This paper presents work on scaleable design for the automated synchronization of large collections of files replicated across multiple hosts. Unlike conventional mirroring tools, our approach addresses multiple-site file synchronization by capturing file-tree update information in an output file during an initial file synchronization session. Once the update file is available, it can be transmitted over the network using parallel point-to-point file transfers or reliable multicasting and then processed at the remote sites. This paper outlines of how the above concept has been implemented as a modification to the open-source mirroring tool, rsync. It then presents performance experiments designed to characterize the server-side processing costs and network throughput requirements under realistic workloads on large storage servers. The performance experiments use the WAN testbed of the Internet2 Distributed Storage Infrastructure (I2-DSI) project, the context for this work, and the results provide guidance on the scalability of I2-DSI using the proposed data mirroring scheme. Keywords: mirroring, distributed storage, replication, Internet2, reliable multicast, I2-DSI. 1.