Backup and Recovery Overview.docx

Backup and Recovery Overview.docx

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BackupandRecoveryOverview

BackupandRecoveryOverview

Thischapterprovidesageneraloverviewofbackupandrecoveryconcepts,thefilesinanOracledatabaserelatedtobackupandrecovery,andthetoolsavailableformakingbackupsofyourdatabase,recoveringfromdatalossorothererror,andmaintainingrecordsofyourbackups.

Thischapterincludesthefollowingtopics:

∙WhatisBackupandRecovery?

∙BackupandRecovery:

BasicConcepts

∙TheDatabaseRecoveryProcess:

BasicConcepts

∙FormsofDataRecovery

∙BackupandRecoverywithRMAN

∙MatchingFailurestoBackupandRecoveryTechniques

∙AutomaticDisk-BasedBackupandRecovery:

TheFlashRecoveryArea

∙SystemRequirementsforBackupandRecoveryMethods

∙FeatureComparisonofBackupMethods

WhatisBackupandRecovery?

Ingeneral,backupandrecoveryreferstothevariousstrategiesandproceduresinvolvedinprotectingyourdatabaseagainstdatalossandreconstructingthedatabaseafteranykindofdataloss.

PhysicalBackupsandLogicalBackups

Abackupisacopyofdatafromyourdatabasethatcanbeusedtoreconstructthatdata.Backupscanbedividedintophysicalbackupsandlogicalbackups.

Physicalbackupsarebackupsofthephysicalfilesusedinstoringandrecoveringyourdatabase,suchasdatafiles,controlfiles,andarchivedredologs.Ultimately,everyphysicalbackupisacopyoffilesstoringdatabaseinformationtosomeotherlocation,whetherondiskorsomeofflinestoragesuchastape.

Logicalbackupscontainlogicaldata（forexample,tablesorstoredprocedures）exportedfromadatabasewithanOracleexportutilityandstoredinabinaryfile,forlaterre-importingintoadatabaseusingthecorrespondingOracleimportutility..

Seealso:

OracleDatabaseUtilitiesformoredetailsaboutimportingandexportingdatausingOracleexportandimportutilities.

Physicalbackupsarethefoundationofanysoundbackupandrecoverystrategy.Logicalbackupsareausefulsupplementtophysicalbackupsinmanycircumstancesbutarenotsufficientprotectionagainstdatalosswithoutphysicalbackups.

Unlessotherwisespecified,theterm"backup"asusedinthebackupandrecoverydocumentationreferstophysicalbackups,andtobackuppartorallofyourdatabaseistotakesomekindofphyscialbackup.Thefocusinthebackupandrecoverydocumentationsetwillbealmostexclusivelyonphysicalbackups.

ErrorsandFailuresRequiringRecoveryfromBackup

WhilethereareseveraltypesofproblemthatcanhaltthenormaloperationofanOracledatabaseoraffectdatabaseI/Ooperations,onlytwotypicallyrequireDBAinterventionandmediarecovery:

mediafailure,andusererrors.

OtherfailuresmayrequireDBAinterventiontorestartthedatabase（afteraninstancefailure）orallocatemorediskspace（afterstatementfailuredueto,forinstance,afulldatafile）butthesesituationswillnotgenerallycausedatalossorrequirerecoveryfrombackup.

UserError

Usererrorsoccurwhen,eitherduetoanerrorinapplicationlogicoramanualmis-step,datainyourdatabaseischangedordeletedincorrectly.Datalossduetousererrorincludessuchmisstepsasdroppingimportanttablesordeletingorchangingthecontentsofatable.Whileusertrainingandcarefulmanagementofprivilegescanpreventmostusererrors,yourbackupstrategydetermineshowgracefullyyourecoverthelostdatawhenusererrordoescausedataloss.

MediaFailure

Amediafailureisthefailureofareadorwriteofadiskfilerequiredtorunthedatabase,duetoaphysicalproblemwiththedisksuchasaheadcrash.Anydatabasefilecanbevulnerabletoamediafailure.

Theappropriaterecoveryfromamediafailuredependsonthefilesaffectedandthetypesofbackupavailable.

OracleBackupandRecoverySolutions:

RMANandUser-ManagedBackup

Forperformingbackupandrecoverybasedonphysicalbackups,youhavetwosolutionsavailable:

∙RecoveryManager（RMAN）,atool（withcommand-lineclientandEnterpriseManagerGUIinterfaces）thatintegrateswithsessionsrunningontheOracleservertoperformarangeofbackupandrecoveryactivities,aswellasmaintainingarepositoryofhistoricaldataaboutyourbackups

∙Thetraditionaluser-managedbackupandrecovery,whereyoudirectlymanagethefilesthatmakeupyourdatabasewithamixtureofhostoperatingsystemcommandsandSQL*Plusbackupandrecovery-relatedcapabilities

BothmethodsaresupportedbyOracleCorporationandarefullydocumented.RecoveryManageris,however,thepreferredsolutionfordatabasebackupandrecovery.Itcanperformthesametypesofbackupandrecoveryavailablethroughuser-managedmethodsmoreeasily,providesacommoninterfaceforbackuptasksacrossdifferenthostoperatingsystems,andoffersanumberofbackuptechniquesnotavailablethroughuser-managedmethods.

