8464ch04_Planning.fm Draft Document for Review February 7, 2020 3:15 pm 4.2.4 Retain Copy mode 4.2.5 Defining cluster families 4.2.6 TS7700 cache thresholds and removal policies Cache thresholds for a TS7700 cluster Retain Copy mode is an optional attribute, which is controlled by Management Class  construct configuration in which a volume’s prev iously existing Copy Consistency Points are  accepted rather than applying the Copy Consistency Points defined at the mounting cluster.  This attribute applies to private volume mounts for reads or write appends. It is used to  prevent more copies of a volume from being created in the grid than wanted. This attribute is  important in a grid with three or more clusters that includes two or more clusters online to a  host. If you have a grid with three or more clusters, you can define  cluster families . This function introduces a concept of grouping clusters into families. By using cluster families,  you can define a common purpose or role to a subset of clusters within a grid configuration.  For example, the role that is assigned (production or archive) is used by the TS7700 Licensed  Internal Code to make improved decisions for tasks, such as replication and TVC selection.  For example, clusters in a common family are favored for TVC selection, or replication can  source volumes from other clusters within its family before clusters are used outside of its  family. These thresholds determine the state of the cache as it relates to remaining free space.  There are three thresholds that define the capacity of CP0 in a TS7700T and the active cache  capacity in a TS7700D. These thresholds determine the state of the cache as it relates to  remaining free space.  The following thresholds are available in ascending order of occurrence: The policy removes the oldest logical volumes from the TS7700D and CP0 in TS7700T  cache if a consistent copy exists elsewhere in the grid. This state occurs when the cache  is 3 TB below the out-of-cache-resources threshold. In the automatic removal state, the  TS7700 automatically removes volumes from the disk-only cache to prevent the cache  from reaching its maximum capacity.  This state is identical to the limited-free-cache-space-warning state unless the Temporary  Removal Threshold is enabled. You can also lower the removal threshold in the LI REQ.  The default is 4 TB. To perform removal operations in a TS7700T, the size of CP0 must be at least 10 TB.  Consider the following points: – You can disable automatic removal within an y specific TS7700D cluster by using the  following  LIBRARY REQUEST  command, as shown in the following example: – The default automatic removal threshold can be changed from the command line by  using the following library request command: LIBRARY REQUEST,library-name,CACHE,REMOVE,{ENABLE|DISABLE} LIBRARY REQUEST,library-name,CACHE,REMVTHR,{VALUE} Chapter 4. Preinstallation planning and sizing  161 GLYPH<SM590000> Automatic Removal