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SAN consolidation with director-class switches

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Directors defined

The fundamental architectural difference between FC switches and FC directors is that FC directors are designed and built to scale up, and to provide high bandwidth and high availability. Today's FC directors are built with a blade-type design, so that additional ports can be added as needed by slotting an additional blade. The current crop of directors can scale up to several hundred Fibre Channel ports in a single unit by adding blades that contain various increments of ports.

The Brocade DCX Backbone family of directors can be deployed in single- or dual-chassis configurations, with support for up to 384 FC ports in the single chassis or a maximum of 768 FC ports in the dual chassis. A smaller version in the Brocade DCX line, the DCX-4S, scales up to 192 ports. Ports for the Brocade DCX Backbone directors are added by inserting 16-, 32- or 48-port blades that support 8 Gbps Fibre Channel. The Brocade 48000 Director is the previous-generation director that's available in a single-chassis-only configuration; however, it doesn't have the local switching capabilities of the DCX Backbone.

Cisco MDS 9500 Series Multilayer Directors are available in three models, the 9506, 9509 and 9513, and can scale up to 192, 336 and 528 ports, respectively. The ports for the Cisco MDS 9500 Series are added in different increments depending on whether you need 4 Gbps or 8 Gbps Fibre Channel. For 4 Gbps FC, the ports are added in 12-, 24- or

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48-port increments. For 8 Gbps FC, the ports are added in 24- or 48-port increments, or a hybrid 4/44 model that has four 8 Gbps ports and 44 4 Gbps ports.

By contrast, regular Fibre Channel switches typically have a fixed physical port count, although blocks of ports may be enabled by a license key to keep the initial acquisition cost low. Port counts for these switches range from eight ports up to 64 or 80 ports for Brocade switches, and 128 ports for the QLogic 9000 in a single switch chassis.

Another advantage that directors provide is that with the large port count in a single unit, it may be possible to design a SAN fabric with minimal or no inter-switch links (ISLs). ISLs, sometimes known as "hops," contribute to increased latency and have the potential to degrade performance if they're not configured optimally. Often, smaller fixed-port count switches are deployed to get a SAN started. As storage growth occurs and the single switch is outgrown, a second switch is added and ISLs are used between the switches to expand the SAN fabric. As growth continues, a third switch is typically added and so on. At some point in this growth pattern with many smaller switches and several ports on each switch dedicated to ISLs, many of the ports won't be available to use as device ports (to connect host servers and storage units), so several hops may be necessary to connect a host server at one end of the SAN to a storage device at the other end of the SAN. Directors can alleviate this situation by removing many or all of the ISLs so that most of the switch ports can be used for devices rather than dedicating ports to the switching infrastructure.

Click here to get a PDF of the Cisco and Brocade director-class switches chart.

This was first published in April 2010

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