Dell PowerVault MD3260i - Manual (Page 51)

• 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 used with
the create snapGroup command allows you to choose to either fail the write attempt or automatically
purge a specified number of older snapshot images.
create snapGroup
userLabel="snapGroupName"
sourceVirtualDisk="virtualDiskName"
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