操作系统有责任高效地使用硬件对于磁盘设备

Chapter14:Mass-StorageSystemsDiskStructureDiskSchedulingDiskManagementSwap-SpaceManagementRAIDStructureDiskAttachmentStable-StorageImplementationTertiaryStorageDevicesOperatingSystemIssuesPerformanceIssues 1 4/11/第1页DiskStructureDiskdrivesareaddressedaslarge1-dimensionalarraysoflogicalblocks,wherethelogicalblockisthesmallestunitoftransfer.

The1-dimensionalarrayoflogicalblocksismappedintothesectorsofthedisksequentially.Sector0isthefirstsectorofthefirsttrackontheoutermostcylinder.Mappingproceedsinorderthroughthattrack,thentherestofthetracksinthatcylinder,andthenthroughtherestofthecylindersfromoutermosttoinnermost. 2 4/11/第2页DiskSchedulingTheoperatingsystemisresponsibleforusinghardwareefficiently—forthediskdrives,thismeanshavingafastaccesstimeanddiskbandwidth.

操作系统有责任高效地使用硬件——对于磁盘设备，这意味着很短访问时间和磁盘带宽。Accesstimehastwomajorcomponents访问时间包含两个主要部分Seektimeisthetimeforthediskaretomovetheheadstothecylindercontainingthedesiredsector.寻道时间是指把磁头移到所需柱面时间。Rotationallatencyistheadditionaltimewaitingforthedisktorotatethedesiredsectortothediskhead.旋转延迟是指等候磁盘上所需要扇区旋转到磁头下面时间。 3 4/11/第3页DiskSchedulingMinimizeseektime最小寻道时间Seektimeseekdistance寻道时间

寻道距离Diskbandwidthisthetotalnumberofbytestransferred,dividedbythetotaltimebetweenthefirstrequestforserviceandthecompletionofthelasttransfer.

磁盘带宽，是用传输总位数，除以第一个服务请求与最终传输完成之间总时间。 4 4/11/第4页DiskScheduling(Cont.)SeveralalgorithmsexisttoscheduletheservicingofdiskI/Orequests.Weillustratethemwitharequestqueue(0-199).

98,183,37,122,14,124,65,67 Headpointer53 5 4/11/第5页FCFSIllustrationshowstotalheadmovementof640cylinders. 6 4/11/第6页SSTFSelectstherequestwiththeminimumseektimefromthecurrentheadposition.SSTFschedulingisaformofSJFscheduling;maycausestarvationofsomerequests.Illustrationshowstotalheadmovementof236cylinders. 7 4/11/第7页SSTF(Cont.) 8 4/11/第8页SCANThediskarmstartsatoneendofthedisk,andmovestowardtheotherend,servicingrequestsuntilitgetstotheotherendofthedisk,wheretheheadmovementisreversedandservicingcontinues.Sometimescalledtheelevatoralgorithm(电梯算法).Illustrationshowstotalheadmovementof208cylinders. 9 4/11/第9页SCAN(Cont.) 10 4/11/第10页C-SCANProvidesamoreuniformwaittimethanSCAN.Theheadmovesfromoneendofthedisktotheother.servicingrequestsasitgoes.Whenitreachestheotherend,however,itimmediatelyreturnstothebeginningofthedisk,withoutservicinganyrequestsonthereturntrip.Treatsthecylindersasacircularlistthatwrapsaroundfromthelastcylindertothefirstone. 11 4/11/第11页C-SCAN(Cont.) 12 4/11/第12页C-LOOKVersionofC-SCANArmonlygoesasfarasthelastrequestineachdirection,thenreversesdirectionimmediately,withoutfirstgoingallthewaytotheendofthedisk. 13 4/11/第13页C-LOOK(Cont.) 14 4/11/第14页SelectingaDisk-SchedulingAlgorithmSSTFiscommonandhasanaturalappealSCANandC-SCANperformbetterforsystemsthatplaceaheavyloadonthedisk.Performancedependsonthenumberandtypesofrequests.Requestsfordiskservicecanbeinfluencedbythefile-allocationmethod.Thedisk-schedulingalgorithmshouldbewrittenasaseparatemoduleoftheoperatingsystem,allowingittobereplacedwithadifferentalgorithmifnecessary.EitherSSTForLOOKisareasonablechoiceforthedefaultalgorithm. 15 4/11/第15页DiskManagementLow-levelformatting,orphysicalformatting—Dividingadiskintosectorsthatthediskcontrollercanreadandwrite.Touseadisktoholdfiles,theoperatingsystemstillneedstorecorditsowndatastructuresonthedisk.Partitionthediskintooneormoregroupsofcylinders.Logicalformattingor“makingafilesystem”.Bootblockinitializessystem.ThebootstrapisstoredinROM.Bootstraploaderprogram.Methodssuchassectorsparingusedtohandlebadblocks. 16 4/11/第16页MS-DOSDiskLayout 17 4/11/第17页Swap-SpaceManagementSwap-space—Virtualmemoryusesdiskspaceasanextensionofmainmemory.Swap-spacecanbecarvedoutofthenormalfilesystem,or,morecommonly,itcanbeinaseparatediskpartition.Swap-spacemanagement4.3BSDallocatesswapspacewhenprocessstarts;holdstextsegment(theprogram)anddatasegment.Kernelusesswapmapstotrackswap-spaceuse.Solaris2allocatesswapspaceonlywhenapageisforcedoutofphysicalmemory,notwhenthevirtualmemorypageisfirstcreated. 18 4/11/第18页4.3BSDText-SegmentSwapMap 19 4/11/第19页4.3BSDData-SegmentSwapMap 20 4/11/第20页RAIDStructureRAID–multiplediskdrivesprovidesreliabilityviaredundancy.RAIDisarrangedintosixdifferentlevels. 21 4/11/第21页RAID(cont)Severalimprovementsindisk-usetechniquesinvolvetheuseofmultipledisksworkingcooperatively.

