Data replication considerations - Backup/Restore

Computer Technology Review, Sept, 2003 by Patty Barkley

Data replication technology is finding its way into every aspect of the IT application infrastructure. Moving data between servers, a task that once meant physically carrying tapes between locations, can now be streamlined by transmitting data directly over a network. Enterprise, metro-area and wide-area networks (available at relatively low cost) offer the bandwidth and speed necessary to efficiently transfer data across the data center--or across the world--for disaster recovery, content distribution, hardware migration or system consolidation.

Data replication has revolutionized enterprise disaster preparedness. Organizations large and small rely increasingly on software applications for every facet of business activity. Any interruption of access to application data can be devastating. Using data replication technologies, organizations are now able to safeguard the IT infrastructure from disaster.

For IT staff involved in configuring replication, staying abreast of business process vulnerability to data loss and changes in replication technology is essential. This paper aims to provide an overview of data replication alternatives and a review of the considerations enterprises must weigh in determining the appropriate use of replication technology.

Data Replication Alternatives

The term data replication covers a broad spectrum of methods for creating duplicate copies of operational data, either locally or at a remote data center. Like other applications of data replication technology, disaster recovery planning requires comprehensive and universally applicable data copy functionality that offers high-performance movement of data across significant distances.

The storage infrastructure provides multiple platforms for hosting enterprise data replication processing. Whether operating from an application server, the controller of a high-end storage array, or from a network-resident dedicated device, the data replication application creates mirror images of primary system data on secondary volumes located anywhere on the network. The backup copies can then be made available to business applications if an outage disrupts access to the primary source of data.

Server-Based Replication

Server-based data replication, referred to by some vendors as host-based or application-based, is installed on, and operates from, an application server platform. Often running on the same server as the business application, server-based solutions are frequently co-located with volume management Products--Volume Replicator from VERITAS offers an example. The replication software intercepts application I/O and duplicates write activity to a secondary volume. Replicated volumes can be mounted to either the application server or to a backup application server located elsewhere on the network. Although server-based solutions consume valuable CPU and memory resources at the application server, they provide the storage administrator with significant flexibility when determining the topology of data replication. Any storage hardware supported by the application server can be configured to receive replicated data.

If the pool of application servers requiring replication is relatively homogenous, a single software product will likely satisfy all data copy needs. However, the more varied the pool of servers becomes, the more likely that multiple replication products will be required to accommodate all operating system software and hardware configurations. Storage administrators, tasked with managing these solutions, must learn the ins and outs of each product and address the inevitable product compatibility issues.

The close integration between existing storage management applications and same-vendor replication software has definite advantages. Replication utilities will probably have a familiar look and feel, and product support will be integrated, speeding resolution of potential problems.

The most significant drawback to server-based replication is limited scalability. Software must be installed and managed at each server with data volumes to be replicated, adding to replication costs and to the administration burden of storage managers. For small-scale configurations, however, server-based replication presents a cost-effective approach.

Controller-Based Replication

High-end storage arrays offer a wide range of proprietary storage management functionality that leverages the array controller as an operating environment. Logical unit (LUN) management, performance monitoring and management, and bulk data movement are just some of the functions commonly built into the storage array. The tight integration between hardware and software allows vendor developers unmatched scope for optimizing the speed and functionality of storage management applications.

All vendors of enterprise-class storage arrays now offer data replication as a controller-based (sometimes called disk-based) application. Products include: SRDF from EMC, PPRC from IBM, Truecopy from Hitachi Data Systems, and ContinuousAccess from HP.