Replication alternatives


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Contemporary network-based replication offerings are either inline appliances or fabric based. With inline appliances, all I/Os need to pass through the replication device. Technically, the appliances terminate all incoming I/Os and initiate new I/Os that are forwarded to the primary and, in case of write I/Os, to replicated storage targets. The inline approach has been plagued by performance and scalability issues. The poster child for inline appliances is IBM's SAN Volume Controller (SVC).

A scalable architecture and plenty of cache have not only enabled it to overcome performance and scalability limitations but, aided by the simplicity of the inline appliance approach compared to the more complex fabric-based implementations, it has become one of the successes in the network-based replication and virtualization market.

In fabric-based replication products, the splitting and forwarding of I/Os is performed within an FC fabric. By taking advantage of FC switching and the separating data and control path, it's the best performing and most scalable approach. The majority of fabric-based replication products run on intelligent switches from Brocade Communications Systems Inc. and Cisco Systems Inc. Even though both Brocade and Cisco offer Data Mobility Manager (DMM) for local data center replication, third-party vendors like EMC and FalconStor Software Inc. offer more advanced fabric-based replication products that run on Brocade and Cisco intelligent switches.

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A case in point is EMC RecoverPoint, which provides fabric-based, asynchronous continuous data protection (CDP) with application integration that's on par with commensurate host-based CDP products. Despite obvious benefits, fabric-based replication has seen lackluster adoption. "Switch-based replication and virtualization have been over-hyped, but there are people who are working on it and over time it will become more common," said Greg Schulz, founder and senior analyst at Stillwater, Minn.-based StorageIO Group.

LSI Corp.'s StoreAge Storage Virtualization Manager (SVM) straddles the line between inline appliances and fabric-based products that depend on expensive intelligent switches. The combination of SVM and LSI's Data Path Module, which plugs into existing Fibre Channel switches to perform switch-based forwarding and eliminates the need for intelligent switches, combines the simplicity of IBM SVC with the performance and scalability benefits of a split-path architecture. HP seems to concur, and is offering the LSI product as HP StorageWorks SAN Virtualization Services Platform (SVSP) to complement its host- and array-based replication offerings with a network-based replication and virtualization product.

Even though the market share for array-, host- and network-based replication will shift over time, there will be appropriate places for all three approaches. While each has its own set of advantages and shortcomings, specific environments and situations will best determine where replication should occur.

BIO: Jacob Gsoedl is a freelance writer and a corporate director for business systems. He can be reached at jgsoedl@yahoo.com.

This was first published in April 2009

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