Administrator Guide

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.
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 by allowing both nodes to
concurrently access data.
Virtual disk read/write permissions
After the virtual disk copy is complete, the target virtual disk automatically becomes read-only to the hosts. The target virtual disk rejects
read and write requests while the virtual disk copy operation has a status of Pending or In Progress or if the operation fails before
completing the copy. Keep the target virtual disk Read-only enabled if you want to preserve the data on the target virtual disk for reasons
such as the following:
• If you are using the target virtual disk for backup purposes.
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Premium feature—virtual disk copy