Distance benchmarks for data replication

User question: I currently have two EMC Symmetrix systems about 200 miles apart. When we tried to do synchronous replication the production boxes were frozen. A few quick calls around suggested that there

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are distance limitations of around 100 Kilometers for the amount of bandwidth. We are trying to do this over a T3. Is this true?

Answer: Is it true? No.

Until recently, Fibre Channel fabrics connected by dark fiber, T3, and/or DWDM (dense wave division multiplexing) could only span metropolitan distances, typically well under 100 miles. Today, new IP SAN technologies have abolished those limits, with demonstrated connectivity of thousands of miles.

Keep in mind, however, that DR applications such as synchronous data replication are sensitive to latency, which is tied directly to distance. Speed of light propagation dictates about one millisecond of latency for every 100 miles. A thousand mile span between primary and DR sites, for example, would inject roughly 10 milliseconds of latency each way, or 20 milliseconds latency round trip. For synchronous applications, vendors typically recommend a maximum of 150-200 miles between sites as a maximum based on the performance impact incurred. In practice, I have customers who have pushed synchronous data replication more than twice that distance.

Why not use asynchronous data replication. Asynchronous data replication, in contrast, is highly tolerant of latency and can be driven across thousands of miles. It also will significantly reduce your bandwidth requirements versus traditional synchronous solutions.

For more information:

Expert Advice: Replicating data overseas for reliability, availability

Tip: Seven steps to data replication

Tip: The truth about synchronous replication

About the author: Brendan Reilly is a 15 year storage industry veteran and currently serves as the Chief Technology Officer and the Vice President of Consulting Services for SANZ Incorporated, a publicly traded storage consulting and integration firm based in Castle Rock, Colorado. He is also SearchStorage.com's storage hardware expert.

This was first published in March 2004

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