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One of the prevalent trends in emerging large scale multi-tenant datacenters is network virtualization using overlays. Here we investigate application performance degradation in such an overlay applied to commodity 10 Gigabit Ethernet networks. We have adopted partition/aggregate as a representative commercial workload that today is deployed on bare metal servers and is notoriously sensitive to latency and TCP incast congestion. Using query completion time as the primary metric, we evaluate the degree to which a software-defined network (SDN) overlay impacts this application's behavior, the performance bounds of partition/aggregate with an SDN overlay, and whether active queue management (AQM) such as random early detection (RED) can benefit this environment. We introduce a generic SDN overlay framework, which we measure in hardware and simulate using a real TCP stack extracted from FreeBSD v9, running over a detailed Layer 2 commodity 10G Ethernet fabric network simulator. To further alleviate TCP incast congestion and support legacy congestion control, we propose an AQM translation scheme called v-RED. Finally, we report results concerning SDN's benefits in addressing TCP incast. Contrary to our expectations, we found that latency-sensitive applications do not necessarily suffer from performance degradation when deployed over SDN overlays.