Access Cost Estimation for Unified Grid Storage Systems

In typical Data Grids large amounts of replicated data are stored all over the globe in different storage systems with access latencies ranging from seconds to hours. The task of a replica management system is not only to keep track of the replicas but also to select those replicas that can be accessed by an application program with a minimal response or transfer time. Most wide-area replication research focuses on network-based replica selection. However, our past experience with Data Grids has shown that often hierarchical storage systems are the main bottleneck rather than network links. This is due to the fact that access latencies of hierarchical storage systems can be of the order of seconds up to hours in case the data resides on a tape that is not mounted yet. In this paper we give an overview of our replica management framework called Reptor and a storage system cost estimator that is used. Furthermore, we give details on access estimation of file replicas that reside on hierarchical storage systems. The results show that the access estimates provide a good basis for a replica management system to perform efficient replica selection.