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42 CrashConsistency:FSCKandJournaling Aswe’veseenthusfar,thelesystemmanagesasetofdatastruc turesto implementtheexpectedabstractions:les,directories,and alloftheother metadataneededtosupportthebasicabstractionthatweexpec tfroma lesystem.Unlikemostdatastructures(forexample,thosefoun din memoryofarunningprogram),lesystemdatastructuresmust persist , i.e.,theymustsurviveoverthelonghaul,storedondevicesthat retain datadespitepowerloss(suchasharddisksorash-basedSSDs). Onemajorchallengefacedbyalesystemishowtoupdatepersis – tentdatastructuresdespitethepresenceofa powerloss or systemcrash . Specically,whathappensif,rightinthemiddleofupdatingon -disk structures,someonetripsoverthepowercordandthemachineloses power?Ortheoperatingsystemencountersabugandcrashes?Bec ause ofpowerlossesandcrashes,updatingapersistentdatastructu recanbe quitetricky,andleadstoanewandinterestingprobleminle system implementation,knownasthe crash-consistencyproblem . Thisproblemisquitesimpletounderstand.Imagineyouhaveto up- datetwoon-diskstructures, A and B ,inordertocompleteaparticular operation.Becausethediskonlyservicesasinglerequestatat ime,one oftheserequestswillreachthediskrst(either A or B ).Ifthesystem crashesorlosespowerafteronewritecompletes,theon-diskstruc ture willbeleftinan inconsistent state.Andthus,wehaveaproblemthatall lesystemsneedtosolve: T HE C RUX :H OW T O U PDATE T HE D ISK D ESPITE C RASHES Thesystemmaycrashorlosepowerbetweenanytwowrites,and thustheon-diskstatemayonlypartiallygetupdated.Afterth ecrash, thesystembootsandwishestomountthelesystemagain(inorder to accesslesandsuch).Giventhatcrashescanoccuratarbitra rypoints intime,howdoweensurethelesystemkeepstheon-diskimagei na reasonablestate? 1
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