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Cascaded SAN switch architectures

A discussion of the simplest of the SAN switch connection schemes

Cascaded SAN switch architectures
Rick Cook

How should you connect your SAN? There are a variety of ways, and this tip discusses one of them, one with medium complexity that may be suitable for your smaller installations. Further tips on this subject will be forthcoming.

There are four major ways of connecting switches together in a SAN, according to Bill Peldzus manager of storage consulting marketing at Imation's Storage Professional Services Group, each of them with its own advantages and drawbacks

The simplest method, Peldzus says, is to cascade the switches. That is, for example, switch A is connected to switch B and switch B is connected to switch C, like plugging one string of Christmas lights into another. "If you're not familiar with how a fabric works it sounds good," Peldzus says. The problem is it's not reliable, not scalable and it doesn't perform well. As a result it is seldom used in a well-designed SAN.

The reliability problem is simple. If one switch fails, it interrupts the flow to the switches on either side, effectively cutting the SAN into pieces. The scalability problem arises because so much of the traffic flows through all the switches, which limits the total amount of traffic the network can handle. Performance suffers because the flow of data from storage to the server and back goes through multiple hops as it is handed from switch to switch. Best practice is to minimize the number of switches the data passes through, Peldzus says.

A related design mistake, Peldzus says, is to put all the servers in the SAN on one switch and all the storage on another. This makes for a clean implementation conceptually, he says, but it means multiple hops for data, which cuts performance.

Additional resources:
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* Or try to stump one of our storage networking experts. Post your specific question, and view other Q&As.

* And who is SAM? Check out this article "Storage area management speeds application allocation." Scotts Valley, CA-based InterSAN created storage area management (SAM) to make it easier to allocate space on a server for a new application and store it without mishap. Find out how SAM works and why enterprises may need it in this searchWindowsManageability interview with Karen Dutch, InterSAN's vice president of marketing and Jules Myklebust, director of product management.

About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.

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