## Copies to follow Storage Class Preference (COPYFSC) 

Normally, the TVCs in both TS7700 tape drives in a multi-cluster grid are managed as one TVC to increase the likelihood that a needed volume is in cache. By default, the volume on the TS7700 that is selected for I/O operations is preferred to stay in cache on that TS7700. The copy that is made on the other TS7700 is preferred to be removed from cache as soon as a copy to physical media is completed (through the premigrate/migrate process). Consider the following points: 

GLYPH<SM590000> Preferred to stay in cache  means that when space is needed for new volumes, the oldest volumes are removed first. This algorithm is called the  least recently used (LRU) algorithm. Preferred to stay in cache is also referred to as Preference Group 1  (PG1). 

GLYPH<SM590000> Preferred to be removed from cache  means that when space is needed for new volumes, the largest volumes are removed first, regardless of when they were written to the cache. Preferred to be removed from cache is also referred to as Preference Group 0  (PG0). 

Applied Cache Preference Group (PG0/PG1) to be applied depends on the Storage Class construct that is associated to individual virtual volumes. Therefore, it is possible to use different Cache Preference settings for different virtual volumes. 

Note:  Construct names are assigned to virtual volumes by the attached Host System. They are used to establish Data Management policies to be run by the TS7700 against specific volumes. Constructs (and associated policies) are defined in advance using the TS7700 MI. For more information, see “Defining TS7700 constructs” on page 557. If the Host System assigns a construct name without first defining it, the TS7700 creates the construct with the default parameters. 

In the default Storage Class Case, for a TS7700 running in a dual production multi-cluster grid configuration, virtual tape drives in both TS7700 are selected as the I/O TVCs, and have the original volumes (newly created or modified) preferred in cache. The copies to the other TS7700 are preferred to be removed from cache. Therefore, each TS7700 TVC is filled with unique, newly created, or modified volumes, roughly doubling the amount of cache seen by the host. 

However, for a TS7700 running in a multi-cluster grid configuration that is used for business continuance, particularly when all I/O is preferred to the local TVC, this default management method might not be wanted. If the remote site of the multi-cluster grid is used for recovery, the recovery time is minimized by having most of the needed volumes in cache. What is needed is to have the most recent copy volumes remain in the cache, not being preferred out of cache. 

Based on business requirements, this behavior can be modified for a TS7700 by using the COPYFSC control, as shown in the following example: 

LI REQ, <distributed-library>, SETTING, CACHE, COPYFSC, <ENABLE/DISABLE> 

This control features the following characteristics: 

GLYPH<SM590000> The default is set to disabled . 

GLYPH<SM590000> When disabled , virtual volumes that are copied into the cache from a peer TS7700 are managed as PG0 volumes (prefer largest files out of cache first), regardless of local definition of the associated Storage Class construct. 

GLYPH<SM590000> When set to enabled , virtual volumes copied into the cache from a peer TS7700 are managed by using the actions that are defined for the Storage Class construct that is associated with the volume, as locally defined. 

