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Chapter 6: Using the Snapshot Images Premium Feature 105
A consistency group pools several volumes together so that you can take a snapshot of
all the volumes at the same point in time. This action creates a synchronized snapshot
of all the volumes and is ideal for applications that span several volumes, for example,
a database application that has the logs on one volume and the database on another
volume.
You can use either of the following two methods to create a consistency group:

Create the consistency group, and add the member volumes in one step.

Create the consistency group, and then add the member volumes in a subsequent
step.
Standard volumes and thin volumes are the only type of volumes that can be used for
a consistency group. Non-standard volumes, such as snapshot (legacy) volumes,
cannot be used for consistency groups. The base volume can reside on either a volume
group or a disk pool.
Synchronous Mirroring and Snapshot Consistency Groups

A volume can belong to several consistency groups. You can use consistency
groups for snapshots and Synchronous Mirroring. Make sure to define separate
and specific consistency groups for snapshots and Synchronous Mirroring.

When you add a base volume that contains a consistency group to an
asynchronous mirror group, the system automatically changes the repository full
policy to automatically purge the oldest snapshot image and sets the auto-delete
limit to the maximum allowable snapshot limit for a consistency group.

All member volumes in a consistency group that also belong to an asynchronous
mirror group, must belong to the same asynchronous mirror group.
Characteristics of Consistency Groups
A consistency group has the following characteristics:

You can initially create consistency groups with or without member volumes.

Depending on your configuration, a consistency group has a maximum allowable
number of volumes.

Depending on your configuration, a consistency group has a maximum allowable
number of snapshot images.

You can schedule a snapshot image to be created for a consistency group, which
causes consistent snapshot images to be created for all member volumes.

You can perform a Rollback operation for a consistency group.