MostofthebackupandrecoverydocumentationsetwillfocusonRMAN-basedbackupandrecovery.User-managedbackupandrecoverytechniquesarecoveredinthelaterchaptersofOracleDatabaseBackupandRecoveryAdvancedUser'sGuide.

WhetheryouuseRMANoruser-managedmethods,youcansupplementyourphysicalbackupswithlogicalbackupsofschemaobjectsmadeusingdataexportutilities.Datathussavedcanlaterbeimportedtore-createthisdataafterrestoreandrecovery.However,logicalbackupsareforthemostpartbeyondthescopeofthebackupandrecoverydocumentation.

BackupandRecovery:

BasicConcepts

Thephysicalstructuresofthedatabaseandtheroleeachplaysinthedatabaserecoveryprocessarewhatdeterminetheformsofbackupandrecoveryavailablethroughuser-managedtechniquesandthroughRMAN.

PhysicalDatabaseStructuresUsedinRecoveringData

ThefilesandotherstructuresthatmakeupanOracledatabasestoredataandsafeguarditagainstpossiblefailures.ThissectionintroduceseachofthephysicalstructuresthatmakeupanOracledatabaseandtheirroleinthereconstructionofadatabasefrombackup.Thissectioncontainsthesetopics:

∙DatafilesandDataBlocks

∙RedoLogs

∙UndoSegments

∙ControlFiles

DatafilesandDataBlocks

AnOracledatabaseconsistsofoneormorelogicalstorageunitscalledtablespaces.EachtablespaceinanOracledatabaseconsistsofoneormorefilescalleddatafiles,physicalfilesunderthehostoperatingsysteminwhichthedatabaseisrunning.

Adatabase'sdataiscollectivelystoredinthedatafilesthatconstituteeachtablespaceofthedatabase.ThesimplestOracledatabasewouldhaveonetablespace,storedinonedatafile.Thedatbasemanagesthestoragespaceinthedatafilesofadatabaseinunitscalleddatablocks.Adatablockisthesmallestunitofdatausedbyadatabase.Datablocksarethesmallestunitsofstoragethatthedatabasecanuseorallocate.

Modifiedornewdataisnotwrittentodatafilesimmediately.Updatesarebufferedinmemoryandwrittentodatafilesatintervals.Ifadatabasehasnotgonethroughanormalshutdown（thatis,ifitisopen,orexitedabnormally,asinaninstancefailureoraSHUTDOWNABORT）thentherearetypicallychangesinmemorythathavenotbeenwrittentothedatafiles.Datafilesthatwererestoredfrombackup,orwerenotclosedduringaconsistentshutdown,aretypicallynotcompletelyuptodate.

Copiesofthedatafilesofadatabaseareacriticalpartofanybackup.

Seealso:

OracleDatabaseConceptsformoredetailaboutthestructureandcontentsofdatafilesanddatablocks.

RedoLogs

Redologsrecordallchangesmadetoadatabase'sdatafiles.Withacompletesetofredologsandanoldercopyofadatafile,thedatabasecanreapplythechangesrecordedintheredologstore-createthedatabaseatanypointbetweenthebackuptimeandtheendofthelastredolog.Eachtimedataischangedinthedatabase,thatchangeisrecordedintheonlineredologfirst,beforeitisappliedtothedatafiles.AnOracledatabaserequiresatleasttwoonlineredologgroups,andineachgroupthereisatleastoneonlineredologmember,anindividualredologfilewherethechangesarerecorded.

Atintervals,thedatabaserotatesthroughtheonlineredologgroups,storingchangesinthecurrentonlineredologwhilethegroupsnotinusecanbecopiedtoanarchivelocation,wheretheyarecalledarchivedredologs（or,collectively,thearchivedredolog）.YoucanrunyourdatabaseinARCHIVELOGmode（inwhichthisarchivingofredologfilesisenabled）orNOARCHIVELOGmode（inwhichredologfilesaresimplyoverwritten）.

Preservingthearchivedredologisamajorpartofmostbackupstrategies,astheycontainarecordofallupdatestodatafiles.Backupstrategiesofteninvolvecopyingthearchivedredologstodiskortapeforlonger-termstorage.RunninginNOARCHIVELOGmodelimitsyourdatarecoveryoptions.

Seealso:

OracleDatabaseAdministrator'sGuideformoredetailsabouttheonlineredologs,OracleDatabaseAdministrator'sGuideformoredetailsaboutarchivedredologs,and"DecidingBetweenARCHIVELOGandNOARCHIVELOGMode"foradiscussionoftheimplicationsofarchivingordiscardingyourredologfiles.

ControlFiles

Thecontrolfilecontainsacrucialrecordofthephysicalstructuresofthedatabaseandtheirstatus.Severaltypesofinformationstoredinthecontrolfilearerelatedtobackupandrecovery:

∙Databaseinformation（RESETLOGSSCNandtimestamp）

∙Tablespaceanddatafilerecords（filenames,datafilecheckpoints,read/writestatus,offlineranges）

∙Informationaboutredothreads（currentonlineredolog）

∙Logrecords（logsequencenumbers,SCNrangeineachlog）

∙ArecordofpastRMANbackups

∙Informationaboutcorruptdatafileblocks

Therecoveryprocessfordatafilesisinpartguidedbystatusinformationinthecont