講義教程成果

Chapter11:FileSystemFile-SystemFile-SystemDirectoryAllocation-SpaceEfficiencyandTodescribethedetailsofimplementinglocalfilesystemsdirectoryTodiscussblockallocation -blockalgorithmsandDiskprovides cerewriteandrandomI/Otransfersperformedinblocksofsectors(usually512bytes).ToprovideefficientandconvenientaccesstodiskFilesystemresidesonsecondarystorageProvideduserinterfacetostorage, logicaltophysicalProvidesefficientandconvenientaccesstodiskbyallowingtobestored,located,retrievedFilesystemorganizedintoLayeredFileHigherHigherlevelutilizefunctionalitiesProvidedbylowerleveltofulfillUser’srequestFileSystemDevicedriversmanageI/OdevicesattheI/OcontrolGivencommandslike“readblocks123intomemorylocation1060”,outputslow-levelhardwarespecificcommandstohardwarecontrollerBasicfilesystemissuesgenericcommandstotheappropriatedevicedriverstoreadandwritephysicalblocksFileorganizationmoduleunderstandsfiles,logicalblocks,andphysicalblocksTranslateslogicalblockaddresstophysicalblock space,diskallocation(e.g.,unallocatedAllofthefile-systemAllofthefile-systemexceptrealdataofaLogicalfilesystemmanagesmetadataTranslatesfilenameintofilenumber,filehandle,locationmaintainingfilecontrolDirectoryWehavesystemcalls(e.g.,open(),close()forfiles)attheAPIbuthowdoweimplementtheirOn-diskandin-memorystructuresareusedtoOnDisk(filesystemcontainsinfoBootcontrolblock(pervolume)containsinfoneededbysystemtobootOSfromthatvolumeNeededifvolumecontainsOS,usuallyfirstblockofVolumecontrolblock(pervolume)containsvolumeTotal#ofblocks,#of blocks,blocksize, blockpointersorDirectorystructureorganizesthePer-fileFileControlBlock(FCB)containsmanydetailsthepermissions,size,In-MemoryFileSystemInMounttablestoringeachmountedDirectorystructurecacheholdsthedirectoryinfoofrecentlyaccesseddirectoriesSystem-wideopen-filetablecontainsacopyoftheFCBofeachopenfilePer-processopen-filetablecontainsapointertotheentryinsystem-wideopenfiletable.ThefollowingfigureillustratesthenecessaryfilesystemprovidedbytheoperatingFigure11-3(a)referstoopeningaFigure11-3(b)referstoreadingaOpenreturnsafilehandleforsubsequentDatafromreadeventuallycopiedtospecifieduserprocessmemoryIn-MemoryFileSystemSystem-wideopenfiletablecheckedbeforeAfilePartitionsandMountingPartitioncanbeavolumecontainingafilesystem(“cooked”)raw–justasequenceofblockswithnofileBootblockcanpointtobootvolumeorbootloadersetofblocksthatcontainenoughcodetoknowhowtoloadthekernelfromthefileOrabootmanagementprogramformulti-OSRootpartitioncontainstheOS,otherpartitionscanholdotherOSes,otherfilesystems,orberawMountedatbootOtherpartitionscanmountautomaticallyorAtmounttime,filesystemconsistencyIsallmetadataIfnot,fixit,tryIfyes,addtomounttable,allowVirtualFileVirtualFileSystems(VFS)onUnixprovideanobject-wayofimplementingfileVFSallowsthesamesystemcallinterface(theAPI)tobefordifferenttypesoffileSeparatesfile-systemgenericoperationsimplementationImplementationcanbeoneofmanyfilesystemstypes,networkfileThendispatchesoperationtoappropriatefileimplementationVirtualFileSystemsTheAPIistotheVFSinterface,ratherthananyspecifictypefileBasedonopen(),Write(),close()systemDirectoryDirectory-allocationDirectory-allocationanddirectory-managementalgorithmssignificantlyaffectsefficiency,performance,andreliabilityofafilesystemLinearlistoffilenameswithpointertothedataSimpletoTime-consumingtoLinearsearchtime(forafileCouldkeeporderedalphabeticallyvialinkedHashTable–linearlistwithhashdataDecreasesdirectorysearchtime(forafileCollisions–situationswheretwofilenameshashtothesamelocationOnlygoodifentriesarefixedsize,orusechained-overflowmethod(changesizerequiresnewhashfunction)AllocationMethods-Anallocationmethodreferstohowdiskblocksareallocatedfor【importantforutilizingdiskefficientlyandaccessingfilequickly(1)Contiguousallocation–eachfileoccupiessetofcontiguousBestperformanceinmostSimple–onlystartinglocation(block#)andlength(numberblocks)arerequired(b,b+1,b+2,…,b+n-RandomProblemsfindingspacefornewfile(dynamicallocationproblem:first-externalfragmentation,needforcompactionoff-(downtime)oron- fromlogicaltoExtent-BasedManynewerfilesystems(i.e.,VeritasFileSystem)usemodifiedcontiguousallocationExtent-basedfilesystemsallocatediskblocksinAnextentisacontiguousblockofExtentsareallocatedforfileAfileconsistsofoneormoreAllocationMethods-(2)Linkedallocation–eachfileisalinkedlistofdiskblockscanbescatteredanywhereintheFileendsatnilNoexternalEachblockcontainspointertonextNocompaction,external

