How far apart can SAN locations be?
Storage locations can potentially be very far apart, separated by thousands of miles, even around the globe. The real consideration in selecting distance is that of latency
Latency is the enemy of distance. If you're doing synchronous remote mirroring
-- copying data in real time with no loss of data -- you're striving for a recovery point objective
(RPO) and recovery time objective
(RTO) near zero. In synchronous mode, you need to keep distances short to minimize latency and use a network with enough bandwidth to handle your real-time data load. This all changes if you run in asynchronous mode. Since you're not operating in real time -- several minutes or even several hours of delay between locations -- latency is no longer a factor, allowing us to span hundreds or thousands of miles.
Latency can exist in a data center, a campus or a metropolitan area even where you have fast dedicated networks. If there is any network bottleneck or congestion, or anything that causes delays, delays are latency, and those work against distance. So, when you're looking at long distances, don't just consider bandwidth; also evaluate the effective latency and understand the application's sensitivity to that delay.
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This was first published in January 2007