Dell PowerVault MD3000 - Manual (Page 67)

Snapshot Virtual Disks
67
Premium Feature—Snapshot Virtual
Disks
NOTE:
If you ordered this feature, you received a Premium Feature Activation card
shipped in the same box as your Dell PowerVault MD storage array. Follow the
directions on the card to obtain a key file and to enable the feature.
A snapshot virtual disk is a point-in-time image of a virtual disk in a storage
array. It is not an actual virtual disk containing data; rather, it is a reference to
the data that was contained on a virtual disk at a specific time. A snapshot
virtual disk is the logical equivalent of a complete physical copy. However, you
can create a snapshot virtual disk much faster than a physical copy, using less
disk space.
The virtual disk on which the snapshot is based, called the source virtual disk,
must be a standard virtual disk in your storage array. Typically, you create a
snapshot so that an application, such as a backup application, can access the
snapshot and read the data while the source virtual disk remains online and
accessible.
NOTE:
No I/O requests are permitted on the source virtual disk while the virtual
disk snapshot is being created.
A snapshot repository virtual disk containing metadata and copy-on-write
data is automatically created when a snapshot virtual disk is created. The only
data stored in the snapshot repository virtual disk is that which has changed
since the time of the snapshot.
After the snapshot repository virtual disk is created, I/O write requests to the
source virtual disk resume. Before a data block on the source virtual disk is
modified, however, the contents of the block to be modified are copied to the
snapshot repository virtual disk for safekeeping. 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. The
snapshot repository uses less disk space than a full physical copy, because the
only data blocks that are stored in the snapshot repository virtual disk are
those that have changed since the time of the snapshot.