ABSTRACT: Los Alamos and Sandia National Laboratories have formed the Alliance for Computing at the Extreme Scale (ACES). The two labs will jointly architect, develop, procure and operate capability systems for the DOE’s Advanced Simulation and Computing Program—in support of national security. The first of these systems is Cielo. It’s a peta-scale system built by Cray and deployed at the LANL computing facility. One of the key requirements is the integration of the Cray cluster with LANL’s lane-oriented Parallel Scalable backbone (PaSCalBB) Ethernet network. PaScalBB provides a storage backbone that connects several compute clusters with storage implemented by the Panasas parallel file system. The scale of Cielo motivates the use of I/O forwarding from compute nodes through I/O nodes, but naïve approaches lead to inefficient use of the asymmetric PaScalBB network. This project introduces topology aware striping to the I/O forwarding layer to provide high storage throughput to Cielo.