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Figure 2-13 shows a four-cluster grid where Cluster 0 replicates to Cluster 2, and Cluster 1 replicates to Cluster 3. The goal is that only two copies of data remain in the grid after the volume is accessed. One of the copies is local and one of the copies is remote. Later, when a host wants to mount the volume that is written to Cluster 0. On systems where DAA is supported, DAA is used to determine which cluster is the best cluster from which to request the mount. DAA asks the grid from which cluster to allocate a virtual drive. The host then attempts to allocate a device from the best cluster (Cluster 0). 

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Remember:  DAA support for JES3 was added in z/OS V2R1. 

On systems where DAA is not supported, the host allocates 50% of the time to the cluster that does not have a copy in its cache. When the alternative cluster is chosen, the copies remain present, and more copies are made to the new Copy Consistency Points that are defined in the Management Class, which results in more copies. 

If host allocation selects the cluster that does not have the volume in cache, one or two extra copies are created on Cluster 1 and Cluster 3 because the Copy Consistency Points indicate that the copies must be made to Cluster 1 and Cluster 3. 

