TS7700 Cache migration threshold

This topic describes the preference groups that manage virtual volume migration from cache in a TS7700, and the threshold that determines when migration begins.

Virtual volumes are managed by preference group (PG). These groups determine how soon volumes are migrated from cache following their copy to backend physical tape and are classified either as PG0 or PG1. The migration threshold applies only to PG1, which is the last group of volumes migrated from disk cache.
Preference Group 0 (PG0)
When space is needed, volumes are migrated from cache according to size. Largest volumes are migrated first.
This group is assigned to volumes that are unlikely to be accessed after being created. There is no need to keep them in cache any longer than is necessary to copy them to physical tape. When a volume is assigned as PG0, the TS7700 gives it preference to be copied to physical tape. When space is needed in the tape volume cache (TVC), the TS7700 first selects a PG0 volume that has been copied to a physical volume and then deletes it from cache, leaving a copy only on physical tape. PG0 volumes are selected by largest size first, independent of how long they have been resident in cache. If no PG0 volumes remain that are resident in cache and have also been copied to physical tape, the TS7700 begins to select preference group 1 (PG1) volumes for migration.
The TS7700 also migrates PG0 volumes from cache if the subsystem is relatively idle. There is a small amount of internal processing impact to migrating a volume from cache, so there is some benefit in migrating them when extra processing capacity is available. In the case where the TS7700 migrates PG0 volumes during idle times, it selects them by smallest size first.
Preference Group 1 (PG1)
When space is needed, volumes are migrated from cache according to last recently used (LRU) access time.
This group is assigned to volumes that are likely to be accessed after being created. One example of volumes in this category are those that contain master files created as part of the nightly batch run. Because the master files are likely to be used as input for the next night's batch run, it is beneficial for these volumes to stay in the TVC for as long as possible. When a volume is assigned as PG1, the TS7700 adds it to the queue of volumes to be copied to physical tape soon after the volume is demounted. It is copied to tape with a lower priority than PG0 volumes that have not yet been copied to tape. When space is needed in cache, the TS7700 first determines whether any PG0 volumes can be migrated. If not, the TS7700 selects PG1 volumes to be migrated based on an LRU algorithm. The LRU algorithm causes volumes that have already been copied to physical tape and that have been resident in cache the longest without access to be migrated first. The selected volumes are deleted from disk cache, leaving a copy of the volume only on physical tape.
The migration threshold for PG1 is 95% of full capacity or full capacity minus 300 GB, whichever value permits more active content to remain in cache.
When a preference group is assigned to a volume, that assignment is persistent until the volume is reused for scratch or remounted and a new preference group is assigned. Thus, a volume that is assigned as PG0 maintains that preference group when it is subsequently recalled into cache. You can use the Storage Class panel of the management interface to set a volume preference group as PG0 or PG1.
Note: When a second physical tape copy is requested within a volume's management class, copies to both physical tape pools must complete before the volume is a candidate for migration.

Virtual volumes in a Tape Attach environment can be designated for time-delayed premigration.