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Tie your private loops to SAN fabric

A look at some products that will make the connection between the private FC-AL loop and the public SAN fabric.


Tie your private loops to SAN fabric
Rick Cook

While a SAN fabric offers considerable performance advantages over FC-AL, it is also considerably more expensive. In addition many enterprises have a considerable investment in FC-AL devices, especially disk systems, which cannot be upgraded to run on a fabric.

One way to preserve the existing investment in FC-AL is to tie existing FC-AL loops (private loops) to the SAN fabric. Products to make the connection between the private FC-AL loop and the public fabric are available from a number of vendors, including Vixel, Brocade and Qlogic. These products can also increase performance of the private loop.

Although all these products have the same aim they differ considerably in how they do it.

Typically, they all use a switch to tie the private loop to the fabric, often in place of the FC-AL hub. In addition to tying the fabric to the FC-AL loop, putting a switch in a private loop in place of a hub considerably increases the bandwidth available to the FC-AL devices. All of the products involve trade-offs between cost, flexibility and reliability. For that reason it is important to match the vendor's device to your needs.

A data sheet on Brocade's QuickLoop is available at

A white paper "What is Loop Switching?" discussing Vixel's approach is available on the Vixel web site.

Qlogic has a white paper titled "Reducing the Complexity of Managing Private Loops" which explains the company's approach on its web site.

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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