Dell PowerVault MD3820f - Manual (Page 135)

• 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 apples:
• 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.
• 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 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.
Consistency Group Repositories
A 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
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