Dell PowerVault MD3820f - Manual (Page 173)

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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.
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