## 2.3.3 Resident-only TVC cache management 

In a stand-alone disk-only TS7700D, virtual volumes always remain in the TVC because no physical tape or cloud is attached. In a TS7700T and TS7700C configuration, contents in the CP0 partition are also not candidates for moving to physical tape or cloud. Lastly, a DS8000 object partition retains all objects in disk cache and does not yet support movement to physical tape or cloud. For virtual volumes in this state, only Delete Expire processing, a logical volume EJECT, or a reuse can free up the logical volume TVC space. 

For a DS8000 Object partition, SMS-initiated object deletes free up the space for the offloaded object by requesting an attached DS8000 to delete the object and any metadata objects that is associated with it. In a TS7700T or TS7700C, LI REQ PARTRFSH  can be used to move resident-only volumes from CP0 to a CPx partition as another method to free up CP0 capacity. 

If a disk-only TS7700D or resident-only partition begins to run out of space, warning messages are surfaced to all attached hosts and the library enters a degraded state. If capacity continues to grow and the free space is exhausted, a critical message is surfaced and the TS7700D or resident-only partitions enter a read-only state. The TS7700D enters the Out of Cache condition and moves to a read-only state. If a TS7720T/TS7760C CP0 partition becomes full, it becomes read-only regarding workloads that target CP0. 

Important: Monitor your cache in a TS7700D stand-alone environment to avoid an Out of Cache Resources situation. 

## 2.3.4 Premigration of CPx TVC content 

The TS7700T and TS7700C support the ability to automatically offload logical volumes from TVC cache to back-end attached tape or cloud. Premigration  is the term that is used for putting logical volumes onto a physical tape or object store. The next few sections describe how this premigration process works. 

## Premigration with multiple disk cache partitions 

You can specify 1 - 7 independent disk cache partitions (CPx). Each partition features its own independent cache management that allows different workloads to have different disk cache footprint characteristics. For example, workloads that benefit from a high disk cache hit ratios can use a large disk cache partition, while those workloads that are rarely accessed and benefit little from disk cache residency can use a smaller disk cache partition. 

By providing each workload type its own partition, the hierarchical storage management concepts can apply to other volumes with similar workload characteristics only. 

The Storage Group (SG) construct is used to determine which partition a workload targets. After it is in a CPx partition, it is then a candidate for premigration or the copying to back-end tape or cloud. By default, the queuing of the premigration occurs within minutes of volume close. But, it can be delayed by using Time-delayed premigration. 

The total amount of logical volume content that can be queued for premigration to tape or cloud is limited by the number of FC 5274 and FC 5279 features. The LI REQ PMTHLVL  value is used to determine when the TS7700T or TS7700C begins to slow down inbound host activity to prevent the premigration queue from growing much further. This concept is referred to as Host Throttling  and is a normal behavior for the TS7700T and TS7700C. 

