SQL Server High AvailabilityAlways On Technologies.docx
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SQL Server High AvailabilityAlways On Technologies.docx
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SQLServerHighAvailabilityAlwaysOnTechnologies
HighAvailability-AlwaysOnTechnologies
SQLServerTechnicalArticle
Writer:
GeoffAllix(ContentMaster)
TechnicalReviewer:
MichaelRaheem
ProjectEditor:
JoanneHodgins
Published:
November2007
AppliesTo:
SQLServer2008
Summary:
Microsoft®SQLServer®2008reducesdowntimeofbusiness-criticaldatabasesbyprovidingawiderangeofhighavailabilitysolutions.Thesesolutionsresultindatabeingavailabletocustomersandemployeestwenty-fourhoursaday,sevendaysaweek,andstilldelivercost-effectivehighperformance.TheAlwaysOnfeaturesinSQLServer2008increaseprotectionagainstsystemandhardwareerrors,reducethenumberandrecoverytimesofplannedorunplanneddowntime,anddecreaseconflictscausedbylargenumbersofuserswhoareaccessingthesamedatabase.
Copyright
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TableofContents
Introduction
Anyapplicationdowntimecanadverselyaffectbusinessandresultinlostrevenue,customerdissatisfaction,anddamagetothecredibilityofthebusiness.Manydatabaseapplications,especiallyenterprisebusinessapplications,requireminimumsystemdowntime.Systemdowntimecanbecausedbyavarietyofissuessuchashumanerror,naturaldisaster,hardwareorsoftwarefailure,andapplicationupgrade.MicrosoftSQLServer2008AlwaysOntechnologiesprovideafullrangeofoptionstominimizedowntimeandmaintainappropriatelevelsofapplicationavailability.
AvailabilityScenarios
Achievinghighavailabilityisacommonrequirementinmanybusinesses.Theappropriatesolutionforaparticularscenariodependsonanumberoffactors,andthesolutionthatworksbestinonecasemightnotbethemostsuitablesolutionforanothercase.Forexample,considerthefollowingscenarios.
∙AnInternet-basedsportinggoodsretailerwantstoprovidenearcontinuousavailabilitytoitscustomers.TheWebserversarealreadyinaWebfarm,butdatabasefailurescauseerrorsandmostofthesesituationsresultinthecompanylosingcustomers.Thecompanyhasanapprovedstandardserverbuildandideallywantstoavoidmajorcosts,infrastructurechanges,orapplicationre-writes.
∙Arecruitmentcompanyisconcernedaboutusererroronitsclientdatabase.Thecompanywouldalsoliketohaveawarmstandbyserverthatcouldbebroughtonlinequicklyalthoughinstantfailoverisunnecessary.
∙Aninsurancecompanywantstoprovidenearcontinuousavailabilityforitsclaimsdatabase.Theperformanceofthesystemmustnotbeaffectedanddocumentationtosupporttheclaim,whichiscurrentlystoredinthefilesystem,mustalsobemadeavailable.
∙Amanagementtrainingcompanyhasthreesiteswithasalesteamateachsite.ThesalesteamsenterrecordsintotheOrdersdatabasefortheirownsites.Toimproveperformance,eachsiterequiresaccesstoalloftheordersforthecompany,ideallyonitslocalserver.Intheeventofafailure,toprovideavailability,eachsitemustbeabletoswitchtotheserveratanothersite.
∙Anaccountancycompanywantstoprovideavailabilitytoitsclients,evenifthereisawiderangingfailuresuchasanaturaldisaster.Informationdoesnotneedtobemadeinstantlyavailable,butshouldbeavailablewithinhoursratherthandays.Thecompanywouldalsoliketoretainadailyversionofitsdatabasesforregulatorycompliance.Thecopydatadoesnotneedtobeinstantlyavailable,butmustbeavailableforanumberofyears.
∙Atoymanufacturerwantstorunreportsagainstitssalesdatabasebut,becausethisisconstantlyinuse,thereportsarenormallyblockedbycurrentlyrunningtransactions.Thecompanyhaslookedintoreportingagainstuncommittedtransactions,butthishasresultedinincorrectresultsbecausesomeofthetransactionsdidnotcomplete.
