9.2 Fibre Channel Analyzers

As with network analyzers developed for LAN and WAN traffic analysis, Fibre Channel analyzers provide protocol capture and decode on the transmission link. Even with enormous buffers, it is not possible to capture more than a few seconds of traffic at 1Gbps and 2Gbps speeds. You can, however, capture protocol events that may last some microseconds or milliseconds of time. Typically, an analyzer is used to trace a specific process, such as a fabric login problem. Instead of randomly sampling traffic in the hopes of capturing a specific event, analyzers let you trigger the start of data capture on the basis of designated ordered sets and addresses. Trigger criteria can be complex, with combinations of protocol states and source/destination address, or they can be restricted to certain bit patterns.

Through either manual control or preprogrammed triggers, the analyzer can capture several seconds, or several hundred megabytes, of activity at a time. After the capture is performed, the analyzer decodes the protocols and data and displays the results, usually on an integrated monitor. Capture is typically run against one port at a time, although some vendor implementations allow data capture on multiple ports. The latter is particularly useful for capturing a transaction from the initiator as well as the target perspective.

The maximum amount of data that can be captured varies from vendor to vendor but generally does not exceed several seconds. Because IDLEs and other primitives may occur in series, the memory needed to buffer captured data is reduced simply by counting the number of repeated occurrences and logging those to the decode display. Thousands of consecutive IDLEs are thus collapsed into a single entry, and that also makes it easier to read a trace.

After a capture is executed, the decode display will contain a sequential listing of all ordered set and frame activity. Depending on the vendor's design, the trace will appear in one or more windows, with drill-down screens for details of specific events. Trace field headers usually include a time stamp (to nanosecond granularity), a time delta from one event to the next, the analyzer port that saw the event, and ordered set and frame information, as shown in Figure 9-3. Most Fibre Channel analyzers supply SCSI frame decode by default, with IP and other protocols as options. The drill down for frame decode contains all frame fields, including routing control, source and destination IDs, frame controls, and data payload.

Figure 9-3. Generic Fibre Channel analyzer trace (courtesy of Finisar Corporation)

graphics/09fig03.jpg

Analyzers may include, as integrated or accessory features, the capability to generate ordered sets and frames as well as to induce protocol or frame errors for test purposes. Data generators are particularly useful for verifying proper operation of end devices, although this type of activity is usually conducted in a test lab environment. Vendors may also offer client/server versions of their products to allow multiple diagnostic workstations to utilize a central analyzer resource.

In addition to diagnostic functions, analyzers provide performance statistics that can be used to monitor the impact of a particular application or configuration on the SAN. Because overloading a SAN segment can be one source of errors, port utilization data is very useful for understanding both link and upper protocol issues. Some fabric switches also provide performance data in graphical form, something that is useful for monitoring capacity without the need to physically insert an analyzer.

Although the cost of Fibre Channel analyzers remains high, the ongoing cost for sophisticated troubleshooting is in the training and retention of experts who can make sense of captured data. Having the tools to capture and decode data at gigabit speeds is only part of the equation; maintaining the expertise to productively use the tools is still a challenge for many enterprise networks.



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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