HP StorageWorks SAN Virtualization Services Platform Best Practices Guide (5697-0935, May 2011)

2 Fabric topology
This chapter describes specific SVSP guidelines for SAN configuration. For additional information
about designing the SVSP fabric configuration, see the following documents:
• The HP StorageWorks SAN Design Reference Guide provides SVSP and SAN design
information, and is available from http://www.hp.com/go/sandesignguide.
• The "Zoning" and “Configuration best practices" chapters in the HP StorageWorks SAN
Virtualization Services Platform Administrator Guide provide basic fabric, zoning, and switch
best practices to add SVSP to the SAN.
Fibre Channel congestion
Congestion in Fibre Channel (FC) is likely to cause problems, because implementations of the
protocol do not provide effective mechanisms for relieving the congestion. The most common cause
of Fibre Channel congestion is interswitch links as shown in Figure 1 (page 10). In the top half of
the figure, the input stream is directed to a limited number of links. In the bottom half of figure, the
output may be directed over a long distance, making it difficult to keep the link full. The problem
is compounded if there were limited credits needed to support the bandwidth. Other causes of
congestion can be a lack of resources on nodes in the fabric, aborts and retries, queue full or busy
responses, and miscommunication by nodes in the fabric. The HP StorageWorks SAN Design
Reference Guide contains many suggestions to help size interswitch links.
Figure 1 Causes of congestion
NOTE: Servers running N_port ID Virtualization (NPIV) and virtual machines may present more
FC load than a typical server and may cause downstream links to be congested. Monitoring is the
best way to determine the load in these situations.
Although Fibre Channel congestion is independent of the SVSP solution, the high performance
nature of the SVSP solution means fabric congestion can dramatically influence performance and
the stability of the solution. Dropped frames and frame errors result in frames delivered out of order
to the storage arrays. This can cause additional work and resources on the part of the array,
resulting in even slower operation. All fabric topologies must follow the guidance of the current
HP StorageWorks SAN Design Reference Guide.
10 Fabric topology