Administrator Guide

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.
Snapshot consistency groups
To perform the same snapshot image operations on multiple virtual disks, you can create a consistency group containing the virtual disks.
Any operation performed on the consistency group is performed simultaneously on all of the virtual disks in that group, which creates
consistent copies of data between each virtual disk. Consistency groups are commonly used to create, schedule, or rollback virtual disks.
Each virtual disk belonging to a consistency group is referred to as a member virtual disk. When you add a virtual disk to a consistency
group, the system automatically creates a new snapshot group that corresponds to this member virtual disk. You can set up a schedule for
a consistency group to automatically create a snapshot image of each member virtual disk in the group at a specific time in the future or
on a regular basis.
A consistency group, pools multiple virtual disks together, enabling you to take a snapshot of all virtual disks at the same point in time. This
creates a synchronized snapshot of all the virtual disks and is especially suitable for applications that span multiple virtual disks. For
example, a database application containing log files on one virtual disk and the database on another.
For consistency groups, the following applies:
• Consistency groups can be created initially with or without member virtual disks.
• Snapshot images can be created for a consistency group to enable consistent snapshot images between all member virtual disks.
• Consistency groups can be rolled back.
• A virtual disk can belong to multiple consistency groups.
• Only standard virtual disks and thin virtual disks can be included in a consistency group.
• A base virtual disk can reside on either a disk group or disk pool.
Understanding snapshot repositories
Repositories are system-created virtual disks used to hold write data for a snapshots, snapshot groups and consistency groups. During
creation of either group or write-enabled snapshot virtual disk, an associated repository is automatically created. By default, one individual
repository virtual disk is created for each group or snapshot. You can create the overall repository automatically using the default settings,
or you can manually create the repository by defining specific capacity settings.
A snapshot virtual disk allows the host access to a copy of the data contained in a snapshot image. A snapshot image is not directly read
or write accessible to the host and is used only to save data captured from the base virtual disk.
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Premium feature—Snapshot Virtual Disk