Dell PowerVault MD3820f - Manual (Page 134)

• 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 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.
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