Feature

Guidelines to troubleshooting your SAN.

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FC3. The FC3 layer contains the supporting services in the FCP. Such services as the Simple Name Server (SNS), Alias Server and Time Server live at this layer.

Because there are user interfaces in this layer, whether from the command line or a third-party application, you'll find the solution to most of your problems at this and higher layers. Depending on the error being reported, and the FCP functions being used in your SAN, you'll start to interrogate the failing service with the tools provided by your switch vendor.

Most problems in this layer have to do with the SNS. And just like in the IP world where a DNS corruption may cause an end device to go off the air, a corruption in the SNS will have a similar affect in the FC world.

In all likelihood, you're not using all of the functionality of the FC3 layer. For instance, the Alias Server supports multicasting on an FC network. Therefore, if you're not utilizing multicasting, there's no need to take that path during the exercise.

FC4. Protocol mapping occurs at the FC4 layer. According to the standard, FC4 defines the program structures that third-party vendors must adhere to when sending and receiving data off of the FC transport.

What this means is this is the layer in which SCSI device drivers interface with the FCP. This area is undergoing constant change. Pressure to get products to market and mental lapses often don't give hardware engineers and

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application developers the ability to thoroughly test different scenarios that we are experiencing in the field.

That's why you must verify that the proper device drivers and firmware revisions are loaded on the devices comprising your SAN. Most hardware vendors have matrices indicating what device drivers and firmware revisions work best with other certified solutions. Still, it's a good idea to standardize on device driver and firmware revisions for like equipment. For example, if your group is managing 75 QLogic HBAs-and there isn't an extenuating reason not to-all 75 HBAs should be managed at the same device driver and firmware revision level.

If you follow these guidelines, you'll have a good start to resolving problems inside the SAN infrastructure. By breaking apart the functionality of the different layers of the FCP reference model, the troubleshooter should be able to reduce the time necessary to solve the problem by limiting their explorations to the one or two layers that are responsible for the lost functionality that you're experiencing in your SAN.

This was first published in November 2002

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