Dell POWERVAULT MD3600I - Manual (Page 166)

the base virtual disk of an older (Legacy) snapshot virtual disk as your target virtual disk, you must first disable all
snapshot (Legacy) virtual disks that are associated with the base virtual disk.
Types Of Virtual Disk Copies
You can perform either offline or online virtual disk copies. To ensure data integrity, all I/O to the target virtual disk is
suspended during either type of virtual disk copy operation. After the virtual disk copy is complete, the target virtual disk
automatically becomes read-only to the hosts.
Offline Copy
An offline copy reads data from the source virtual disk and copies it to a target virtual disk, while suspending all updates
to the source virtual disk when the copy is in progress. In an offline virtual disk copy, the relationship is between a
source virtual disk and a target virtual disk. Source virtual disks that are participating in an offline copy are available for
read requests, while the virtual disk copy displays the In Progress or Pending status. Write requests are allowed only
after the offline copy is complete. If the source virtual disk is formatted with a journaling file system, any attempt to issue
a read request to the source virtual disk may be rejected by the storage array RAID controller modules and result in an
error message. Make sure that the Read-Only attribute for the target virtual disk is disabled after the virtual disk copy is
complete to prevent error messages from being displayed.
Online Copy
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.
NOTE: If the snapshot virtual disk that is used as the copy source is active, the source virtual disk performance
degrades due to copy-on-write operations. When the copy is complete, the snapshot is disabled and the source
virtual disk performance is restored. Although the snapshot is disabled, the repository infrastructure and copy
relationship remain intact.
Creating A Virtual Disk Copy For An MSCS Shared Disk
To create a virtual disk copy for a Microsoft Cluster Server (MSCS) shared disk, create a snapshot of the virtual disk,
and then use the snapshot virtual disk as the source for the virtual disk copy.
NOTE: An attempt to directly create a virtual disk copy for an MSCS shared disk, rather than using a snapshot
virtual disk, fails with the following error: The operation cannot complete because the selected virtual disk is not a
source virtual disk candidate.
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