General Terms

We explore the nature of traffic in data centers, designed to sup-port the mining of massive data sets. We instrument the servers to collect socket-level logs, with negligible performance impact. In a server operational cluster, we thus amass roughly a petabyte of measurements over two months, from which we obtain and re-port detailed views of traffic and congestion conditions andpatterns. We further consider whether trafficmatrices in the clustermight be obtained instead via tomographic inference from coarser-grained counter data