Dell PowerVault MD3820f - Manual (Page 136)

consistency group, the system automatically creates a new snapshot group that corresponds to this
member virtual disk. A consistency group repository must be created for each member virtual disk in a
consistency group in order to save data for all snapshot images in the group.
A consistency group snapshot image comprises multiple snapshot virtual disks. Its purpose is to provide
host access to a snapshot image that has been taken for each member virtual disk at the same moment
in time. A consistency group snapshot image is not directly read or write accessible to hosts; it is used
only to save the data captured from the base virtual disk. The consistency group snapshot virtual disk can
be designated as either read-only or read-write. Read-write consistency group snapshot virtual disks
require a repository for each member virtual disk in order to save any subsequent modifications made by
the host application to the base virtual disk without affecting the referenced snapshot image. Each
member repository is created when the consistency group snapshot virtual disk is created.
Ranking Repository Candidates
If you choose to create a repository manually, you can filter repository candidates for each member
virtual disk by selecting either a percentage of the base virtual disk capacity or by specifying a preferred
capacity in the Snapshot Group Settings window. Based on your selection, best repository candidates are
displayed. Repository candidates shown contain both new and existing repository virtual disks residing on
a disk group or disk pool.
For more information, see the online help topics.
Using Snapshot Consistency Groups With Remote Replication
Although a virtual disk can belong to multiple consistency groups, you must create separate consistency
groups for snapshot images and Remote Replication.
Adding a base virtual disk containing a consistency group to a Remote Replication (non-legacy,
asynchronous), the repository automatically purges the oldest snapshot image and sets the auto-delete
limit to the maximum allowable snapshot limit for a consistency group.
Additionally, all member virtual disks belonging to both a snapshot consistency group and a Remote
Replication group must belong to the same Remote Replication group.
Creating Snapshot Images
A snapshot image is a logical point-in-time image of the content of an associated base virtual disk. With
snapshot images, you can create multiple copies of production data on your storage array much more
quickly than full copies. Snapshot images track source changes from the time each snapshot image is
created. You can create snapshot images for the following storage objects:
• Standard virtual disks
• Thin virtual disks
• Consistency groups
Before creating a snapshot image, consider these guidelines:
• If you attempt to create a snapshot image on a snapshot group and that snapshot group has reached
its maximum number of snapshot images, you can retry creating snapshot images after doing one of
the following:
– Enable automatic deletion of snapshot images in the Advanced Options section of the Create
wizard.
– Manually delete one or more snapshot images from the snapshot group.
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