8464ch02_Architecture.fm Draft Document for Review February 7, 2020 3:15 pm GLYPH<SM590000> Prefer Keep: When Space is needed Group 1 (LRU) – An appropriate number of copies exist on peer clusters. – The pinning duration (in number of hours) elapsed since the last access or creation. – The available free space on the cluster fell below the removal threshold.  – Volumes with the Prefer Remove (LRU Group 0) policy were exhausted. The copy of a private volume is removed if the following conditions exist: The order in which volumes are removed under this policy is based on their LRU access  times. Volumes in Group 0 are removed before the removal of volumes in Group 1, except  for any volumes in scratch categories, which are always removed first. Prefer Remove and Prefer Keep policies are similar to cache preference groups PG0 and  PG1, except that removal treats both groups as LRU versus the use of their volume size. In  addition to these policies, volumes that are assigned to a scratch category and that were not  previously delete-expired are also removed from cache when the free space on a cluster falls  below a threshold. Regardless of their removal policies, scratch category volumes are always  removed before any other removal candidates in descending volume size order.  Volume retention time is also ignored for scratch volumes. Only if the removal of scratch  volumes does not satisfy the removal requirements do PG0 and PG1 volumes become  candidates for removal. If an appropriate number of volume copies exist elsewhere, scratch  removal can occur. If one or more peer copies cannot be validated, the scratch volume is not  removed.  Figure 2-12 shows the TS7720 cache removal priority. Figure 2-12 TS7700 cache removal priority Host command-line query capabilities are supported that help override automatic removal  behaviors and disable automatic removal within a TS7700D cluster, or for the CP0 in a  TS7700T or TS7700C. For more information, see  IBM Virtualization Engine TS7700 Series  z/OS Host Command Line Request User’s Guide , WP101091. 80  IBM TS7700 Release 5.0 Guide