HP StorageWorks 4000/6000/8000 .HP StorageWorks SAN Design Reference Guide, Pa - Page 180

Infrastructure factors A single-switch fabric provides the highest level of performance. In a fabric with multiple switches, the following factors can affect performance: • Latency Switch latency is less than 5% (at 1 Gb/s) of the data transfer time; therefore, the number of switches and hops between devices is not a major performance factor. However, as devices send frames through more switches and hops, other data traffic in the fabric routed through the same ISL or path can cause oversubscription. • Oversubscription Oversubscription degrades Fibre Channel performance. When devices must contend for the same ISL or path, each device receives an equal share or 1/nth of the available bandwidth on the path (where n is the number of contending devices). Oversubscription occurs when one or more devices sends more data than the total bandwidth available on the ISL or path. • Fabric interconnect speeds Fibre Channel supports 8 Gb/s, 4 Gb/s, 2 Gb/s, and 1 Gb/s speeds. For optimum performance, configure a fabric with all components at the same, highest available speed. Additional factors such as distance, number of switch and device port buffers, and device response times can also affect performance. • Mixed Fibre Channel speeds For fabrics consisting of 8 Gb/s, 4 Gb/s, and 2 Gb/s or 4 Gb/s, 2 Gb/s, and 1 Gb/s switches and devices, the fabric segment connections negotiate the speed at which specific devices communicate. The presence of lower speed devices in a fabric does not force other independent higher speed devices or paths to a lower speed. Fibre Channel requires that all 8 Gb/s ports be able to negotiate to 4 Gb/s and 2 Gb/s, and all 4 Gb/s ports to 2 Gb/s and 1 Gb/s speeds. Switch ports or user ports in a fabric communicate at the highest mutually supported speed. Performance guidelines Although the topology and size of the fabric affect performance, adhering to the rules and recommendations outlined in this guide minimizes these factors. The topology designs have been defined to accommodate specific data access types. Recommendations on the number of ISLs based on device-to-device access ratios ensure that adequate bandwidth is available across the fabric, minimizing oversubscription. To maximize fabric performance, HP recommends the following guidelines: • Implement dual-fabric SANs. • In a cascaded or core-edge fabric, position switches with the highest port speeds near the center of the fabric. • Use the highest speed available for all infrastructure components and devices. • Ensure that communicating devices have the same speed connectivity path through the fabric. • Connect devices that communicate frequently to the same Fibre Channel switch. • When possible, ensure that there is an equal number of high-bandwidth application servers and storage systems (for one-to-one access). • Ensure that FCC is enabled on all C-series switches. 180 SAN fabric connectivity and switch interoperability rules