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Data centers and synchronous replication

SAN/NAS expert Greg Schulz answers a reader's question regarding synchronous replication for data centers.

I am planning to connect two data centers by dense wavelength division multiplexing (DWDM), and the distance between them is 70 kilometers (km). I am also planning to buy an EMC Symmetrix box and would like to have synchronous replication for 20% of my applications. Will the distance be an issue for synchronous replication?
In theory, it is possible to span distances of 70 km when using DWDM over a dark or dedicated fiber optic cable/circuit or lambda service. However, there are many caveats. First, your focus on latency vs. bandwidth is a good start and there are a few things you might want to check on. Decibel loss is the amount of optical signal strength degradation over distance due to a weak light source, or light traveling over cable with many connections. What is the decibel loss and actual distance spanned by your fiber optic cable? Your two sites may be 70 km apart, however the fiber optic path may be longer. Likewise, if there are many breaks and connections, you may incur decibel loss which degrades latency over distance.

Your actual bandwidth or optical carrier provider can attach test equipment and tell you the actual distance as well as decibel loss of the cable. Depending on the amount of data being moved, how much it changes and other factors, it is possible that you could do the synchronous replication. However, check with EMC or DWDM (e.g. ADVA, Ciena, Cisco, Nortel, among others) suppliers to get a better assessment. Also, make sure that what you are getting for bandwidth is actually a full lambda circuit and not some variation of a slower optical bandwidth service.

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