8464ch12_Performance.fm Draft Document for Review February 7, 2020 4:43 pm 12.5 Considerations for Virtual Device Allocation 12.6 Cache throughput and cache bandwidth In TS7700 GRID and z/OS environment, device allocation assistance (DAA) can be used in  private (specific) mount to select a preferred cluster virtual device in grid, which maximizes  TVC cache hit and optimizes performance and workload by minimizing remote mounts and  remote cache accesses. When DAA is enabled (default setting), in the grid configuration with TS7700T and/or  TS7700C the virtual device is selected preferentially at the cluster where the volume already  cached in the TVC. Conversely, when DAA is disabled, z/OS randomly selects a virtual  device. It may cause a cache miss on TVC and need longer time to access data by recalling  from tape or cloud or by remote-mounted if other cluster have the data in the TVC. If you have  TS7700T or TS7700C in the grid configuration, DAA function prevents cache miss and leads  maximizing performance. For more information about DAA, see 2.4.15, “Allocation assistance” on page 73. The TS7700 cache features a finite bandwidth, depending on your actual environment  (number of installed cache drawers and models). Other influences are compression ratio and  blocksizes that are used. For example, The difference in cache bandwidth (depending on the  number of cache drawers that are installed and the compression method that is used) is  shown in Figure 12-7. Figure 12-7 Performance versus Drawer Counts 718  IBM TS7700 Release 5.0 Guide