This section summarizes the relationship between transaction volumes and policies, storage pools, device classes, and drives.
A policy is defined for each transaction volume. The policy defines an arbitrary storage pool to decide on the tapes to be used. In addition, a device class is defined for each storage pool. The device class defines the maximum number of drives that can be used concurrently.
In the following figure, seven transaction volumes and five drives are used. If backup is executed concurrently without defining the maximum number of drives that can be used concurrently, the backup operations by Server B may occupy four drives and Server A may be able to use only one drive.
The maximum number of drives that can be used simultaneously by Server A and Server B can be set using the device class.
In this example, Server A can use two drives simultaneously to backup three transaction volumes, and Server B can use three drives simultaneously to backup four transaction volumes. If the backup operations of all volumes are performed concurrently, the backup operation for any one of the transaction volumes of each server is made to wait for an idle drive. When an idle drive becomes available, writing to tape begins.
Figure 3.3 Relationships between transaction volumes and definitions
