Dell POWERVAULT MD3600I - Manual (Page 115)

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Premium Feature—Snapshot Virtual Disk
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 using PiT groups. If you are using the Snapshot Virtual Disk
(Legacy) premium feature, see
Premium Feature—Snapshot Virtual Disks (Legacy) .
A snapshot image is a logical image of the content of an associated base virtual disk created at a specific point-in-time,
often known as a
restore point . This type of image is not directly readable or writable to a host since the snapshot image
is used to save data from the base virtual disk only. To allow the host to access a copy of the data in a snapshot image,
you must create a snapshot virtual disk. This snapshot virtual disk contains its own 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 Virtual Disk Vs. Snapshot Virtual Disk (Legacy)
Any snapshot virtual disk image is a logical point-in-time image of a virtual disk. However, the Snapshot Virtual Disk
premium feature differs from the Snapshot Virtual Disk (Legacy) premium feature in the following ways:
• The Snapshot Virtual Disk (Legacy) premium feature uses one data repository for each snapshot. The non-
legacy Snapshot Virtual Disk premium feature uses one data repository for all snapshot images associated with
a base virtual disk. As a result, only one write operation per virtual disk is required (not multiple sequential write
operations).
• Snapshot Virtual Disk adds the concept of a snapshot group. Because there is only one repository for multiple
snapshot images, the repository is associated with the snapshot group instead of with the snapshot image.
• Unlike a Snapshot Virtual Disk (Legacy), a snapshot image created with the Snapshot Virtual Disk premium
feature is not directly read/write accessible by a host since the snapshot image is used only to save changed
data for a base virtual disk.
• Either Snapshot Virtual Disk or Snapshot Virtual Disk (Legacy) images can be created from a base virtual disk,
but not both.
• A snapshot image created using Snapshot Virtual Disk is created almost instantaneously and initially uses no
disk space.
• Snapshot Virtual Disk images are always created inside snapshot groups; each snapshot image is associated
with only one snapshot group.
Snapshot Images And Groups
Snapshot images can be created manually or automatically by establishing a schedule that defines the date and time
you want the snapshot image created. The following objects can be included in a snapshot image:
• Standard virtual disks
• Thin provisioned virtual disks
• Consistency groups
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