Snapshot Virtual Disks 697Premium Feature—Snapshot VirtualDisksNOTE: If you ordered this feature, you received a Premium Feature Activation cardshipped in the same box as your Dell PowerVault MD storage array. Follow thedirections on the card to obtain a key file and to enable the feature.NOTE: The basic snapshot feature allows up to four snapshots to be present at thesame time. If the enhanced snapshot premium feature was ordered, then up to eightsnapshots may be present at the same time.A snapshot virtual disk is a point-in-time image of a virtual disk in a storagearray. It is not an actual virtual disk containing a copy of the original data;rather, it is a reference to the data that was contained on a virtual disk at aspecific time. A snapshot virtual disk is the logical equivalent of a completephysical copy. However, you can create a snapshot virtual disk much fasterthan 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 asnapshot so that an application, such as a backup application, can access thesnapshot and read the data while the source virtual disk remains online andaccessible.NOTE: No I/O requests are permitted on the source virtual disk while the virtualdisk snapshot is being created.A snapshot repository virtual disk containing metadata and copy-on-writedata is automatically created when a snapshot virtual disk is created. The onlydata stored in the snapshot repository virtual disk is that which has changedsince the time of the snapshot.After the snapshot repository virtual disk is created, I/O write requests to thesource virtual disk resume. Before a data block on the source virtual disk ismodified, however, the contents of the block to be modified are copied to thesnapshot repository virtual disk for safekeeping. Because the snapshotrepository 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
