Dell POWERVAULT MD3600I - Manual (Page 116)

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 Disk Read/Write Properties
A snapshot virtual disk can be designated as either read-only or read-write.
The following are the differences between the two:
• Read-Only snapshot virtual disks provide the host read access to a copy of the data contained in the snapshot
image. However, the host cannot modify the snapshot image. A Read-Only snapshot virtual disk does require an
associated repository.
• Read-Write snapshot virtual disks require an associated repository to provide the host write access to a copy of
the data contained in the snapshot image. A Read-Write snapshot virtual disk requires its own repository to save
any subsequent modifications made by the host application to the base virtual disk without affecting the
referenced snapshot image. The snapshot is allocated from the storage pool from which the original snapshot
image is allocated. All I/O writes to the snapshot image are redirected to the snapshot virtual disk repository that
was allocated for saving data modifications. The data of the original snapshot image remains unchanged. For
more information, see Understanding Snapshot Repositories .
Snapshot Groups And Consistency Groups
The Snapshot Virtual Disk premium feature supports the following types of snapshot groups:
• Snapshot groups — A snapshot group is a collection of point-in-time images of a single associated base virtual
disk.
• Consistency groups — A consistency group is a group of virtual disks that you can manage as a single entity.
Operations performed on a consistency group are performed simultaneously on all virtual disks in the group.
Snapshot Groups
The purpose of a snapshot group is to create a sequence of snapshot images on a given base virtual disk without
impacting performance. You can set up a schedule for a snapshot group to automatically create a snapshot image at a
specific time in the future or on a regular basis.
When creating a snapshot group, the following rules apply:
• Snapshot groups can be created with or without snapshot images.
• Each snapshot image is can be a member of only one snapshot group.
• Standard virtual disks and thin virtual disks are the only types of virtual disks that can contain a snapshot group.
Non-standard virtual disks, such as snapshot virtual disks, cannot be used for snapshot groups.
• The base virtual disk can reside on either a disk group or a disk pool.
• Snapshot virtual disks and snapshot groups cannot exist on the same base virtual disk.
A snapshot group uses a repository to save all data for the snapshot images contained in the group. A snapshot image
operation uses less disk space than a full physical copy because the data stored in the repository is only the data that
has changed since the latest snapshot image.
A snapshot group is created initially with one repository virtual disk. The repository initially contains a small amount of
data, then increases over time with subsequent data updates. You can increase the size of the repository by increasing
the capacity of the repository, or add virtual disks to the repository.
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