Dell PowerVault MD3000 - Manual (Page 69)

Snapshot Virtual Disks 69
7
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.
NOTE: The basic snapshot feature allows up to four snapshots to be present at the
same time. If the enhanced snapshot premium feature was ordered, then up to eight
snapshots may be present at the same time.
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 a copy of the original 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