Dell POWERVAULT MD3620I - Manual (Page 181)

Configuration: Premium Feature—Virtual Disk Copy 181
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 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.
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