On-line volume coalesce operation to enable on-line storage subsystem volume consolidation

A mechanism to permit consolidation of storage subsystem volumes into larger, more easily managed volumes and an operating system device driver which includes a trap mechanism for intercepting calls from a host into logical unit devices that were previously consolidated into a single physical volume. A map converts such calls to a logical unit device into a corresponding offset in the single physical volume. A driver accesses the single physical volume with corresponding offsets to transfer data associated with a particular logical unit device. So, logical unit devices can be consolidated and reconsolidated to single physical volumes and other physical volumes without requiring changes to the operating system or its application software and without requiring application or host downtime.