Migration thresholds specify when the server should begin and stop migrating data to the next storage pool in the storage hierarchy. Migration thresholds are defined as a percentage of total storage-pool data capacity.
You can use the defaults for the migration thresholds, or you can change the threshold values to identify the maximum and minimum amount of space for a storage pool.
To control how long files must stay in a storage pool before they are eligible for migration, specify a migration delay for a storage pool. For details, see Keeping files in a storage pool.
If you decide to enable cache for disk storage pools, files can temporarily remain on disks even after migration. When you use cache, you might want to set lower migration thresholds.
For more information about migration thresholds, see How the server selects files to migrate and Migration thresholds. For information about using the cache, see Minimizing access time to migrated files and Caching in disk storage pools.