Dell PowerVault MD3420 - Manual (Page 57)

• 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.
• Snapshots created using the Snapshot Virtual Disk (Legacy) premium feature cannot 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 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
snapshot group or snapshot image. 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.
Snapshot Consistency Group Repositories
A snapshot consistency group is made up of simultaneous snapshots of multiple virtual disks. Each virtual
disk that belongs 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. A consistency group repository is also 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.
Consistency Groups And Remote Replication
Although a virtual disk can belong to multiple consistency groups, you must create separate consistency
groups for snapshot images and Remote Replication.
When a base virtual disk containing a consistency group is added to Remote Replication (non-legacy,
asynchronous), the repository will automatically purge the oldest snapshot image and set 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
Guidelines before creating a snapshot image:
• If you attempt to create a snapshot image on a snapshot group and that snapshot group has reached
its maximum number of snapshot images. The failBaseWrites or purgeSnapImages parameters
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