Dell EMC PowerVault MD 34XX/38XX Series Storage Arrays Administrator's Guide

An online copy creates a point-in-time snapshot copy of any virtual disk within a storage array, while still allowing writes to the virtual disk when the copy is in progress. This is achieved by creating a snapshot of the virtual disk and using that snapshot as the actual source virtual disk for the copy. In an online virtual disk copy, the relationship is between a snapshot virtual disk and a target virtual disk. The virtual disk for which the point-in-time image is created (the source virtual disk) must be a standard virtual or thin disk in the storage array.

A snapshot virtual disk and a snapshot repository virtual disk are created during the online copy operation. The snapshot virtual disk is not an actual virtual disk containing data; instead, it is a reference to the data contained on the virtual disk at a specific time. For each snapshot taken, a snapshot repository virtual disk is created to hold the copy-on-write data for the snapshot. The snapshot repository virtual disk is used only to manage the snapshot image.

Before a data block on the source virtual disk is modified, the contents of the block to be modified are copied to the snapshot repository virtual disk. Because the snapshot repository virtual disk stores copies of the original data in those data blocks, further changes to those data blocks write only to the source virtual disk.