Dell PowerVault MD3000 - Manual (Page 89)

Using the Virtual Disk Copy Feature
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After you create a virtual disk copy, the target virtual disk automatically
becomes read-only to hosts to ensure that the data is preserved. Hosts that
have been mapped to a target virtual disk do not have write access to the
virtual disk, and any attempt to write to the read-only target virtual disk
results in a host I/O error.
If you want hosts to have write access to the data on the target virtual disk,
use the set virtualDiskCopy command to disable the read-only attribute for
the target virtual disk.
Snapshot Virtual Disks
A snapshot virtual disk is a point-in-time image of a virtual disk. It is typically
created so that an application, such as a backup, can access the snapshot
virtual disk and read the data while the source virtual disk remains online and
accessible to hosts.
NOTICE:
Before using the source virtual disk of a snapshot virtual disk as your
target, you must disable all snapshot virtual disks associated with the source virtual
disk. By disabling the snapshot virtual disks, you avoid altering the snapshot data if
the source virtual disk is changed.
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.