8464ch03_business requirements.fm Draft Document for Review February 7, 2020 3:15 pm 3.3.5 Synchronous mode copy  3.3.6 Override policies 3.3.7 Cluster family Synchronous mode copy creates a copy of the data whenever an explicit or implicit sync point  is written from an application. This enables a much more granular copy than all other  consistency points, such as Run or Deferred.  This consistency point is ideal for applications that move primary data to tape, such as  DFSMShsm or OAM Object Support, which can remove the primary instance in DASD after  issuing an explicit sync point. Therefore, you should use Synchronous mode copy for this type of applications.  The synchronous mode copy offers three options for how to handle private mounts:  GLYPH<SM590000> Always open both instances on private mount. GLYPH<SM590000> Open only one instance on private mount. GLYPH<SM590000> Open both instances on z/OS implied update. Plan the usage of this option. Dual open is necessary for workloads that can append to  existing tapes. When only reads are taking place, the dual open can introduce unnecessary  resource use, especially when one of the instances requires a recall from a physical tape.  Using the dual open z/OS implied update helps reduce any resource use to only those  mounts where an update is likely to take place.  In addition, synchronous mode copy provides an option to determine its behavior when both  instances cannot be kept in sync. One option is to move to the synch-deferred state. Another  option is to fail future write operations. Depending on your requirements, determine whether  continued processing is more important than creating synchronous redundancy of the  workload. For more information, see 2.4.5, “Copy consistency points” on page 65. Override policies overrule the explicit definitions of Copy policies.  Note:  Synchronous mode is not subject to override policies. The Override policies are cluster-based. They cannot be influenced by the attached hosts or  policies. With Override policies, you can help influence the behavior on how the TS7700  cluster chooses a TVC selection during the mount operation, and whether a copy must be  present in that cluster (for example, favoring the local mount point cluster). Copy Count Override enables the client to define for this cluster that at least two or more  copies exist at RUN time, but the client does not care which clusters have a copy. If you use  Copy Count Override, the grid configuration and available bandwidth between locations likely  determines which RUN copies meet the count criteria. Therefore, the limited numbers of  copies can be within the closest locations versus at longer distances. Remember this if you  use this override. Cluster families  can be introduced to help with TVC selection or replication activity. You might  want to use them for the following conditions: GLYPH<SM590000> You have an independent group or groups of clusters that serve a common purpose within  a larger grid. 122  IBM TS7700 Release 5.0 Guide