Selecting thresholds for volumes

When you choose thresholds for your volumes, consider the following:

• Size of data sets
• Rate that users require data space
• Frequency of interval migration

Follow this guideline when you set the thresholds of occupancy: The larger the data set is, the lower the threshold. Set the thresholds so users have space for their data sets but data set thrashing is minimized. Thrashing is a condition in which the system can do little useful work because of excessive data movement between primary and migration volumes. Data set thrashing can be caused by any of the following reasons:

• Not enough primary volume space
• The high threshold is set too low, causing DFSMShsm to migrate data sets more often than is necessary during interval migration.
• The low threshold is set too low, causing DFSMShsm to migrate too many data sets from primary volumes.
• The minimum migration age is set too low. DFSMShsm uses the minimum migration age for volume processing as a lower limit to determine which data sets are eligible for migration. If the minimum migration age is too low, automatic primary space management forces the migration of data sets that were recently referred to and are likely to be referred to again soon.