Dense wavelength division multiplexing for disaster recovery


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Dealing with distance

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Something to keep in mind as we delve deeper into this solution space is the distance in miles between the sites supporting your applications. The latency associated with the speed of light over an optical cable equates to a single data or management frame taking one millisecond per 100 miles. To address this latency, switch vendors recommend that you allocate more frame buffers at both endpoints of the long-distance storage area network (SAN) link. Increasing the frame buffers at both ends will allow the transmitting E_Port to have more frames in flight without waiting for an acknowledgment (ACK) from the receiving E_Port for having received previously sent frames.

How many buffers should your E_Port support? Well, it depends on two things. For one, it depends on whether the application in question is synchronous or asynchronous in nature. The more synchronous, the more buffers you will need. It also depends on the speed and distance among connecting E_Ports. The greater the speed and distance, the more buffer credits you will need to keep frames flowing in opposite directions without stalling.

It's been two years now since the attacks on the World Trade Center and the Pentagon, and the disaster recovery considerations for storage area networks (SANs) have changed tremendously--for the better.

Back then, SAN islands were expensive, yet gaining in popularity because of the many benefits they yielded in scalability and performance, and because of the distances that could be placed between attached storage enclosures and their respective application servers. Still, only the most lucrative companies--such as large financial and insurance institutions--could afford to put space between redundant pieces of their storage infrastructures.

Since then, hardware vendors have slashed prices, new protocols have been developed and standardized and the intricacies of long-distance SAN links are better understood. Now you have the opportunity to determine which solution will best ensure that when your storage islands are bridged together (for enhanced management and business continuity), you don't compromise day-to-day results for something (disaster recovery) we all agree is an exception to our normal business processes.

This article looks at how to do that analysis. We'll also begin looking at solutions by exploring dense wavelength division multiplexing (DWDM), the most comprehensive and flexible solution. In the October issue of Storage, we'll look at more focused and less-costly alternatives.

The effect on the U.S. economy that the Sept. 11 attacks and the recent accounting scandals has had is staggering, to say the least. As a result, capital expenditures in many shops were almost nonexistent, and IT managers have been walking around with their hands in their pockets to signify how money-conscious they have become. For example, at a recent presales technical engagement I participated in, the person in charge of spending IT budget dollars walked into the meeting with the lining of her pockets inside out, turned her pocketbook upside down and rolled out a quarter. At this very moment, I remembered why I stay on the technical side of the solution. My job--both before and after the budget director's presentation--was simply to present the facts.

Yet while most IT organizations still haven't rolled out a significant amount of SAN hardware, some larger organizations haven't stopped scaling their SANs since their initial implementation. Department after department has heard of the gains experienced by application owners down the hall and across the country who moved their applications onto a SAN. Realizing the benefit, those on the sidelines quickly scraped up the capital to do the same for their applications.

At the same time, many IT managers are being asked to stretch the resiliency of their data center applications beyond the scope of human and natural disasters. Most organizations need to coordinate their strategies for growth and redundancy when it comes to aggregating the management of many regional or national SAN islands or extending the data center. Selecting the right long-distance SAN link between E_Ports will ultimately determine the success of the project. If you're conducting a SAN assessment, include this issue in your considerations.

This was first published in September 2003

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