Understanding DAG命令详细Word格式.docx
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Understanding DAG命令详细Word格式.docx
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2010-08-20
Adatabaseavailabilitygroup(DAG)isthebasecomponentofthehighavailabilityandsiteresilienceframeworkbuiltintoMicrosoftExchange
Server
2010.ADAGisagroupofupto16Mailboxserversthathostasetofdatabasesandprovideautomaticdatabase-levelrecoveryfromfailuresthataffectindividualserversordatabases.
ADAGisaboundaryformailboxdatabasereplication,databaseandserverswitchovers,andfailovers,andforaninternalcomponentcalledActiveManager.ActiveManagerisanExchange
2010componentwhichmanagesswitchoversandfailoversthatrunsoneveryserverinaDAG.FormoreinformationaboutActiveManager,seeUnderstandingActiveManager.
AnyserverinaDAGcanhostacopyofamailboxdatabasefromanyotherserverintheDAG.WhenaserverisaddedtoaDAG,itworkswiththeotherserversintheDAGtoprovideautomaticrecoveryfromfailuresthataffectmailboxdatabases,suchasadiskfailureorserverfailure.
Contents
DatabaseAvailabilityGroupLifecycle
UsingaDatabaseAvailabilityGroupforHighAvailability
UsingaDatabaseAvailabilityGroupforSiteResilience
ClientExperienceWhenUsingDatabaseAvailabilityGroups
DAGsleverageafeatureofExchange
2010knownasincrementaldeployment,whichistheabilitytodeployserviceanddataavailabilityforallMailboxserversanddatabasesafterExchange
isinstalled.Afteryou'
vedeployedExchange
2010,youcancreateaDAG,addMailboxserverstotheDAG,andthenreplicatemailboxdatabasesbetweentheDAGmembers.
ADAGiscreatedbyusingtheNew-DatabaseAvailabilityGroupcmdlet.ADAGisinitiallycreatedasanemptyobjectinActive
Directory.ThisdirectoryobjectisusedtostorerelevantinformationabouttheDAG,suchasservermembershipinformation.WhenanadministratoraddsthefirstservertoaDAG,afailoverclusterisautomaticallycreatedfortheDAG.ThisfailoverclusterisusedexclusivelybytheDAG,andtheclustermustbededicatedtotheDAG.Useoftheclusterforanyotherpurposeisnotsupported.
Inadditiontoafailoverclusterbeingcreated,theinfrastructurethatmonitorstheserversfornetworkorserverfailuresisinitiated.ThefailoverclusterheartbeatmechanismandclusterdatabasearethenusedtotrackandmanageinformationabouttheDAGthatcanchangequickly,suchasdatabasemountstatus,replicationstatus,andlastmountedlocation.
Duringcreation,theDAGisgivenauniquename,andeitherassignedoneormorestaticIPaddressesorconfiguredtouseDynamicHostConfigurationProtocol(DHCP).YoucanspecifyasingleIPaddressoracomma-separatedlistofIPaddressesbyusingtheDatabaseAvailabilityGroupIPAddressesparameter.
ConsideraDAGthatwillhavethreeservers;
twoservers(EX1andEX2)areonthesamesubnet(10.0.0.0)andthethirdserver(EX3)isonadifferentsubnet(192.168.0.0).Theadministratorrunsthefollowingcommands:
CopyCode
New-DatabaseAvailabilityGroup-NameDAG1-DatabaseAvailabilityGroupIPAddresses10.0.0.5,192.168.0.5
Add-DatabaseAvailabilityGroupServer-IdentityDAG1-MailboxServerEX1
Add-DatabaseAvailabilityGroupServer-IdentityDAG1-MailboxServerEX2
Add-DatabaseAvailabilityGroupServer-IdentityDAG1-MailboxServerEX3
Note:
ConfiguringtheDatabaseAvailabilityGroupIPAddressesparameterwithavalueof0.0.0.0configurestheDAG(cluster)touseDHCPforitsIPaddressesorIPaddressresources.
TheclusterforDAG1iscreatedwhenEX1isaddedtotheDAG.Duringclustercreation,theAdd-DatabaseAvailabilityGroupServercmdletretrievestheIPaddressesconfiguredfortheDAGandignorestheonesdon'
tmatchanyofthesubnetsfoundonEX1.Inthisexample,theclusterforDAG1iscreatedwithanIPaddressof10.0.0.5,and192.168.0.5isignored.
Then,EX2isadded.Again,theAdd-DatabaseAvailabilityGroupServercmdletretrievestheIPaddressesconfiguredfortheDAG.Therearenochangestothecluster'
sIPaddressesbecauseEX2isonthesamesubnetasEX1.
Then,EX3isadded.Again,theAdd-DatabaseAvailabilityGroupServercmdletretrievestheIPaddressesconfiguredfortheDAG.Becauseasubnetmatching192.168.0.5ispresentonEX3,the192.168.0.5addressisaddedasanIPaddressresourceintheclustergroup.Inaddition,anORdependencyfortheNetworkNameresourceforeachIPaddressresourceisautomaticallyconfigured.The192.168.0.5addresswillbeusedbytheclusterwhentheclustergroupmovestoEX3.
