Draft Document for Review February 7, 2020 3:15 pm 8464ch02_Architecture.fm The selection algorithm orders the clusters in the following sequence: 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.  74  IBM TS7700 Release 5.0 Guide 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.