.Part 2 Fabric Infrastructure Rules HP SAN Design Reference Guide 785352-001

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 16 Gb/s, 8 Gb/s, and 4 Gb/s; or 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 16 Gb/s ports be
able to negotiate to 8 Gb/s and 4 Gb/s, all 8 Gb/s ports 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.
FCC allows C-series switches to intelligently regulate traffic across ISLs and ensure that each
initiator-target pair of devices has the required bandwidth for data transfer. C-series switches
can also prioritize frames using the QoS feature.
SAN performance considerations 163