Configuration: Premium Feature—Snapshot Virtual Disks 14911Configuration: Premium Feature—Snapshot Virtual DisksNOTE: 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 snapshot feature allows up to 16 snapshots per LUN and 256 per arrayto 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; it isa 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 physicalcopy, 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, the contents of the block to be modified are copied to the snapshotrepository virtual disk for safekeeping. Because the snapshot repository virtualdisk stores copies of the original data in those data blocks, further changes tothose data blocks write only to the source virtual disk. The snapshot
