The selection algorithm orders the clusters in the following sequence: 

1. Clusters with the highest Copy Consistency Point. 

2. Clusters that have the volume already in cache. 

3. Clusters in the same cluster family. 

4. Clusters that have a valid copy on tape. 

5. Clusters without a valid copy. 

If the mount is directed to a cluster without a valid copy, a remote mount can be the result. Therefore, in special cases, remote mounts and recalls can still occur (even if DAA is enabled). 

Later, host processing attempts to allocate a device from the first cluster that is returned in the list. If an online non-active device is not available within that cluster, it moves to the next cluster in the list and tries again until a device is chosen. This process enables the host to direct the mount request to the cluster that results in the fastest mount, which is typically the cluster that has the logical volume resident in cache. 

DAA improves a grid’s performance by reducin g the number of cross-cluster mounts. This feature is important when copied volumes are treated as Preference Group 0 (removed from cache first), and when copies are not made between locally attached clusters of a common grid. With DAA, the use of the copy policy overrides to Prefer local TVC for Fast Ready mounts provides the best overall performance. 

Without DAA, configuring the cache management of replicated data as PG1 (prefer to be kept in cache with an LRU algorithm) is the best way to improve private (non-Fast Ready) mount performance by minimizing cross-cluster mounts for TS7700T/TS7700C configurations. However, this performance gain includes a reduction in the effective grid cache size because multiple clusters are maintaining a copy of a logical volume in disk cache. To regain the same level of effective grid cache size, an increase in physical cache capacity might be required. 

DAA (JES2) functions are included in z/OS V1R11 and later. DAA (JES3) is available starting with z/OS V2R1. 

## Scratch allocation assistance 

With the grid configuration, different clusters might have a benefit over others for certain workload types. For example, OAM or DFSMShsm Migration Level 2 (ML2) migration might favor a large TS7700D disk-only cluster over a smaller configured TS7700T/C cluster. Or, an archive workload might favor a TS7700T or TS7700C cluster so that the data can offload quickly to back-end tape or cloud. 

SAA functions extend the capabilities of DAA to the scratch mount requests. SAA filters the list of clusters in a grid to return to the host a smaller list of candidate clusters that are designated as scratch mount candidates. By identifying a subset of clusters in the grid as sole candidates for scratch mounts, SAA optimizes scratch mounts to a TS7700 grid. 

