There are two types of migration, disk migration and tape migration . The main purpose of disk migration is to free up disk storage. Migration selects the qualified bitfiles and copies these bitfiles to the next storage level defined in the hierarchy.
Purge later frees the original bitfiles from the disk storage.
The main purpose of tape migration is to free up tape volumes, and not just migrate bitfiles. The active bitfiles in the target virtual volumes are moved laterally to the free tape volumes in the same storage level. The inactive bitfiles in the target virtual volumes are migrated to the free tape volumes in the next storage level. The client APIs provide the vehicle for the Storage System Manager to request the server to start migration and purge whenever it is necessary.