SANbox switches can be linked to create larger automated fabrics servicing thousands of Fibre Channel storage and compute devices. To meet the needs of various installations, QLogic offers a unique range of connecting options, allowing customers to choose levels of resiliency, scalability, and performance appropriate to their specific project and budget parameters.

All SANbox multi-switch fabrics provide in-order delivery of frames through any number of links, enabling load-sharing, fault failover and other configuration possibilities. Customers can select from virtually unlimited wiring alternatives in three distinct categories:

• Cascaded (Low cost, smaller installations, low aggregate bandwidth)
• Mesh (Higher performance, more resilient)
• Multistage™ (QLogic exclusive: Highest performance, lowest cost-per-gigabit, highest resiliency, ideal for larger installations and/or tape drives* -- see Notes.)

Although well suited to certain applications, cascaded architectures do not scale well. Adding switches can significantly reduce performance, due to compound latency caused by multiple switch-to-switch "hops," and to the limited, shared E_port "pipeline." Bandwidth and latency vary unpredictably depending on where messages enter and exit the fabric, which means network planners using this approach must pay close attention to changing traffic patterns when designing and redesigning their system.

Option 2: Mesh ("Cascaded Plus") For projects requiring a more predictable environment, mesh architectures offer a cost-effective entry to true high-performance multi-switch topologies. Because each switch is directly connected to every other switch in the fabric, this configuration maintains a low hop-count even as the fabric scales, minimizing bandwidth loss and the effects of compound latency.

Eventually, however, the high number of required inter-switch connections (E_ports) overwhelms the ability to add more I/O ports, creating the need for a more flexible, robust strategy.

Option 3: Multistage™ Cross-Connect For fabrics above 8-10 switches or for fabrics that require a high degree of redundancy, QLogic's exclusive Multistage architecture provides the ultimate high-bandwidth solution. In this topology, some switches are dedicated to perform "cross-connect" functions, creating a variety of performance, resiliency and cost-per-gigabit benefits not found in any other configuration.

• Performance and Expandability
  Multistage fabrics maintain the highest aggregate bandwidth by allowing network designers to control the ratio of I/O ports to cross-connect ports, creating as many redundant paths as necessary to achieve a given performance goal.

  Multistage networks can be expanded in three ways:

  1. By adding more cross-connect links (Inexpensive Wiring-only Expansion). This raises the overall fabric capacity and non-blocking percentage (the percentage of time each port has uncontested network access) without requiring additional switches, but does lower the number of available I/O ports.
  2. By adding additional I/O switches (I/O Expansion). This increases the number of available end ports and overall fabric capacity while incurring only a minimum of additional blocking, depending on the number of cross-connect links used.
  3. By adding additional cross-connect switches (Performance Expansion). This raises overall system performance considerably while maintaining approximately the same number of I/O ports.
  When the number of cross-connect links equals the number of I/O ports, a 100% non-blocking configuration is achieved, and every node can experience simultaneous full-duplex gigabit-per-second access to the fabric. Large non-blocking configurations are not possible with any other architecture.

• Resiliency
  Multistage makes it easy to configure systems with no single point of failure, since each I/O switch is linked to multiple cross-connect switches. Redundant data paths mean that even in the unlikely event of a total switch outage, the network as a whole continues to function at normal or near-normal capacity. The following illustration shows the effect of outages at different locations of a multi-stage network.

  
   

• Cost-per-gigabit
  Finally, although Multistage environments can involve extra hardware costs, their enhanced performance often makes up for this initial expense when cost is measured on a per-gigabit basis. When the economic impact of easy expandability and a truly resilient, fail-safe infrastructure is factored in, multi-stage topologies provide the best return on investment for a wide range of large and mid-sized installations.

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  The three expansion methods (Cascaded, Mesh and Multistage) can be applied individually or in combination, depending on the desired performance and cost-per-gigabit affect.

  Summary Points

  QLogic provides three distinct methods of connecting multi-switch fabrics: Cascaded, Mesh and Multistage.

  All SANbox multi-switch fabrics provide in-order delivery of frames through any number of links.

  QLogic's exclusive Multistage cross-connect architecture offers the following benefits:

  • Predictable latency and performance (guaranteed bandwith)
  • Highest aggregate bandwith
  • Greatest expandability (Create large non-blocking fabrics)
  • Greatest fabric resiliency (Create highly redundant configurations.)

  For more information on how QLogic's multi-switch scalability options can affect the cost and connectivity of your IT project, please call or us.

  Notes

  * Fibre Channel-attached tape drives require guaranteed bandwidth to maintain a consistent data stream to each drive's recording head. Predictable performance, high reliability, and the ability to create large 100% non-blocking architectures makes multi-stage ideal for any environment that includes tape drives.

  ** Refers to the fabric's end-to-end capacity. Actual capacities are even higher for each of these examples when each I/O switch's internal capacity is added.

  *** Switch illustrations and examples used in this brief are based on 16-port units. However, the capabilities discussed pertain to all SANbox products.

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  SANbox, Multistage, QLogic and the QLogic logo are the marks and property of QLogic, Inc..