Data locality is the basic architectural decision.
You will get the best storage performance in EC2 by localizing the data to be as close to the compute workload as is possible.
EC2 is divided into various zones. Compute, storage and support software can all be placed in pre-defined availability zones. Typically these reflect the timezone location of the data center, as well as a further subdivision into a physical instance of the data center within one region or time zone. Kdb+ will achieve the lowest latency and highest bandwidth in the network by using nodes and storage hosted in the same availability zone.