Implementing Multi-Site Clusters

April 14 20111

Trần Văn HuệNhất 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 separate physical locations

Site ASite A Site BSite 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 ASite A Site BSite B

Changes are made on Site A and replicated

to Site B

ReplicaReplica

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

PrimaryStorage

SecondaryStorage

WriteRequest

WriteComplete

Replication

Acknowledgement

Data Data

Site ASite A Site BSite B

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

PrimaryStorage

SecondaryStorage

WriteRequest

WriteComplete

Replication

Data Data

Site ASite A Site BSite B

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

• Some applications might require all of the nodes to be in the same Active Directory site

• Active Directory replication latency might effect application data availability

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

Third-Party Replication Mechanism

Site CSite C File Share Witness

SAN

Site ASite A Site BSite B

SAN

WAN

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

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