操作系统英文教学课件：Chapter-10-File-System-Interface

Chapter10:File-SystemInterfaceChapter10:File-SystemInterfaceFileConceptAccessMethodsDirectoryStructureObjectivesToexplainthefunctionalitiesoffilesystemsTodescribetheinterfacestofilesystemsTodiscussfile-systemdesigntradeoffs,includingaccessmethods,filesharing,filelocking,anddirectorystructuresFileConceptOSprovidesauniformlogicalviewofinfomationstoragedespitethevariousstoragemedia(nonvolatile).Afileisalogicalstorageunit.Afileisanamedcollectionofrelatedinformationthatisrecordedonsecondarystorage.Types:Data:numeric;character;binaryProgramIngeneral,afileisasequenceofbits,bytes,lines,orrecords.Themeaningisdefinedbythefile’screatoranduser.Afilehasacertaindefinedstructure,whichdependsonitstype.Example:textfiles,sourcefiles,objectfiles,executablefilesContiguouslogicaladdressspaceFileConceptFileconcept1.Fileattributes2.Fileoperations3.Filetypes4.Filestructures5.InternalfilestructureFileAttributesFileAttributesInformationaboutfilesarekeptinthedirectorystructure,whichismaintainedonthediskTypically,adirectoryentryonlyconsistsofthefile’snameanditsuniqueidentifier.Theidentifierinturnlocatestheotherfileattributes.FileOperationsFileisanabstractdatatype.OSprovidesthe6basicsystemcalls1.Create:allocatespace+createandirectoryentry2.Write:writepointer3.Read:readpointer4.Repositionwithinfile:alsoknownasseek5.Delete:releasespace+erasethedirectoryentry6.Truncate:filelen=0;releasespace;allotherattributesremainunchangedothers:Forfile:append,renameForfileattribute:chown,chmod,:::Fordirectory&directoryentries:Open(Fi)–searchthedirectorystructureondiskforentryFi,andmovethecontentofentrytomemoryClose(Fi)–movethecontentofentryFiinmemorytodirectorystructureondiskFileOperationsOpenFiles&Open-FileTableOpen-filetable,OFT:asmalltablecontaininginformationaboutallopenfilesSeveralprocessesmayopenthesamefileatthesametime2-levels:aper-processtable&asystem-widetablewithprocess-independentinformationOpenFilesSeveralpiecesofdataareneededtomanageopenfiles:Filepointer:pointertolastread/writelocation,perprocessthathasthefileopenFile-opencount:counterofnumberoftimesafileisopen–toallowremovalofdatafromopen-filetablewhenlastprocessesclosesitDisklocationofthefile:cacheofdataaccessinformationAccessrights:per-processaccessmodeinformationOpenFileLockingProvidedbysomeoperatingsystemsandfilesystemsallowoneprocesstolockafileandpreventotherprocessesfromgainingaccesstoitfunctionalityissimilartoreader-writerlocksOS-orFS-dependentMandatoryoradvisoryMandatory（强制）:accessisdenieddependingonlocksheldandrequested;OSensureslockingintegrityWindowsOsesAdvisory（建议）：processescanfindstatusoflocksanddecidewhattodouptosoftwaredevelopersUNIXFileTypes–Name,ExtensionFileStructureSometimes,filetypescanindicatetheinternalstructureoffileFilestructures(文件结构)(逻辑上)None-sequenceofwords,bytesSimplerecordstructureLinesFixedlength;VariablelengthComplexStructuresFormatteddocumentRelocatableloadfilefilestructureSystem-supportedfilestructuresMostmodernOSessupportaminimalnumberoffilestructuresdirectlyExample:UNIXseeseveryfileasasequenceof8-bitbytesBenefits:ApplicationshavemoreflexibilitySimplifiestheOSInternalfilestructureInternelfilestructureHowtolocateanoffsetwithinafile?Logicalfile(record)(varyinlength)

