While the cost per megabyte of magnetic disk storage is economical, organizations are alarmed by the increasing cost of managing storage. Storage Area Network (SAN) architectures strive to minimize this cost by consolidating storage devices. A SAN is a special-purpose network that interconnects different data storage devices with servers. While there are many definitions for a SAN, there is a general consensus that it provides access at the granularity of a block and is typically used for database applications.
In this study, we focus on SAN switches that include an embedded storage management software in support of virtualization. We describe an On-line Re-organization Environment, ORE, that controls the placement of data to improve the average response time of the system. ORE is designed for a heterogeneous collection of storage devices. Its key novel feature is its use of “time” to quantify the benefit and cost of a migration. It migrates a fragment only when its net benefit exceeds a pre-specified threshold. We describe a taxonomy of techniques for fragment migration and employ a trace driven simulation study to quantify their tradeoff. Our performance results demonstrate a significant improvement in response time (order of magnitude) for those algorithms that employ ORE’s cost/benefit feature. Moreover, a technique that employs bandwidth of all devices intelligently is superior to one that simply migrates data to the fastest devices.