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Troubleshooting McData 2640 SAN router issues

Storage expert Greg Schulz offers guidelines for troubleshooting SAN router issues.

We have just installed two McData 2640 SAN routers, one in each of our data centers, but we are seeing really bad performance over the WAN. We have set the MTU sizes to 1500, but the network guys keep seeing a 4 K negotiation, is this something the network guys can control? The connection is stable other than the retransmitted segments, the network team believes we are overloading the destination with packets before it can process them.
Without knowing more detail about your environment's network, WAN, applications and workload it would be difficult to determine where the issue exists. It is possible that the source is overloading the destination somehow. However, there are some questions that come to mind to help identify where the issue exists. How are the McData 2640 SAN routers being used? For example, are they being used for remote mirroring from an EMC, HDS, HP, IBM or Sun/STK type storage device or some other function?

How are you determining that you are getting bad performance? Is this based upon latency, low bandwidth, errors or some other indicator? And, what is or should be normal?

What is the effective speed of the WAN interface compared to the theoretical (quoted bandwidth)? How are the buffers configured on the destination devices, and what type of devices are they? Have you spoken with the McData technical support folks and/or the technical support folks of storage devices that are using the routers?

Lastly, can other applications get better bandwidth and utilize a larger MTU on the same WAN interface to determine if it is the routers or a combination of destination and source devices?

This was first published in January 2006
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