Types Of Virtual Disk CopiesYou can perform either offline or online virtual disk copies. To ensure data integrity, all I/O to the target virtual disk issuspended during either type of virtual disk copy operation. After the virtual disk copy is complete, the target virtual diskautomatically becomes read-only to the hosts.Offline CopyAn offline copy reads data from the source virtual disk and copies it to a target virtual disk, while suspending all updatesto the source virtual disk when the copy is in progress. In an offline virtual disk copy, the relationship is between asource virtual disk and a target virtual disk. Source virtual disks that are participating in an offline copy are available forread requests, while the virtual disk copy displays the In Progress or Pending status. Write requests are allowed onlyafter the offline copy is complete. If the source virtual disk is formatted with a journaling file system, any attempt to issuea read request to the source virtual disk may be rejected by the storage array RAID controller modules and result in anerror message. Make sure that the Read-Only attribute for the target virtual disk is disabled after the virtual disk copy iscomplete to prevent error messages from being displayed.Online CopyAn online copy creates a point-in-time snapshot copy of any virtual disk within a storage array, while still allowing writesto the virtual disk when the copy is in progress. This is achieved by creating a snapshot of the virtual disk and using thatsnapshot as the actual source virtual disk for the copy. In an online virtual disk copy, the relationship is between asnapshot virtual disk and a target virtual disk. The virtual disk for which the point-in-time image is created (the sourcevirtual disk) must be a standard virtual disk in the storage array.A snapshot virtual disk and a snapshot repository virtual disk are created during the online copy operation. The snapshotvirtual disk is not an actual virtual disk containing data; instead, it is a reference to the data contained on the virtual diskat a specific time. For each snapshot taken, a snapshot repository virtual disk is created to hold the copy-on-write datafor 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 thesnapshot repository virtual disk. Because the snapshot repository virtual disk stores copies of the original data in thosedata 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 performancedegrades due to copy-on-write operations. When the copy is complete, the snapshot is disabled and the sourcevirtual disk performance is restored. Although the snapshot is disabled, the repository infrastructure and copyrelationship remain intact.Creating A Virtual Disk Copy For An MSCS Shared DiskTo 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 snapshotvirtual disk, fails with the following error: The operation cannot complete because the selected virtual disk is not asource virtual disk candidate.NOTE: When creating a snapshot virtual disk, map the snapshot virtual disk to only one node in the cluster.Mapping the snapshot virtual disk to the host group or both nodes in the cluster may cause data corruption byallowing both nodes to concurrently access data.110
