Intelligent SANs: issues to consider when selecting an enterprise-class network storage controller

Computer Technology Review, Oct, 2004 by Rick Walsworth

Despite the fact that storage costs continue to drop, most data centers still find it too costly to store all enterprise data on a single platform. Yet the alternative--selecting a best-of-breed storage solution for each application--while certainly cost-effective, leaves data centers with a disparate environment where different vendors' SANs, storage management software, and media types exist as isolated islands.

To resolve the complexity of managing heterogeneous storage architectures, the concept of delivering intelligence within the SAN was developed. This "intelligent SAN" presents a storage abstraction layer to servers within the environment, enabling integration of disparate resources into a virtualized pool that is available horizontally across the entire data center and all applications When selecting an intelligent storage network solution, data center managers face a growing array of choices, each promising to deliver the virtualization and storage management capabilities they require.

Homogenous Management of Heterogeneous Resources

Consider at the outset that although intelligent storage networks were originally conceived of as a way to simply virtualize disparate resources, they can also offer a number of value-added functionalities, the most notable being data mirroring and replication across heterogeneous systems. In addition, some solutions can also facilitate load balancing across storage media, and support implementation of a quality of service hierarchy that enables cost effective service level agreements without the need to over-provision storage assets or network bandwidth.

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But before evaluating what specific features are required or desirable, remember that meeting the fundamental objective of consolidating heterogeneous resources in a data center environment requires that the infrastructure can scale as data center storage volumes grow--without performance bottlenecks or single points of failure. Unfortunately, however, not all solutions meet these fundamental requirements.

External server-based appliances that are implemented within a storage network, for example, actually create a bottleneck that can negatively impact application performance. When this occurs, additional appliances have to be added--each becoming a separate management point in a costly move that, at best, can only solve the overload problem temporarily. In other words, as storage volumes increase and new resources are implemented, additional appliances will have to be implemented as well, adding to management complexity and overhead. Additionally, these appliances also create a single point of failure within the data path which, in turn, affects application availability.

To address the availability issue, some server appliances are implemented "out-of-band", providing virtualization and management capabilities out of the primary data path. But users of such solutions are still left with a never-ending cycle of adding resources that defeats the key benefits of an intelligent storage network--management simplicity and reduced storage costs in a highly available, highly scalable architecture.

In an attempt to rectify the shortcomings of external server-based appliances, some SAN switch vendors have introduced in-band application modules within the switch chassis--an approach that eliminates the need for an external device. Unfortunately, though, this approach does not solve the performance, scalability or availability problems. The reason the scalability problem remains is that virtualization and management functions are implemented on individual application modules (server blades) within the switch, each with a limited capacity, yet each having to serve a potentially large number of ports. These application modules also create a significant cost barrier, costing as much as $200,000 each, supporting as little as two terabytes of data. Furthermore, in some cases only a very limited number of modules can be supported per chassis. As a result, users have large upfront costs and may run out of chassis slots (and ports) before the bottleneck problem can be resolved.

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Of equal concern is the fact that, as with external appliances, each blade can only manage and virtualize storage resources attached directly to it. Or, put another way, appliance and switch-based intelligent storage networks both suffer from discrete capacity limitations that prevent enterprise-wide resource sharing, do not lower storage resource costs, and do not optimize data lifecycle management across the entire data center.

Delivering on the Promise of Intelligent SANs

The only way to completely resolve the capacity limitations of appliance and switch-based intelligent storage networks without continually facing the need for adding servers or blades is with a solution that offers in-band, distributed, port-based intelligence rather than centralized, server-based intelligence. This paradigm, often referred to as an enterprise-class network storage controller, delivers instant scalability as a data center expands and adds storage resources. Storage controllers also allow storage resources to be consolidated and virtualized with a single homogeneous management interface that can uniformly allocate, replicate and mirror data across heterogeneous storage resources throughout the enterprise.