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1、VersatilityandUnixSemanticsinaFan-OutUnificationFileSystemPujaGupta,HarikesavanKrishnan,CharlesP.Wright,MohammadNayyerZubair,JayDave,andErezZadokStonyBrookUniversityTechnicalReportFSL-04-01AbstractWebmastersoftenneedtopublishnewcontent,whichcanbeexperimental.Usingunification,thewebmasterAdministr
2、atorsoftenprefertokeeprelatedsetsoffilescanmarktheexistingcontentasread-only,andunifyaindifferentlocationsormedia,asitiseasiertomain-higher-priorityemptydirectorywiththecontent.Whentainthosefilesseparately.Users,ontheotherhand,changesaremadetotheunioneddirectory,theoriginalprefertoseeallfilesinone
3、locationforconvenientac-contentisunchangedandallchangesoccurintheemptycess.Onesolutionthataccommodatesbothneedsisadirectory.Whenthecontentisreadytobepublished,virtualunificationofdisparatedirectories—afilesystemtheWebserver'sdocumentrootcanbeunionedwiththethatprovidesamergedviewofseveraldirectori
4、es,with-newdata—causingallchangesacrossallfilestobeper-outphysicallymergingthem.Unificationcanbeusedtoformedatomically.Ifthereareanyproblems,thentheprovidecopy-on-writesemanticsforread-onlysources,contentcansimplyberevertedtotheoriginalbyremov-mergethecontentsofseveralCD-ROMimageswith-ingthenewco
5、ntentfromtheunion.Unioningcanpro-outunpackingtheimages,mergebinarydirectoriesfromvideasimilarsolutionforsourcecodemanagement.differentpackages,andmore.ItisdifficulttoprovideaperfectillusionofavirtualunificationofasetoffilesSystemadministratorsneedtoknowwhatchangesarewhilemaintainingUnixsemantics.P
6、asteffortstopro-madetothesystemwhileinstallingnewsoftware.Ifvidesuchunificationoftencompromisedonthesetoftheinstallationfailed,thesoftwaredoesnotworkasad-featuresprovidedorcompatibilitywithUnixsemantics.vertised,orisnotneeded,thentheadministratoroftenWedesignedaunificationfilesystemcalledUnionfswa
7、ntstoreverttoapreviousgoodsystemstate.Unifi-whichusesapromisingfan-outstackingtechniquecationcanprovideafilesystemsnapshotthatcarriesoutrarelyusedbefore.UnionfsmaintainsUnixsemanticstheinstallationofnewsoftwareinasandboxeddirec-whil
