Crash Recovery - Texas State University：崩溃恢复-德克萨斯州立大学.ppt

《Crash Recovery - Texas State University：崩溃恢复-德克萨斯州立大学.ppt》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。

1、CrashRecoveryChapter18Review:TheACIDpropertiesAtomicity:AllactionsintheXacthappen,ornonehappen.Consistency:IfeachXactisconsistent,andtheDBstartsconsistent,itendsupconsistent.Isolation:ExecutionofoneXactisisolatedfromthatofotherXacts.Durability:IfaXactcommits,itseffectspersist.

2、TheRecoveryManagerguaranteesAtomicity&Durability.MotivationAtomicity:Transactionsmayabort(“Rollback”).Durability:WhatifDBMSstopsrunning?(Causes?)crash!DesiredBehavioraftersystemrestarts:T1,T2&T3shouldbedurable.T4&T5shouldbeaborted(effectsnotseen).T1T2T3T4T5HandlingtheBufferPoo

3、lForceeverywritetodisk?Poorresponsetime.Butprovidesdurability.Stealbuffer-poolframesfromuncommitedXacts?Ifnot,poorthroughput.Ifso,howcanweensureatomicity?P541,ch16.7:stealingframesForceNoForceNoStealStealTrivialDesiredMoreonStealandForceSTEAL(whyenforcingAtomicityishard)Tostea

4、lframeF:CurrentpageinF(sayP)iswrittentodisk;someXactholdslockonP.WhatiftheXactwiththelockonPaborts?MustremembertheoldvalueofPatstealtime(tosupportUNDOingthewritetopageP).NOFORCE(whyenforcingDurabilityishard)Whatifsystemcrashesbeforeamodifiedpageiswrittentodisk?Writeaslittleasp

5、ossible,inaconvenientplace,atcommittime,tosupportREDOingmodifications.BasicIdea:LoggingRecordREDOandUNDOinformation,foreveryupdate,inalog.Sequentialwritestolog(putitonaseparatedisk).Minimalinfo(diff)writtentolog,somultipleupdatesfitinasinglelogpage.Log:AnorderedlistofREDO/UNDO

6、actionsWrite-AheadLogging(WAL)TheWrite-AheadLoggingProtocol:ChangetodatabaseisfirstrecordedinthelogRecordinthelogmustbewrittentostablestoragebeforethechangetodatabaseiswrittentodisk#1guaranteesAtomicity.#2guaranteesDurability.Exactlyhowislogging(andrecovery!)done?We’llstudythe

7、ARIESalgorithms.WAL&theLogEachlogrecordhasauniqueLogSequenceNumber(LSN).LSNsalwaysincreasing.EachdatapagecontainsapageLSN.TheLSNofthemostrecentlogrecordforanupdatetothatpage.SystemkeepstrackofflushedLSN.ThemaxLSNflushedsofar.WAL:BeforeapageiswrittenpageLSN£flushedLSNLSNsDBpage

8、LSNsRAMflushedLSNpageLSNLogrecordsflushedtodisk“Logtail”inRAM