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ImplementingMulti-Site Clusters

April 14 20111

Trần Văn Huệ Nhất Nghệ CPLS



Overview of Multi-Site Clusters

•What Are Multi-Site Clusters?

•What Makes Multi-Site Clusters Unique?

•Why Use a Multi-Site Cluster?

• Evaluating the Business Need for a Multi-Site Cluster

•Storage Challenges for Multi-Site Clusters

•Synchronous / Asynchronous Replication?

•

Network Challenges for Multi-Site Clusters • Choosing the Appropriate Quorum Mode in Multi-Site Clusters

• Demo: Deploying a Multi-Site Cluster

Agenda:



What Are Multi-Site Clusters?

A multi-site cluster is a cluster that has been extended so that

different nodes in the same cluster reside in separatephysical locations

Site A Site B

SAN SAN



What Makes Multi-Site Clusters Unique?

• Multiple storage arrays, which are independent in each site

• Nodes commonly access storage local to their own sites

• Storage is replicated between sites

• No “true” shared disk is visible to all nodes

Site A Site B

Changes aremade on Site Aand replicated

to Site B

Replica



Why Use a Multi-Site Cluster?

Advantages over cold standby server in a disasterrecovery site:

Automatic failover • Reduced downtime

• Lower complexity

Reducedadministrativeoverhead

•

Automatic synchronization of applicationand cluster changes

• Better service continuity than coldstandby servers

Reduced human

error factor

• Reduced reliance on old, potentially

error-prone procedures



Evaluating the Business Need for a Multi-Site Cluster

• Because of increased cost and complexity, multi-siteclusters aren’t for every company or application

• Evaluate the application type and importance, and consider

available alternatives

• Plan for the application and hardware architecture, andfor routine failover and failback testing



Storage Challenges for Multi-Site Clusters

Challenge Description

Requires aseparate orthird-party datareplicationsolution

• Hardware (block level) storage-basedreplication

• Software (file system level) host-basedreplication

• Application-based replication (such asExchange 2007 Cluster ContinuousReplication)

Can be eithersynchronous orasynchronousreplication

•Synchronous: No acknowledgement of datachanges made in Site A until the data issuccessfully written to Site B

• Asynchronous: Data changes made in SiteA will eventually be written to the storagein Site B



What Is Synchronous Replication?

When you use synchronous replication, the host receives a

“write complete” response from the primary storage after thedata is written successfully to both storage locations

PrimaryStorage

Secondary

Storage

WriteRequest

WriteComplete

Replication

Acknowledgement

Data Data

Site A Site B



What Is Asynchronous Replication?

When you use asynchronous replication, the host receives a

“write complete” response from the primary storage after thedata is written successfully on the primary storage

PrimaryStorage

Secondary

Storage

WriteRequest

WriteComplete

Replication

Data Data

Site A Site B



Comparing Synchronous and AsynchronousReplication

Replication Type Description

Synchronous • Works across shorter distances

• Results in no data loss

• Requires high-bandwidth/low-latencyconnections

• Allows write latencies to impactperformance

Asynchronous • Stretches over longer distances

• Results in potential data loss on hard

failure• Assumes large enough bandwidth to keep

up with data

• Does not significantly impact performance



Network Challenges for Multi-Site Clusters

• Inter-node communications are time sensitive; you mightneed to configure these thresholds to meet the higherWAN latency

• DNS replication might impact client reconnect times when

failover is based on hostname

• Some applications might require all of the nodes to be in the

same Active Directory site

• Active Directory replication latency might effect applicationdata availability



Choosing the Appropriate Quorum Mode inMulti-Site Clusters

Quorum Mode Choose when:

Node and DiskMajority

• Directed by vendor only

No Majority: DiskOnly

• Directed by vendor only

Node Majority • Limited to two sites

Node and File ShareMajority

• Three sites are available



Using Node and File Share Majority in aMulti-Site Cluster

• File share witness allows a 2-node cluster with no shared disk

• Single file server can serve as the witness for multiple clusters

Third-Party Replication

Mechanism

Site C File ShareWitness

SAN

Site A Site B

SAN

WAN



Deploying a Multi-Site Cluster

When deploying multi-site clusters:

Ensure that the business requirements are met bythe solution

Use a hardware vendor to create a full solution for

multi-site clusters

Choose the correct quorum mode to properly maintainfunctionality in the event of failures

Choose the correct storage replication solution to meet

these needs



Demo

Demo:Multi Site Cluster



Demo

Private network: 172.16.1.0/16

Public network: 192.168.1.0/24

PC08: DC, QuorumIP: 192.168.1.8

Datakeeper replicate

PC09:IP: 192.168.1.9IP: 172.16.1.9

PC10:IP: 192.168.1.10IP: 172.16.1.10

Site 1Site 2

Site 3

multi site cluster

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