-only”devices;updatingablockinthemiddleofthetapealsoeffectivelyeraseseverythingbeyondthatblock.AnEOTmarkisplacedafterablockthatiswritten.

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7/27/2023FileNamingTheissueofnamingfilesonremovablemediaisespeciallydifficultwhenwewanttowritedataonaremovablecartridgeononecomputer,andthenusethecartridgeinanothercomputer.ContemporaryOSsgenerallyleavethenamespaceproblemunsolvedforremovablemedia,anddependonapplicationsanduserstofigureouthowtoaccessandinterpretthedata.Somekindsofremovablemedia(e.g.,CDs)aresowellstandardizedthatallcomputersusethemthesameway.

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7/27/2023HierarchicalStorageManagement(HSM)Ahierarchicalstoragesystemextendsthestoragehierarchybeyondprimarymemoryandsecondarystoragetoincorporatetertiarystorage—usuallyimplementedasajukeboxoftapesorremovabledisks.Usuallyincorporatetertiarystoragebyextendingthefilesystem.Smallandfrequentlyusedfilesremainondisk.Large,old,inactivefilesarearchivedtothejukebox.HSMisusuallyfoundinsupercomputingcentersandotherlargeinstallationsthathaveenormousvolumesofdata.

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7/27/2023SpeedTwoaspectsofspeedintertiarystoragearebandwidthandlatency.

Bandwidthismeasuredinbytespersecond.Sustainedbandwidth–averagedatarateduringalargetransfer;#ofbytes/transfertime.

Dataratewhenthedatastreamisactuallyflowing.Effectivebandwidth–averageovertheentireI/Otime,includingseekorlocate,andcartridgeswitching.

Drive’soveralldatarate.

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7/27/2023Speed(Cont.)Accesslatency–amountoftimeneededtolocatedata.Accesstimeforadisk–movethearmtotheselectedcylinderandwaitfortherotationallatency;<35milliseconds.Accessontaperequireswindingthetapereelsuntiltheselectedblockreachesthetapehead;tensorhundredsofseconds.Generallysaythatrandomaccesswithinatapecartridgeisaboutathousandtimesslowerthanrandomaccessondisk.Thelowcostoftertiarystorageisaresultofhavingmanycheapcartridgesshareafewexpensivedrives.Aremovablelibraryisbestdevotedtothestorageofinfrequentlyuseddata,becausethelibrarycanonlysatisfyarelativelysmallnumberofI/Orequestsperhour.

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7/27/2023ReliabilityAfixeddiskdriveislikelytobemorereliablethanaremovablediskortapedrive.

Anopticalcartridgeislikelytobemorereliablethanamagneticdiskortape.

Aheadcrashinafixedharddiskgenerallydestroysthedata,whereasthefailureofatapedriveoropticaldiskdriveoftenleavesthedatacartridgeunharmed.

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