Planning for a SAN on a budget
Wouldn't it be great if we had all the cash that we need for everything? But we don't, the world not being designed for our pleasure. So it may be that you find yourself stuck with implementing a SAN that is less than ideal in terms of your capacity and bandwidth needs. What to do? Well, this tip offers some pretty easy-to-implement schemes that will minimize the negative impact of dealing with less than optimal budget.
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Ideally a SAN should have enough capacity designed in from the beginning. But capacity, in the form of high-bandwidth links and high-performance devices, is expensive. In this less-than-ideal world, storage administrators often need to stay within a budget by getting by with less bandwidth or performance or re-using legacy devices. Getting the maximum performance out of a SAN in these circumstances is a balancing act.
Lower performing legacy equipment such as 1-Gb switches on a 2-Gb network should be placed on a separate path as much as possible. Putting a 1-Gb switch between a pair of 2 Gb switches limits that path's performance to 1 Gb. So if possible, the lower capacity links should be used for less critical paths or used as redundant links for higher capacity paths.
Likewise, the tradeoff between device performance and number of devices should be considered carefully. Speed is important in a SAN, but so is avoiding single points of failure and maintaining redundancy. Often it makes more sense to purchase two lower-performing devices to insure redundancy at lower throughput than to have a single high-performance device whose failure would take down the entire SAN.
SANavigator (www.sanavigator.com) discusses this and other SAN performance issues in a white paper titled "Optimizing SAN Performance" which is available on the company 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.
This was first published in October 2001