A File System for the 21st Century Previewing the Windows NT 50 File System.docx

A File System for the 21st Century Previewing the Windows NT 50 File System.docx

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AFileSystemforthe21stCenturyPreviewingtheWindowsNT50FileSystem

AFileSystemforthe21stCentury:

PreviewingtheWindowsNT5.0FileSystem

JeffreyRichterandLuisFelipeCabrera

BecauseofthewayNTFSstoresattributes,itispossibleforalloftheattributesofasinglefile,includingitsDataattribute,toberesident.Thisimprovesperformancewhenaccessingsmallfiles.NTFSalsostoresthemostcommonattributesofafileinthefile’sdirectoryentry.

Thisarticleassumesyou’refamiliarwithC++andWin32

Codeforthisarticle:

Nov98NTFS.exe（5KB）

JeffreyRichterwroteAdvancedWindows,ThirdEdition（MicrosoftPress,1997）andWindows95:

ADeveloper’sGuide（M&TBooks,1995）.Jeffcanbereachedatwww.JeffreyR.

LuisFelipeCabreraisanarchitectintheWindowsNTBaseDevelopmentgroupatMicrosoft.HisresponsibilitiesareinWindowsNT5.0storagemanagement.

ManyofyourprogrammingtaskswillbesimplifiedwhenyoutakeadvantageofthenewinnovationsintheWindowsNT®5.0filesystem（NTFS）.Let'sgoonawhirlwindtourofthesenewfeatures.Remember,wearediscussingsoftwarethatisinbeta,soeverythingissubjecttochange.PleasecheckMicrosoft'smostrecentdocumentationbeforewritinganycodebasedonthisinformation.

Let'sbeginwithanoverviewoftheNTFSfilesystemlayoutondisk.Whilethisinformationisprogrammaticallyoff-limitstotheapplicationdeveloper,ahigh-levelexplanationwillmakeitmucheasierforyoutounderstandmanyofthenewNTFSfeatures.

AttheheartoftheNTFSfilesystemisaspecialfilecalledthemasterfiletable（MFT）.ThisfileiscreatedwhenyouformatavolumeforNTFS.TheMFTconsistsofanarrayof1KBentries;eachentryidentifiesasinglefileonthevolume.Whenyoucreateafile,NTFSmustfirstlocateanemptyentrywithintheMFTarray（growingthearrayifnecessary）;thenitfillsthe1KBentrywithinformationaboutthefile.Afile'sinformationconsistsofacollectionofattributes.Figure1showsalistofstandardattributesthatcanbeassociatedwithasinglefile（ordirectory）.

Whenafileiscreated,thesystemcreatesthesetofattributesfortheMFT'sfileentryandattemptstoplacetheminsidethe1KBblock.Buttherearetwoproblems:

mostattributesarevariablelength,andmanyattributes（likeName,Data,andNamedData）canbemuchlargerthan1KB.SoNTFScan'tjustsimplythrowalltheattributesinsideanMFTentry.Instead,NTFSmustexaminetheattributes;ifthelengthofanattribute'svalueissmall,theattribute'svalueisplacedinsidetheMFTentry.Thisiscalledaresidentattribute.Iftheattribute'svalueislarge,thenthesystemplacestheattributevalueinanotherlocationonthedisk（makingthisanonresidentattribute）,andsimplyplacesapointertotheattribute'svalueinsidetheMFTentry.

Today,everybodyhaslotsandlotsofsmallfilesstoredthroughouttheirharddrives.Weallhavelotsofshortcut（.LNK）filesandprobablylotsofDESKTOP.INIfilessprinkledaround.BecauseofthewaythatNTFSstoresattributesinanMFTentry,itispossibleforalloftheattributesofasinglefile,includingitsDataattribute,toberesident.Thisgreatlyimprovesperformancewhenaccessingsmallfiles.Inaddition,NTFSalsostoresthemostcommonattributesofafileinthedirectoryentrythatrepresentsthefile.ThismeansthatwhenthesystemdoesaFindFirstFile/FindNextFileoperationtoretrieveafile'snameorbasicattributes,thedatafortheseattributesisfoundinthedirectoryentry,sonootherdiskaccessisneeded.

PriortoWindowsNT4.0,anMFTentryinNTFSwas4KBinsize.This,ofcourse,allowedfileswithslightlymoredatatohavetheirdataresident.InWindowsNT4.0,MicrosoftparedthesizeofanMFTentryto1KB.MicrosoftstudiedthenumberoffilesandtheirsizesonmanytypicalsystemsandsawthatNTFSwaswastingalotofspaceinMFTentriesandthatitwouldbemoreefficienttomaketheMFTentry1KB.

Nowlet'sgooversomeofthefeaturesofferedbyNTFSthatsoftwaredeveloperscan（andshould）takeadvantageof.

Streams

It'slittleknownthatNTFSallowsasinglefiletohavemultipledatastreams.ThisfeaturehasactuallybeeninNTFSsinceitsveryfirstversion（inWindowsNT3.1）buthasbeendownplayedbyMicrosoft.Thisisunfortunatebecausestreamscanbeincrediblyusefulinmanysituations.

Forinstance,let'ssaythatyouaredevelopingabitmap-editorapplication.Whentheusersavestheirdata,youcreateaBMPfileontheharddisk.You'dalsoliketostoreathumbnailversionoftheimageaswell.Thumbnailsaretypicallystoredattheendofafile,afterthemainbitmapimage.Toshowthethumbnailimage,youmustopenthefile,parsetheheaderinformation,seektothebytesfollowingthemainimage'sdata,readinthethumbnailimage'sdata,andthendisplaythethumbnail.Youcouldstorethethumbnaildatainaseparatefile,butit'snotagoodideabecauseit'stooeasyforthemainimagefileandthethumbnailfiletogetseparated.