spacemanagementsystemcalledwhennewReliabilitycanbeaLocatingablockcantakemanyI/Osanddiskseeksefficientforsequential-Spaceforsaving ImproveefficiencybyclusteringblocksintogroupsbutincreasesinternalfragmentationLinkedAllocationMethods–LinkedFAT(FileAllocationTable)Beginningofvolumehastable,indexedbyblockMuchlikealinkedlist,butfasterondiskandNewblockallocationSinceunusedblocksareindicatedbya0tableFindthefirst0-valuedblockAllocationMethods-(3)IndexedEachfilehasitsownindexblock,whichisanarrayofdisk-address,eachpointtotheblockoftheLogicalExampleofIndexedIndexedAllocationNeedindexRandomDynamicaccesswithoutexternalfragmentation,buthaveofindexblock–howlargeafile’sindexblockshouldLinkedIntroducemoreindexblocks,whichareMultilevelIntroducemoreindexblocks,indexofCombinedIndexedAllocation–CombinedScheme:UNIXBestmethoddependsonfileaccessContiguousgreatforsequentialandLinkedgoodforsequential,notForsomesystemthatsupportsboth,declareaccesstypeatcreation->selecteithercontiguousorIndexedmoreSingleblockaccesscouldrequire2indexblockreadsthenblockMultilevelindexesmayrequiremoreindexreads(dependson-SpaceFilesystem -spacelisttotrackavailableUsingterm“block”forWhendeleted,file’sspaceisreused,listissearchedbeforeMethod1:Bitvectororbitmap(n n-…bit[i]

10block[i] )Blocknumber(numberofbitsperword)*(numberof0-valuewords)+offsetoffirst1bitCPUshaveinstructionstoreturnoffsetwithinwordoffirst“1”-SpaceManagementBitmaprequiresextrablocksize=4KB=212bytesdisksize=240bytes(1 n=240/212=228bits(or32MB)ifclustersof4blocks->8MBofEasytogetcontiguous SpaceListonMethod2:Linkedlist CannotgetcontiguousspaceeasilyNowasteofTraverseisinefficient,butnoneedtotraversetheentirelist-SpaceManagementMethod3:Modifylinkedlisttostoreaddressofnextn- blocksinblock,plusapointertonextblockthatcontains pointers(likethisone)Easytofindalargenumber blocksMethod4:Becausespaceisfrequentlycontiguouslyused d,contiguous-allocation,extents,orKeepaddressoffirst blockandcountoffollowingcontiguousblocks spacelistthenhasentriescontainingaddressesandEfficiencyandEfficiencydependentdiskallocationanddirectorytypesofdatakeptinfile’sdirectoryBuffer(