Diskstripingusesagroupofdisksasonestorageunit.

RAIDschemesimproveperformanceandimprovethereliabilityofthestoragesystembystoringredundantdata.Mirroringorshadowingkeepsduplicateofeachdisk.Blockinterleavedparityusesmuchlessredundancy. 22 4/11/第22页RAIDLevels 23 4/11/第23页RAID(0+1)and(1+0) 24 4/11/第24页DiskAttachmentDisksmaybeattachedoneoftwoways:HostattachedviaanI/OportNetworkattachedviaanetworkconnection 25 4/11/第25页Network-AttachedStorage 26 4/11/第26页Storage-AreaNetwork 27 4/11/第27页Stable-StorageImplementationWrite-aheadlogschemerequiresstablestorage.

Toimplementstablestorage:Replicateinformationonmorethanonenonvolatilestoragemediawithindependentfailuremodes.Updateinformationinacontrolledmannertoensurethatwecanrecoverthestabledataafteranyfailureduringdatatransferorrecovery. 28 4/11/第28页TertiaryStorageDevicesLowcostisthedefiningcharacteristicoftertiarystorage.

Generally,tertiarystorageisbuiltusingremovablemedia

CommonexamplesofremovablemediaarefloppydisksandCD-ROMs;othertypesareavailable. 29 4/11/第29页RemovableDisksFloppydisk—thinflexiblediskcoatedwithmagneticmaterial,enclosedinaprotectiveplasticcase.

Mostfloppiesholdabout1MB;similartechnologyisusedforremovabledisksthatholdmorethan1GB.Removablemagneticdiskscanbenearlyasfastasharddisks,buttheyareatagreaterriskofdamagefromexposure. 30 4/11/第30页RemovableDisks(Cont.)Amagneto-opticdiskrecordsdataonarigidplattercoatedwithmagneticmaterial.Laserheatisusedtoamplifyalarge,weakmagneticfieldtorecordabit.Laserlightisalsousedtoreaddata(Kerreffect).Themagneto-opticheadfliesmuchfartherfromthedisksurfacethanamagneticdiskhead,andthemagneticmaterialiscoveredwithaprotectivelayerofplasticorglass;resistanttoheadcrashes.

