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RDCs and SDCs: How to replicate data between clustered nodes

In a replicated data cluster (RDC), data is shared between the clustered nodes by replicating it from one node to the other, rather than by sharing disks between the nodes. For the purposes of this discussion, you need only understand the differences between shared data (where all systems in a cluster can see and access the very same disks) and replicated data (where each system in the cluster, usually a two-node cluster, has its own independent copy of the data, and updates are sent between the systems across a network.

We believe that when there is a choice between replicated data clusters and shared data clusters (SDCs), SDCs are always preferable. The differences and advantages of shared data clusters are quite clear.

RDCs require the extra step of keeping the data in sync between the two clusters at all times, and in real time. Keeping real-time synchronization between two systems' data sets requires synchronous replication (where the data must be sent to the remote system as part of each write to the local system).

Synchronous replication cannot help but introduce I/O overhead, slowing down all the production work. As we've discussed, performance is an important element of availability. Slow performance decreases availability. The performance impact is generally a bigger problem with hardware-based replication than it is with software-based replication. What's more, since RDCs require additional processing steps, additional components, and a functioning

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LAN or WAN over which the replicated data is sent, RDCs add complexity when compared to an SDC, as well as more lements that can fail, thus violating Blueprints for high availability, Second edition," authored by Evan Marcus and Hal Stern, Wiley Publishing, Inc. All rights reserved.

About the authors: Evan Marcus is a frequent SearchStorage.com contributor and an expert at answering readers' questions related to availability, backup and disaster recovery-related issues. He is also a principal engineer for Veritas Software and the industry's data availability maven, with over 12 years of experience in this area. He is also a frequent speaker at industry technical conferences.

Hal Stern is the vice president and chief technology officer for the Services business unit of Sun Microsystems. He has worked on reliability and availability issues for some of the largest online trading and sports information as well as several network service providers.

Do you have a question for Evan Marcus? You can find him in our High Availability category.

This was first published in December 2003

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