WindowsFailoverClusteringregisterstheIPaddressesfortheclusterinDomainNameSystem(DNS)whentheNetworkNameresourceisbroughtonline.Inaddition,aclusternetworkobject(CNO)iscreatedinActive
Directory.Thename,IPaddressesandCNOfortheclusterareusedonlyinternallybythesystemtosecuretheDAGandforinternalcommunicationpurposes.Administratorsandend
usersdon'
tneedtointerfacewithorconnecttotheDAGnameorIPaddressforanyreason.
InadditiontoanameandoneormoreIPaddresses,theDAGisalsoconfiguredtouseawitnessserverandawitnessdirectory.Thewitnessserverandwitnessdirectoryareeitherautomaticallyspecifiedbythesystem,ortheycanbemanuallyspecifiedbytheadministrator.
Bydefault,aDAGisdesignedtousethebuilt-incontinuousreplicationfeaturetoreplicatemailboxdatabasesbetweenserversintheDAG.Ifyou'
reusingthird-partydatareplicationthatsupportstheThirdPartyReplicationAPIinExchange
2010,youmustcreatetheDAGinthird-partyreplicationmodebyusingtheNew-DatabaseAvailabilityGroupcmdletwiththeThirdPartyReplicationparameter.Afterthismodeisenabled,itcan'
tbedisabled.
AftertheDAGhasbeencreated,MailboxserverscanbeaddedtotheDAG.WhenthefirstserverisaddedtotheDAG,aclusterisformedforusebytheDAG.DAGsmakelimiteduseofWindowsFailoverClusteringtechnology,namelytheclusterheartbeat,clusternetworks,andtheclusterdatabase(forstoringdatathatchangesorcanchangequickly,suchasdatabasestatechangesfromactivetopassiveorviceversa,orfrommountedtodismountedandviceversa).AseachsubsequentserverisaddedtotheDAG,it'
sjoinedtotheunderlyingcluster(andthecluster'
squorummodelisautomaticallyadjustedbythesystem,asneeded),andtheserverisaddedtotheDAGobjectinActive
Directory.
AfterMailboxservershavebeenaddedtoaDAG,youcanconfigureavarietyofDAGproperties,suchaswhethertousenetworkencryptionornetworkcompressionfordatabasereplicationwithintheDAG.YoucanalsoconfigureDAGnetworksandcreateadditionalDAGnetworks,asneeded.
Afteryou'
veaddedmemberstoaDAGandconfiguredtheDAG,theactivemailboxdatabasesoneachservercanbereplicatedtotheotherDAGmembers.Afteryou'
vecreatedmailboxdatabasecopies,youcanmonitorthehealthandstatusofthecopiesusingavarietyofbuilt-inmonitoringtools.Inaddition,youcanperformdatabaseandserverswitchovers,asneeded.
FormoreinformationaboutcreatingDAGs,managingDAGmembership,configuringDAGproperties,creatingandmonitoringmailboxdatabasecopies,andperformingswitchovers,seeManagingHighAvailabilityandSiteResilience.
DatabaseAvailabilityGroupQuorumModes
UnderneatheveryDAGisaWindowsfailovercluster.Failoverclustersusetheconceptofquorum.QuorumisnotanewconceptforExchange
2010.HighlyavailablemailboxserversinpreviousversionsofExchangealsousefailoverclusteringanditsconceptofquorum.Thethreemainreasonswhyquorumisimportantaretoensureconsistency,toactasatie-breakertoavoidpartitioning,andtoensureclusterresponsiveness:
∙EnsuringconsistencyAprimaryrequirementforaWindowsfailoverclusteristhateachofthemembersalwayshasaviewoftheclusterthatisconsistentwiththeothermembers.Theclusterhiveactsasthedefinitiverepositoryforallconfigurationinformationrelatingtothecluster.IntheeventthattheclusterhivecannotbeloadedlocallyonaDAGmember,theClusterservicedoesnotstart,becauseitisnotabletoguaranteethatthemembermeetstherequirementofhavingaviewoftheclusterthatisconsistentwiththeothermembers.
∙Actingasatie-breakerAquorumwitnessresourceisusedinDAGswithanevennumberofmemberstoavoidsplit-brainscenariosandtoensurethatonlyonecollectionofthemembersintheDAGisconsideredofficial.Whenthewitnessisneededforquorum,anymemberoftheDAGthatcancommunicatewiththewitnessservercanplaceaServerMessageBlock(SMB)lockonthewitnessserver'
switness.logfile.TheDAGmemberthatlocksthewitness(referredtoasthe"
lockingnode"
)retainsanadditionalvoteforquorumpurposes.TheDAGmembersthatareincontactwiththelockingnodeareinthemajorityandmaintainquorum.AnyDAGmembersthatcannotcontactthelockingnodeareintheminorityandthereforelosequorum.
∙EnsuringresponsivenessToensureresponsiveness,thequorummodelensuresthatwhenevertheclusterisrunning,enoughmembersofthedistributedsystemareoperationalandcommunicative,andatleastonereplicaofthecluster'
scurrentstatecanbeguaranteed.Thismeansthatnoadditionaltimeisrequiredtobringmembersintocommunicationortodeterminewhetheragivenreplicaisguaranteed.
DAGswithanevennumberofmemberswillusethefailovercluster'
sNodeandFileShareMajorityquorummode,whichemploysanexternalwitnessserverthatactsasatie-breaker.Inthisquorummode,eachDAGmemberandthewitnessservergetavote.Thereplicaisstoredbydefaulton
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