Why you want to choose the optimal SAN Extension for your Brocade SAN
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First of all: the following blog is about some SAN extension considerations related to Brocade SAN Switches. The described problems may affect other vendors as well but will not be discussed here. It will also not cover all sub topics and consideration but describes a specific problem.
There are a lot of different SAN extensions out there in the field and Brocade supports a considerable proportion of them. You can see them in the Brocade Compatibility Matrix in the "Network Solutions" section. As offsite replication is one of the key items of a good DR solution, I see many environments spread over multiple locations. If the data centers are near enough to avoid slower WAN connections usually multiplexers like CWDM, TDM or DWDM solutions are used to bring several connections on one long distance link.
From a SAN perspective these multiplexers are transparent or non-transparent. Transparent in this context means that:
While the first point is true for most of the solutions, the second point is the crux. With "everything" I mean all the traffic. Not only the frames, but also the ordered sets. So it should be really the same. Bit by bit by bit exactly the same. If the multiplexing solution can only guarantee the transfer of the frames it is non-transparent.
So how could that be a problem?
In most cases the long distance connection is an ISL (Inter Switch Link). An ISL does not only transport "user frames" (SCSI over FC frames from actual I/O between an initiator and a target) but also a lot of control primitives (the ordered sets) and administrative communication to maintain the fabric and distribute configuration changes. In addition there are techniques like Virtual Channels or QOS (Quality of service) to minimize the influence of different I/O types and techniques to maintain the link in a good condition like fillwords for synchronization or Credit Recovery. All these techniques rely on a transparent connection between the switches. If you don't have a transparent multiplexer, you have to ensure that these techniques are disabled and of course you can't benefit from their advantages. Problems start when you try to use them but your multiplexer doesn't meet the prerequirements.
What can happen?
You see, using the wrong (or at least "non-optimal") equipment can lead to severe problems. It's even more provoking the used multiplexer in fact is transparent but the wrong settings are used in the switches. So if you see such problems or other similar issues and you use a multiplexer on the affected paths, check if your multiplexer is transparent (with the matrix linked above) and if you use the correct configuration (refer to the FabOS Admin Guide). And if you have a non-transparent multiplexer and no possibility to get a transparent one, don't hesitate to contact your IBM sales rep and ask him about consultation on how to deal with situations like this (e.g. with traffic shaping / tuning, etc).