## 2.4.23 TVC management for disk-only TS7700 clusters in a multi-cluster grid 

Disk-only TS7700 clusters in a grid feature several unique options for cache management. 

## Enhanced Removal Policies 

The Enhanced Volume Removal Policy provides tuning capabilities in grid configurations where one or more disk only TS7700D clusters are present. Traditionally, other TS7700T or TS7700C clusters also exist which have a much larger available capacity. Enhanced removal policy provides a method to retain only certain workloads within the smaller capacity TS7700 solutions as data ages. 

Because the TS7700D has a maximum capacity (the size of its TVC), after this cache fills, the Volume Removal Policy enables logical volumes to be automatically removed from this TS7700D TVC while a copy is retained within one or more peer clusters in the grid. When coupled with copy policies, TS7700D Enhanced Removal Policies provide various automatic data migration functions between the TS7700 clusters within the grid. This is also true for a TS7700T/TS7700C CP0. 

In addition, when the automatic removal is run, it implies an override to the current Copy Consistency Policy in place, which results in a lowered number of consistency points compared with the original configuration that is defined by the user. 

When the automatic removal starts, all volumes in scratch categories are removed first because these volumes are assumed to be unnecessary. To account for any mistake where private volumes are returned to scratch, these volumes must meet the same copy count criteria in a grid as the private volumes. The pinning option and minimum duration time criteria that are described next are ignored for scratch (Fast Ready) volumes. 

To ensure that data always is in a TS7700D or CP0 for at least a minimal amount of time, a volume retention time can be associated with each removal policy. This  volume retention time (in hours) enables volumes to remain in a TVC for a certain time before the volume becomes a candidate for removal. The time varies 0 - 65,536 hours. A volume retention time of zero assumes no minimal requirement. 

In addition to the volume retention time, the following policies are available for each volume in a TS7700 CP0: 

GLYPH<SM590000> Pinned 

The copy of the volume is never removed from this cluster. No volume retention time is applicable and it is implied as infinite. After a pinned volume is moved to scratch, it becomes a priority candidate for removal similar to the next two policies. This policy must be used cautiously to prevent cache overruns. 

GLYPH<SM590000> Prefer Remove: When Space is Needed Group 0 (LRU) 

The copy of a private volume is removed if the following conditions exist: 

– An appropriate number of copies exist on peer clusters. 

– The pinning duration (in number of hours) has elapsed since the last access or creation. 

– The available free space on the cluste r has fallen below the removal threshold. 

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. Archive and backup data can be a good candidate for this removal group because it is not likely accessed after it is written. 

