Dell PowerVault MD3620f Command Line Interface Manual page 92

Creating a snapshot virtual disk automatically creates a snapshot repository
virtual disk. The snapshot repository virtual disk stores information about the
data that has changed since the snapshot virtual disk was created. Snapshot
repository virtual disks cannot be selected as a source virtual disk or target
virtual disk in a virtual disk copy.
The virtual disk for which the point-in-time image is created is the source
virtual disk and must be a standard virtual disk in the storage array.
You can select snapshot virtual disks as the source virtual disk for a virtual disk
copy. Selecting a snapshot virtual disk is a good use of this feature, because it
enables complete backups without significant impact to the storage array I/O.
However, some I/O processing resources are lost to the copy operation.
The Snapshot Virtual Disk feature can be used with the Virtual Disk Copy
feature to back up data on the same storage array and to restore the data on the
snapshot virtual disk back to its original source virtual disk.
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Using the Virtual Disk Copy Feature