Physicalblock(fixedsize)Solution:Packing–packinganumberoflogicalrecordsintophysicalblocks.Pack&unpack:convertbetweenlogicalrecordsandphysicalblocksInternalfragmentationwilloccurAccessMethodsFilesstoreinformation.Whenitisused,thisinformationmustbeaccessedandreadintocomputermemoryOnalogicalperspectiveofusers,accessafileofrecords1.SequentialAccess(顺序访问方式)2.DirectAccess(直接访问方式)3.IndexedAccess(索引访问方式)SequentialAccessSequentialAccess(顺序访问方式):thesimplestaccessmethod.Informationinthefileisprocessedinorder,onerecordaftertheother.Thisisamostcommonaccessmode.Forexample:editors,compilersAtapemodeloffileFileoperations&theeffectonfilepointerread/writenextresetrewind/forwardnDirectAccessDirectAccess(直接访问方式)Informationinthefileisprocessedinnoparticularorder.Fileismadeupofanumberedsequenceoffixed-lengthlogicalrecordsAdiskmodelofafile,allowrandomaccess,immediateaccessForexample:databasesCanmovequicklytoanyrecordlocationbysupplyingarelativerecordnumber(n)readn writen positionton readnext writenext rewriten

n=relativeblocknumberSimulationofSequentialAccessonDirect-accessFileIndexedAccessToimprovessearchtimeandreduceI/O1.Makeanindexfileforthefile,whichcontainspointerstovariousrecords2.Searchtheindexfilefirst,3.andthenusethepointertoaccessthefiledirectlyandtofindthedesiredrecord.Withlargefiles,theindexfileitselfmaybecometoolargetobekeptinmemory)Multi-levelindextableExampleofIndexandRelativeFilesDirectoryStructureAcollectionofnodescontaininginformationaboutallfilesF1F2F3F4FnDirectoryFilesBoththedirectorystructureandthefilesresideondiskBackupsofthesetwostructuresarekeptontapesInformationinadirectoryentryInformationinadirectoryentryFileattributesNameTypeAddressCurrentlengthMaximumlengthDatelastaccessedDatelastupdatedOwnerIDProtectioninformationOperationsPerformedonDirectorySearchforafileCreateafileDeleteafileListadirectoryRenameafileTraversethefilesystemOrganizetheDirectory(Logically)toObtainEfficiency–locatingafilequicklyNaming–convenienttousersTwouserscanhavesamenamefordifferentfilesThesamefilecanhaveseveraldifferentnamesGrouping–logicalgroupingoffilesbyproperties,(e.g.,allJavaprograms,allgames,…)DirectoryStructures1.Single-leveldirectory(单层目录)2.Two-leveldirectory(双层目录)3.Tree-structureddirectory(树型结构目录)4.Acyclic-graphdirectory(无环图目录)5.General-graphdirectory(通用图目录)Single-LevelDirectoryAsingledirectoryforallusersNamingproblem

GroupingproblemTwo-LevelDirectorySeparatedirectoryforeachuserPathnameCanhavethesamefilenamefordifferentuserEfficientsearchingNogroupingcapabilityTree-StructuredDirectoriesTree-StructuredDirectories(Cont)EfficientsearchingGroupingCapabilityCurrentdirectory(workingdirectory)AbsoluteorrelativepathnameCreatinganewfileisdoneincurrentdirectoryTree-StructuredDirectories(Cont)Deleteafile

rm<file-name>Creatinganewsubdirectoryisdoneincurrentdirectory

mkdir<dir-name> Example:ifincurrentdirectory/mail

mkdircountmailprogcopyprtexpcountDeleting“mail”deletingtheentiresubtreerootedby“mail”Acyclic-GraphDirectoriesHavesharedsubdirectoriesandfiles,withnocyclesAcyclic-GraphDirectoriesImplementation1.Symboliclinks(符号链接)Aspecialnewdirectoryentry(link)Thecontentofsuchfileisthepathnameoftherealfile/directoryHowtotraverseadirectorycontainssymboliclinks?Theoperatingsystemignorestheselinkswhentraversingdirectorytreestopreservet