AnNTFSnamedstreamoffersthebestofbothworlds.Whenyourapplicationcreatesitsfile,youcanwritethemainimage'sdatatothedefault（unnamed）streamandthencreateanother（named）datastreaminsidethesamefileforthethumbnailimage'sdata.Youhaveonlyonefile,butitcontainstwodatastreams.

Tounderstandhowthisworks,let'sperformanexperiment.OnaWindowsNT-basedmachine（anyversion）openacommandshell.ThenchangetoanNTFSpartitionandenterthefollowing:

C:

\>ECHO"HiReader">XX.TXT:

MyStream

Whenyouexecutethiscommand,thesystemcreatesafilecalledXX.TXT.Thisfilecontainstwostreams:

anunnamedstreamthatcontains0bytesandanamedstream（calledMyStream）thatcontainsthetext"HiReader".Ifyouhaven'tguessedbynow,youaccessafile'snamedstreambyplacingacolonafterthefilenamefollowedbythenameofthestream.Aswithfilenames,Win32®functionstreatstreamnamesascase-preservedandsearchesarecase-insensitive.

Unfortunately,thetoolssuppliedwiththesystemtreatstreamsassecondclasscitizensatbest.Forexample,executethefollowingcommand:

C:

\>DIRXX.TXT

VolumeindriveCisWizard

VolumeSerialNumberis40E5-92D4

DirectoryofC:

\

03/18/9808:

36a0XX.TXT

1File（s）0bytes

0Dir（s）3,399,192,576bytesfree

Asyoucansee,DIRreportsthatthefilesizeis0bytes,butthisisnottrue.TheDIRcommandonlyreportstotheuserthesizeofafile'sunnamedstream;thesizesofnamedstreamswithinthefilearenotshowntotheuser.Bytheway,Exploreralsoreportsafilesizeof0bytes.Thisallowsforsomegeekypartygameswhereyoucanallocatealargestreaminafileonafriend'sdisk.Thefriendwon'tbeabletodiscoverwhereallthediskspacehasgonebecauseallofthetoolsreportthatthefileoccupiesonly0bytes!

Whenworkingwithstreams,rememberthatit'sonlythetoolsthatdon'ttreatstreamswiththerespectthattheydeserve;NTFShasfullsupportforstreams（theyevencountagainstyourstoragequota）.

Now,toseethecontentsofthestream,executethiscommand:

C:

\>MORE

MyStream

"HiReader"

Here'sanotherwaytousestreams.Saythatyouarewritingaword-processingapplication.Whentheuseropensupanexistingdocument,youwillprobablycreateatemporaryfilethatholdsalloftheuser'schanges.Then,whentheuserdecidestosavethechanges,youwillwritealloftheupdatedinformationtothetemporaryfile,deletetheoriginalfile,andfinallymovethetemporaryfilebacktotheoriginalfile'slocationwhilerenamingthefile.

Thissoundsfairlysimpleandstraightforward,butyou'reprobablyforgettingafewthings.Thefinalfileshouldhavethesamecreationtimestampastheoriginal,soyou'llhavetofixthat.Thefinalfileshouldalsohavethesamefileattributesandsecurityinformationastheoriginal.Itisveryeasytomissproperlyupdatingsomeoftheseattributesduringthisfile-saveoperation.

Ifyouusestreams,alloftheseproblemsgoaway.Allstreamswithinasinglefilesharethefile'sattributes（timestamp,security,andsoon）.Youshouldreviseyourapplicationsothattheuser'stemporaryinformationiswrittentoanamedstreamwithinthefile.Then,whentheusersavesthedata,renamethetemporarynameddatastreamtotheunnameddatastream,andNTFSwilldeletetheoldunnameddatastreamanddotherenameinanall-or-nothingmanner.Youwon'thavetodoanythingtothefile'sattributesatall;they'llalljustbethesame.

Beforeweleavestreams,letusjustpointoutafewmorethings.First,ifyoucopyafilecontainingstreamstoafilesystemthatdoesn'tsupportstreams（liketheFATfilesystemusedonfloppydisks）,onlythedataintheunnamedstreamiscopiedover;thedatainanynamedstreamsdoesnotgetcopied.

Second,nameddatastreamscanalsobeassociatedwithadirectory.Directoriesneverhaveanunnameddatastreamassociatedwiththembuttheycertainlycanhavenamedstreams.SomeofyoumaybefamiliarwiththeDESKTOP.INIfileusedbytheExplorer.IftheExplorerseesthisfileinadirectory,itknowstoloadashellnamespaceextensionandallowstheshellnamespaceextensiontoparsethecontentsofthedirectory.ThesystemusesthisforfolderssuchasMyDocuments,Fonts,InternetChannels,andmanymore.SincetheDESKTOP.INIfiledescribeshowtheExplorershoulddisplaythecontentsofadirectory,wouldn'titmakemoresenseforMicrosofttoplacetheDESKTOP.INIdataintoanamedstreamwithinadirectory?

ThereasonMicrosoftdoesn'tdothisisbackwardcompatibility.StreamsareimplementedonlyonNTFSdrives;theydonotexistonFATfilesystemsoronCD-ROMdrives.For