Section IV: Data Migration

Introduction

Storage system stores data objects on different storage devices. When these storage devices are of the same type, the objects may be stored and retrieved with similar access latency. When these storage devices are of different types, the objects may be stored and retrieved with different access latencies. Thus, the type of storage devices that contain the stored object affects the access latency in an accessed stored object.

A common method to arrange the storage devices of different types is the hierarchical storage systems (HSS). All or most objects are stored on the storage devices with longer access latency. When these data objects are accessed, the objects are moved from these storage devices with longer access latency to the storage devices with shorter access latency. This is called data migration. Similar to the chapters on disk scheduling of Part III, data migration on HSS also improves the performance of HSS, especially at the response time of request streams.