∙Arealestatecompanywantstoprotectitssystemagainstusererrorbybeingabletorollrecordsbacktothevaluesofthepreviousday.Thecompanywouldalsoliketobeabletorunreportsagainstyesterday’sdatathatareunaffectedbytransactionstakingplacetoday.Ideally,thecompanywouldliketoimplementasolutionthathastheminimumpossiblediskoverhead.
Withsuchawiderangeofavailabilityrequirements,businessesneedadatabaseplatformthatprovidesflexibleoptionsforhighavailabilitysolutions.Inthiswhitepaper,youwillseehowSQLServer2008AlwaysOntechnologiesprovidetheseflexibleoptionsforhighavailability,andenablesbusinessestoimplementthebestsolutionfortheirparticularrequirements.
IncreasedAvailability
Manydatabaseapplicationsaremission-critical,andthereforedemandminimumdowntimeandquickrecoveryintheeventofadisaster.SQLServer2008offersafullrangeofAlwaysOntechnologiestominimizedowntimeandachievetheappropriatelevelofavailability.
DatabaseMirroring
DatabaseMirroringisatechnologyforincreasingdatabaseavailability.Aschangesareappliedtotheprincipaldatabasetheyareautomaticallyappliedtothemirror.Intheeventofafailureontheprincipalserver,clientapplicationsareautomaticallyredirectedtothemirrorserverwithoutanychangestotheapplication.
YoucanconfigureDatabaseMirroringtoapplychangestothemirrorinrealtime,knownassynchronous,ornearrealtime,knownasasynchronous,soyoucanspecifythelevelofprotectionandtheperformanceoverheadonyoursystem.
WithSQLServer2008EnterpriseEditionthemirroringprotectionisextendedtopagesofdata.Ifapageisfoundtoeitherbecorruptontheprincipalserverormirrorserver,thecorrespondingpageisretrievedfromitspartnerserveranddatabaseoperationcontinuesseamlessly.SQLServer2008alsoimprovesdatabasemirroringbycompressingthedataflowbetweenprincipalandmirrorserversandbyremovingtheneedforadatabaserestartifamanualfailoverisperformed.
InthescenariooftheInternet-basedsportinggoodscompany,DatabaseMirroringwouldprovideaveryfastfailoverintheeventofaserver,database,orevenpagelevelfailure.Applicationswouldnotneedtoberewrittenand,althoughanewserverwouldhavetobepurchased,thestandbyserverwouldbestandardequipmentandwouldnotrequireaSQLServerlicense.
LogShipping
Logshippingisahighavailabilitytechnologythatprovidesawarmstandbyserver.Abackupistakenofthemasterserverandthisisrestoredononeormoresecondaryservers.Logshippingthenappliesperiodiclogbackupstothesecondaryservers.Althoughdatabasemirroringcanonlyhaveonemirrorserver,logshippingcanhavemanysecondaryservers,whichincreasesthelevelofprotection.
Logshippingoccursonascheduleandthereforethereisatimedelaybetweendatachangingonthemasterserverandthesechangestransferringtothesecondaryserver.Thisdelaycanresultindataloss,butinsomescenariosthedelaycanbeusedasameanstoundousererrorsbydelayingtheapplicationoflogdatatothesecondaryserverand(leavingtheoriginaldataonthesecondaryserver).
Toreducetheworkloadoftheprimaryserver,thesecondaryservercanactasaread-onlyreportingserver,althoughreportscannotberunwhilelogbackupsarebeingapplied.
Inthescenariooftherecruitmentcompany,logshippingcouldbeappliedwithadelaybetweenthelogbackupsandtherestoreonthestandbyserver.Thisallowsusererrortobereversedand,ifthereisafailureontheprimaryserver,thelogscouldbeappliedstraightawaytoreducedataloss.
FailoverClustering
FailoverClusteringisaWindowsfeaturethatprovidesprotectiontothewholeserver,notjustthedatabases.Multipleservers,knownasnodes,shareadiskarray,soifonenodefailsitsservicescanbestartedonanot
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