5.6 Switching Hubs

Switching hubs are a hybrid SAN interconnection, occupying a middle ground between shared loop hubs and fabrics. As with the functionality of private loop stealth or phantom mode for some fabric switches, switching hubs leverage the simplicity of arbitrated loop with the high-performance bandwidth of switches. By design, switching hubs are not fabric-capable and so do not support fabric login, SNS, or state change notification. This reduces the engineering requirements and additional cost associated with fabric switches. Switching hubs are therefore more expensive than standard loop hubs, but more economical than fabrics. For configurations that require high bandwidth but no more than 126 total devices, switching hubs offer a reasonable price and performance solution.

Switching hubs typically provide 6 to 12 ports, each of which supports 1Gbps or 2Gbps throughput. The attached loop nodes are configured into one virtual loop composed of multiple loop segments. By assigning high-performance servers to their own ports and RAIDs or JBODs to other ports, switching hubs provide the aggregate bandwidth of fabric switches. The segmentation of one logical loop into smaller virtual loop segments also allows loop behavior to be modified. Loop initialization can be restricted to a single port, and that removes a source of potential disruption for sensitive applications such as streaming video or streaming tape backup. Additionally, concurrent conversations within a single virtual loop, as shown in Figure 5-12, are now possible. A server on port 3, for example, could be accessing storage on port 7 while a server on port 1 could access data on port 6. Because neither normal loop protocols such as ARBs or OPNs nor the data frames themselves have to traverse the entire loop, switching hubs also accommodate long hauls (up to 10km) without impacting overall loop performance.

Figure 5-12. Switching hubs allow multiple concurrent loop transactions

graphics/05fig12.gif

At the high end of the hub product offering, switching hubs support SNMP, SCSI Enclosure Services, or other management features. Depending on the vendor's design, some products offer advanced diagnostic features, including the ability to direct, via a management interface, data capture traffic on any other port without disrupting the topology.



Designing Storage Area Networks(c) A Practical Reference for Implementing Fibre Channel and IP SANs
Designing Storage Area Networks: A Practical Reference for Implementing Fibre Channel and IP SANs (2nd Edition)
ISBN: 0321136500
EAN: 2147483647
Year: 2003
Pages: 171
Authors: Tom Clark

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