DFSMShsm checks the COMMAND OR AUTO MIGRATE attribute in the management class to which the data set is associated. If the value of the attribute is BOTH, DFSMShsm determines whether the data set is eligible for a class transition or migration.
A data set is eligible for a class transition if the data set has met or exceeded any of the three class transition criteria specified within the assigned management class (TIME SINCE CREATION, TIME SINCE LAST USE, or PERIODIC). If a data set transitions to a new management class, then the data set will be eligible for another transition when the volume is next space managed, based on the criteria within the newly assigned management class. If a different management class is not assigned during a transition, then a data set will not transition again if either TIME SINCE CREATION or TIME SINCE LAST USE are the criteria that is used. If a PERIODIC value is specified, then a data set will transition as often as needed.
Rolled-off generation data sets that belong to a management class with the attribute ROLLED-OFF GDS ACTION=MIGRATE are always identified as migration candidates with the highest migration priority.
Non-rolled-off generation data sets that exceed the value specified with the # GDG ELEMENTS ON PRIMARY attribute for their management class are always identified as candidates with priority for early migration. The priority of such data sets is just below that for rolled-off generation data sets.
For both command and event-driven migration, DFSMShsm determines whether the data set being migrated is a checkpointed data set. DFSMShsm migration, including extent reduction, is delayed for a fixed number of days for SMS-managed MVS and IMS GSAM checkpointed data sets. The default delay is five days. If the data set is checkpointed, DFSMShsm determines whether the data set is eligible for migration. A checkpointed data set is eligible for migration when the date-last-referenced, plus the number of days the data set is to be treated as unmovable, are less than or equal to the current date. For data set command migration, error messages are issued for ineligible data sets.
DFSMShsm provides a patch byte that enables users to modify the number of days that must have elapsed since the date-last-referenced for a checkpointed data set to be eligible for migration. For more information about this patch, see the topic about tuning DFSMShsm in z/OS DFSMShsm Implementation and Customization Guide.
Checkpointed data sets must meet a minimum age value before being considered for migration. The default is five days and can be changed using a patch. Once the minimum age value is met, the checkpointed data sets can be identified as migration candidates if the time since they were last used meets or exceeds the value specified by the PRIMARY DAYS NON-USAGE attribute for their management class.
All other data sets, including generation data sets that were not given special migration priority, are identified as migration candidates if the time since they were last used meets or exceeds the value specified by the PRIMARY DAYS NON-USAGE attribute for their management class.
If the COMMAND OR AUTO MIGRATE attribute in a data set's management class is set to BOTH, with the PRIMARY DAYS NON-USAGE attribute set to 9999, and the # GDG ELEMENTS ON PRIMARY attribute set to 1, then relative zero-generation data sets can remain on a user's primary disk for up to 30 years. (The # GDG ELEMENTS ON PRIMARY attribute overrides the PRIMARY DAYS NON-USAGE attribute.) When a generation moves from zero to -1, it becomes eligible to migrate immediately, independent of the PRIMARY DAYS NON-USAGE value.
When a data set is identified as a migration candidate, it is also identified for the target migration volume device type: DASD or tape. The device type chosen depends on the value in the LEVEL 1 DAYS NON-USAGE attribute of its management class. If the value of the attribute is 0, the data set is identified for migration to ML2 tape. If the value of the attribute is 1–9999 or NOLIMIT, the data set is identified for migration to ML1 DASD.
For more information about DFSMShsm support for checkpointed data sets, see the topic about user data sets in z/OS DFSMShsm Implementation and Customization Guide.