Dell PowerVault MD3820f - Manual (Page 31)

repository, which is used to save subsequent modifications made by the host application to the base
virtual disk without affecting the referenced snapshot image.
Snapshot images can be created manually or automatically by establishing a schedule that defines the
date and time you want to create the snapshot image. The following objects can be included in a
snapshot image:
• Standard virtual disks
• Thin provisioned virtual disks
• Consistency groups
To create a snapshot image, you must first create a snapshot group and reserve snapshot repository
space for the virtual disk. The repository space is based on a percentage of the current virtual disk
reserve.
You can delete the oldest snapshot image in a snapshot group either manually or you can automate the
process by enabling the Auto-Delete setting for the snapshot group. When a snapshot image is deleted,
its definition is removed from the system, and the space occupied by the snapshot image in the
repository is released and made available for reuse within the snapshot group.
Snapshot Virtual Disks (Legacy)
A snapshot is a point-in-time image of a virtual disk. The snapshot provides an image of the virtual disk at
the time the snapshot was created. You create a snapshot so that an application (for example, a backup
application) can access the snapshot and read the data while the source virtual disk remains online and
user-accessible. When the backup is completed, the snapshot virtual disk is no longer needed. You can
create up to four snapshots per virtual disk.
Snapshots are used to recover previous versions of files that have changed since the snapshot was taken.
Snapshots are implemented using a copy on write algorithm, which makes a backup copy of data the
instant a write occurs to the virtual disk. Data on a virtual disk is copied to the snapshot repository before
it is modified. Snapshots are instantaneous and take up less overhead than a full physical copy process.
Snapshot (Legacy) Repository Virtual Disk
When you create a snapshot virtual disk, it automatically creates a snapshot repository virtual disk. A
snapshot repository is a virtual disk created in the storage array as a resource for a snapshot virtual disk. A
snapshot repository virtual disk contains snapshot virtual disk metadata and copy-on-write data for a
particular snapshot virtual disk. The repository supports one snapshot only.
You cannot select a snapshot repository virtual disk as a source virtual disk or as a target virtual disk in a
virtual disk copy. If you select a Snapshot source virtual disk as the target virtual disk of a virtual disk copy,
you must disable all snapshot virtual disks associated with the source virtual disk.
CAUTION: Before using the Snapshot Virtual Disks Premium Feature in a Windows Clustered
configuration, you must map the snapshot virtual disk to the cluster node that owns the source
virtual disk. This ensures that the cluster nodes correctly recognize the snapshot virtual disk.
Mapping the snapshot virtual disk to the node that does not own the source virtual disk before the
snapshot enabling process is completed can result in the operating system misidentifying the snapshot
virtual disk. This can result in data loss or an inaccessible snapshot.
Virtual Disk Copy
Virtual disk copy is a premium feature you can use to:
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