Opticaldisksdonotusemagnetism;theyemployspecialmaterialsthatarealteredbylaserlight. 31 4/11/第31页WORMDisksThedataonread-writediskscanbemodifiedoverandover.WORM(“WriteOnce,ReadManyTimes”)diskscanbewrittenonlyonce.Thinaluminumfilmsandwichedbetweentwoglassorplasticplatters.Towriteabit,thedriveusesalaserlighttoburnasmallholethroughthealuminum;informationcanbedestroyedbynotaltered.Verydurableandreliable.ReadOnlydisks,suchadCD-ROMandDVD,comfromthefactorywiththedatapre-recorded. 32 4/11/第32页TapesComparedtoadisk,atapeislessexpensiveandholdsmoredata,butrandomaccessismuchslower.Tapeisaneconomicalmediumforpurposesthatdonotrequirefastrandomaccess,e.g.,backupcopiesofdiskdata,holdinghugevolumesofdata.Largetapeinstallationstypicallyuserobotictapechangersthatmovetapesbetweentapedrivesandstorageslotsinatapelibrary.stacker–librarythatholdsafewtapessilo–librarythatholdsthousandsoftapesAdisk-residentfilecanbearchivedtotapeforlowcoststorage;thecomputercanstageitbackintodiskstorageforactiveuse. 33 4/11/第33页OperatingSystemIssuesMajorOSjobsaretomanagephysicaldevicesandtopresentavirtualmachineabstractiontoapplications

Forharddisks,theOSprovidestwoabstraction:Rawdevice–anarrayofdatablocks.Filesystem–theOSqueuesandschedulestheinterleavedrequestsfromseveralapplications. 34 4/11/第34页ApplicationInterfaceMostOSshandleremovabledisksalmostexactlylikefixeddisks—anewcartridgeisformattedandanemptyfilesystemisgeneratedonthedisk.Tapesarepresentedasarawstoragemedium,i.e.,andapplicationdoesnotnotopenafileonthetape,itopensthewholetapedriveasarawdevice.Usuallythetapedriveisreservedfortheexclusiveuseofthatapplication.SincetheOSdoesnotprovidefilesystemservices,theapplicationmustdecidehowtousethearrayofblocks.Sinceeveryapplicationmakesupitsownrulesforhowtoorganizeatape,atapefullofdatacangenerallyonlybeusedbytheprogramthatcreatedit. 35 4/11/第35页TapeDrivesThebasicoperationsforatapedrivedifferfromthoseofadiskdrive.locatepositionsthetapetoaspecificlogicalblock,notanentiretrack(correspondstoseek).Thereadpositionoperationreturnsthelogicalblocknumberwherethetapeheadis.Thespaceoperationenablesrelativemotion.Tapedrivesare“append-only”devices;updatingablockinthemiddleofthetapealsoeffectivelyeraseseverythingbeyondthatblock.AnEOTmarkisplacedafterablockthatiswritten. 36 4/11/第36页FileNamingTheissueofnamingfilesonremovablemediaisespeciallydifficultwhenwewanttowritedataonaremovablecartridgeononecomputer,andthenusethecartridgeinanothercomputer.ContemporaryOSsgenerallyleavethenamespaceproblemunsolvedforremovablemedia,anddependonapplicationsanduserstofigureouthowtoaccessandinterpretthedata.Somekindsofremovablemedia(e.g.,CDs)aresowellstandardizedthatallcomputersusethemthesameway. 37 4/11/第37页HierarchicalStorageManagement(HSM)Ahierarchicalstoragesystemextendsthestoragehierarchybeyondprimarymemoryandsecondarystoragetoincorporatetertiarystorage—usuallyimplementedasajukeboxoftapesorremovabledisks.Usuallyincorporatetertiarystoragebyextendingthefilesystem.Smallandfrequentlyusedfilesremainondisk.Large,old,inactivefilesarearchivedtothejukebox.HSMisusuallyfoundinsupercomputingcentersandotherlargeinstallationsthathaveenormousvolumesofdata. 38 4/11/第38页SpeedTwoaspectsofspeedintertiarystoragearebandwidthandlatency.

Bandwidthismeasuredinbytespersecond.Sustainedbandwidth–averagedatarateduringalargetransfer;#ofbytes/transfertime.

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

Drive’soveralldatarate. 39 4/11/第39页Speed(Cont.)Accesslatency–amountoftimeneededtolocatedata.Accesstimeforadisk–movethearmtotheselectedcylinderandwaitfortherotationallatency;<35milliseconds.Accessontaperequireswindingthetapereelsuntiltheselectedblockreachesthetapehead;tensorhundredsofseconds.Generallysaythatrandomaccesswithinatapecartridgeisaboutathousandtimesslowerthanrandomaccessondisk.T