Dell PowerVault MD3420 - Manual (Page 56)

• Standard virtual disks
• Thin provisioned virtual disks
• Consistency groups
Snapshot Groups And Snapshot Consistency Groups
The Snapshot Virtual Disk premium feature supports two types of snapshot groups:
• Snapshot groups
• Consistency groups
Snapshot Groups
The purpose of a snapshot group is to create a sequence of snapshot images on a given base virtual disk
without impacting performance. You can set up a schedule for a snapshot group to automatically create
a snapshot image at a specific time in the future or on a regular basis.
The following rules apply when creating a snapshot group:
• Snapshot groups can be created with or without snapshot images.
• Each snapshot image is a member of only one snapshot group.
• Standard virtual disks and thin virtual disks are the only types of virtual disks that can contain a
snapshot group. Snapshot virtual disks cannot contain snapshot groups.
• The base virtual disk of a snapshot group can reside on either a disk group or a disk pool.
• 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 initially created with one repository virtual disk. The repository contains a small
amount of data, which increases over time with subsequent data updates. You can increase the size of
the repository by either increasing the capacity of the repository or adding 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 at a specific time and/or on a regular
basis.
This synchronized snapshot of all the virtual disks is especially suitable for applications that span multiple
virtual disks, such as a database application containing log files on one virtual disk and the database itself
on another.
The following rules apply when creating a consistency group:
• 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.
56