Dell POWERVAULT MD3600I - Manual (Page 153)

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Premium Feature—Snapshot Virtual Disks
(Legacy)
The following types of virtual disk snapshot premium features are supported on the MD storage array:
• Snapshot Virtual Disks using multiple point-in-time (PiT) groups
• Snapshot Virtual Disks (Legacy) using a separate repository for each snapshot
NOTE: This section describes the Snapshot Virtual Disk (Legacy) premium feature.
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 snapshot feature allows up to eight snapshots per LUN and 128 per array to 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; 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, 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.
When you create a snapshot virtual disk, you specify its location, capacity, and other parameters. You can disable or
delete the snapshot virtual disk when it is not required. If you disable a snapshot virtual disk, you can re-create and
reuse it the next time you perform a backup. If you delete a snapshot virtual disk, you also delete the associated
snapshot repository virtual disk.
NOTE: If the Source Virtual Disk is in the offline state, the corresponding Snapshot(s) Repository(ies) and
Snapshot(s) Virtual Disk(s) will be in Failed state.
NOTE: Deleting a snapshot does not affect data on the source virtual disk.
NOTE: The following host preparation sections also apply when using the snapshot feature through the CLI
interface.
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