As computer networks rapid ly increase in size and speed, Internet-distributed systems such as P2P, volun- teer computing, and Grid systems are increasingly com- mon. A precise and accurate characterization of Inter- net resources is important for the design and evalua- tion of such Internet-distributed systems, yet our pic- ture of the Internet landscape is not perfectly clear. To improve this picture, we measure and characterize the time dynamics of availability in a large-scale Internet- distributed system with over 110,000 hosts. Our char- acterization focuses on identifying patterns of corre- lated availability. We determine scalable and accurate clustering techniques and distance metrics for auto- matical ly detecting significant availability patterns. By means of clustering, we identify groups of resources with correlated availability that exhibit similar time ef- fects. Then we show how these correlated clusters of resources can be used to improve resource management for paral lel applications in the context of volunteer com- puting.