Draft Document for Review February 7, 2020 3:15 pm 8464ch02_Architecture.fm 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).  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.  Chapter 2. Architecture, components, and functional characteristics  87 Remember:  DAA support for JES3 was added in z/OS V2R1. Figure 2-13 Four-cluster grid with DAA WAN TS7700 Copy Mode DNDNS TS7700 Copy Mode NDNDS TS7700 TS7700 HA Production Site HA DR Site Host 0 1 2 3