Dell POWERVAULT MD3600I - Manual (Page 117)

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 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
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