The pros and cons of network-based data replication

In network-based data replication, the replication occurs in the network between storage arrays and servers. I/Os are split in an inline appliance or in a Fibre Channel (FC) fabric; the I/O splitter looks at the

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destination address of an incoming write I/O and, if it's part of a replication volume, forwards a copy of the I/O to the replication target. However, as with any technology, network-based data replication has both positive and negative aspects to it.

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Network-based replication combines the benefits of array-based and host-based replication. By offloading replication from servers and arrays, it can work across a large number of server platforms and storage arrays, making it ideal for highly heterogeneous environments. Most network-based replication products also offer storage virtualization as an option or as part of the core product.

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. 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, they're also forwarded to replicated storage targets. The inline approach has been plagued by performance and scalability issues. The most common inline appliance is IBM Corp.'s SAN Volume Controller (SVC).

A scalable architecture and plenty of cache have not only enabled SVC 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 data replication and virtualization market.

In fabric-based replication products, the splitting and forwarding of I/Os is performed within a Fibre Channel fabric. By taking advantage of FC switching and separating the 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 Corp. and FalconStor Software Inc. offer more advanced fabric-based replication products that run on Brocade and Cisco intelligent switches. An example of this 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. However, despite its obvious benefits, fabric-based replication has seen lackluster adoption.

LSI Corp.'s StoreAge Storage Virtualization Manager (SVM) straddles the line between inline appliances and fabric-based products that depend on expensive intelligent switches. When IBM's SVM and LSI's Data Path Module are combined, they plug into the existing Fibre Channel switches to perform switch-based forwarding. This eliminates the need for intelligent switches and also combines the simplicity of SVC with the performance and scalability benefits of split-path architecture.

Hewelett-Packard (HP) Co. 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.

This article originally appeared in Storage magazine.

About this author: Jacob Gsoedl is a frequent contributor to "Storage" magazine.

This was first published in September 2009

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