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Implementing Multi-Site Clusters April 14 20111 Trần Văn Huệ Nhất Nghệ CPLS Overview of Multi-Site Clusters Agenda: • 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 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 separate physical 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 are made on Site A and replicated to Site B Replica Why Use a Multi-Site Cluster? Advantages over cold standby server in a disaster recovery site: Automatic failover • Reduced downtime • Lower complexity Reduced administrative overhead • Automatic synchronization of application and cluster changes • Better service continuity than cold standby 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-site clusters aren’t for every company or application • Evaluate the application type and importance, and consider available alternatives • Plan for the application and hardware architecture, and for routine failover and failback testing Storage Challenges for Multi-Site Clusters Challenge Description Requires a separate or third-party data replication solution • Hardware (block level) storage-based replication • Software (file system level) host-based replication • Application-based replication (such as Exchange 2007 Cluster Continuous Replication) Can be either synchronous or asynchronous replication • Synchronous: No acknowledgement of data changes made in Site A until the data is successfully written to Site B • Asynchronous: Data changes made in Site A will eventually be written to the storage in Site B What Is Synchronous Replication? When you use synchronous replication, the host receives a “write complete” response from the primary storage after the data is written successfully to both storage locations Site A Site B Replication Write Request Write Complete Data Data Primary Storage Acknowledgement Secondary Storage What Is Asynchronous Replication? When you use asynchronous replication, the host receives a “write complete” response from the primary storage after the data is written successfully on the primary storage Site A Site B Replication Write Request Write Complete Data Primary Storage Data Secondary Storage Comparing Synchronous and Asynchronous Replication Replication Type Description Synchronous • Works across shorter distances • Results in no data loss • Requires high-bandwidth/low-latency connections • Allows write latencies to impact performance 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 might need to configure these thresholds to meet the higher WAN latency • DNS replication might impact client reconnect times when failover is based on hostname • Active Directory replication latency might effect application data availability • Some applications might require all of the nodes to be in the same Active Directory site Choosing the Appropriate Quorum Mode in Multi-Site Clusters Quorum Mode Choose when: Node and Disk Majority • Directed by vendor only No Majority: Disk Only • Directed by vendor only Node Majority • Limited to two sites Node and File Share Majority • Three sites are available Using Node and File Share Majority in a Multi-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 File Share Witness Site C Site A Site B WAN SAN SAN Third-Party Replication Mechanism Deploying a Multi-Site Cluster When deploying multi-site clusters: Ensure that the business requirements are met by the solution Use a hardware vendor to create a full solution for multi-site clusters Choose the correct quorum mode to properly maintain functionality in the event of failures Choose the correct storage replication solution to meet these needs Demo Demo: Multi Site Cluster Demo PC08: DC, Quorum IP: 192.168.1.8 Site 3 Public network: 192.168.1.0/24 Site 2 Site 1 PC10: IP: 192.168.1.10 IP: 172.16.1.10 PC09: IP: 192.168.1.9 IP: 172.16.1.9 Private network: 172.16.1.0/16 Datakeeper replicate