Dell PowerVault MD3820f - Manual (Page 186)

Using Virtual Disk Copy With Snapshot Or Snapshot
(Legacy) Premium Feature
After completion of the virtual disk copy of a snapshot (Legacy), the legacy snapshot is disabled. After
completion of the virtual disk copy using a snapshot image, the snapshot image is deleted and the
snapshot virtual disk is disabled.
Snapshots created using older (Legacy) premium feature versions cannot be managed using newer
snapshot premium feature options. Also, a virtual disk in a snapshot group cannot be a target for a virtual
disk copy. If you want to choose 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.
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