With Release 1.5 of the TS7700 Virtualization Engine, by default, during device allocation for a specific (private) mount, the host will query the TS7700 to determine which cluster is the "best" cluster to direct the allocation. (This support is also referred to as device allocation assistance (DAA) or affinity list support). With this query, the TS7700 will return an ordered list of clusters with the preferred cluster being listed first. In doing so, the TS7700 looks at whether a valid copy exists in a cluster (in cache or on back-end tape) and other factors to determine the preferred cluster order. Device allocation will then take into account this ordered list of clusters "subsystems" when allocating a device. The allocation assist "affinity" function improves performance by reducing cross cluster mounts. This function is supported in a JES2-only environment at z/OS V1R8 and above with APAR OA24966, and in z/OS V2R1 with JES3 (refer to JES3 considerations for additional detail).
Starting with Release 1.7 of the TS7700 Virtualization Engine, this support can be disabled either by a tape hardware specialist dialing into the library or starting with Release 2.0 of the TS7700, through the MVS LIBRARY REQUEST command. Refer to Requesting information from the TS7700 Virtualization Engine for information on the LIBRARY REQUEST command. For additional information on device allocation assistance, refer to the IBM Knowledge Center for the TS7700.