Volume shadowing performs four basic functions. The two most important, as with any disk I/O subsystem, are to satisfy read and write requests. The other two functions, copy and merge, are required for shadow set maintenance.

Copy and merge operations are the cornerstone of achieving data availability. Under certain circumstances, Volume Shadowing for OpenVMS must perform a copy or a merge operation to ensure that corresponding LBNs on all shadow set members contain the same information. Although volume shadowing automatically performs these operations, this chapter provides an overview of their operation.

Copy and merge operations occur at the same time that applications and user processes read and write to active shadow set members, thereby having a minimal effect on current application processing.