Parametric Evaluation of Performance Behavior in Hierarchical Storage Architecture

In this article, we investigate the performance behavior of a hierarchical storage architecture. Considering both the size of video files and the skewed file access patterns over the available files, it is not cost-effective to store all video files on secondary storage such as disk arrays, since most users may access only a small subset of the available video files. Using tertiary storage, such as a tape library, is a promising solution for achieving the desired cost effectiveness. Though a hierarchical storage structure is desirable, an unbalanced configuration of the storage hierarchy may result in waste of resources and thus degrade overall server throughput. Our effort is focused on investigating the effect of the various system parameters to obtain an optimal configuration of the hierarchical storage. Throughput of the server is defined along with two orthogonal performance metrics, namely expected service time and system congestion and blocking probability. We establish an analytical formulation for the given performance metrics. Hierarchical storage performance is governed by several factors such as user-access pattern, number of tape drives, capacity of secondary storage, transfer rate of tape drives, and transfer rate of disks. It is observed that speeding up the rate of data transfer may in some cases increase the blocking probability and hence degrade overall server performance. We use a queuing model to obtain an analytical formulation of the server’s performance. This enables us to investigate the effects of different configurations of the storage hierarchy and different data access patterns; it also provides a framework to determine the optimal configuration of storage hierarchies. The results of a simulation-based performance evaluation are also presented.