An overview of storage tiers

DFSMS is enhanced with this new support to provide policy-based data movement between storage classes and storage groups defined within a Primary (L0) Storage hierarchy. This data movement will be performed during the existing space management processing performed by primary space management, on-demand migration, and interval migration, which are performed at the volume level. When a volume is selected for space management processing due to being over threshold, in addition to analyzing each data set on the volume to determine if it is eligible for migration or expiration, these functions will additionally determine if a data set is eligible to be transitioned, based on new management class criteria.

The existing management class "Class Transition Criteria" will now be applied to all data sets, not just data objects managed with OAM. The class transition criteria enable a policy to be assigned which will then determine when a data set should be transitioned, based on its age, the date that it was last referenced, or a periodic value. The class transition criteria are shown below:

                           MANAGEMENT CLASS DISPLAY                Page 4 of 6
                                                                              
CDS Name . . . . . . . . . . : USER.MYSCDS                                  
Management Class Name  . . . : MC1                                       
                                                                              
Class Transition Criteria                                                     
  Time Since Creation Years  :                                                
                      Months :                                                
                      Days . :                                               
                                                                              
  Time Since Last use Years  :                                                
                      Months :                                                
                      Days . :                                                
  Periodic                                                                    
    Monthly   on Day . . . . :                                                
    Quarterly on Day . . . . :                                                
              in Month . . . :                                                
    Yearly    on Day . . . . :                                                
              in Month . . . :

Similar to data set migrations, class transitions will occur during the second phase of volume-level space management processing. The first phase will attempt to free enough space on the volume by expiring and reconnecting data sets. If the first phase does not free enough space on the volume, then the second phase of space management will move eligible data off of the volume. Data sets that are eligible for a class transition will be processed first. If a data set is eligible for both a class transition and migration, then it will be migrated. This prevents the data from being transitioned to another volume, only to be later migrated. After all data sets that are eligible for a class transition have been processed, then standard migration processing will be performed. Class transitions and migrations occur until the volume reaches its low threshold. The DFSMShsm MD exit can be used to bypass the transition of specific data sets.

When a data set is eligible for a class transition, then DFSMShsm invokes the SMS ACS routines to determine how the data set should be transitioned. A new ACS environment of SPMGCLTR is defined to support class transitions. The ACS routines with an ACEROENV of SPMGCLTR may assign any of the following:

Storage Class: When a new storage class is assigned, DFSMShsm will attempt to move the data set to the appropriate storage class. Similar to new allocations, the specified storage class is the "preferred" storage class. If DFSMShsm is unable to move the data set to the new storage class, then the data set will not be transitioned, unless a new storage group has also been specified.
Management Class: When a new management class is assigned to a data set, DFSMShsm will begin using the policies of the newly assigned management class to mange the data set. For example, when a data set is newly allocated, it may be assigned to a management class that specifies that a data set should have two backup copies and that it is not eligible to be migrated. This may be appropriate for the first 180 days of the data sets existence. After 180 days, only a single backup copy is required and the data set is eligible for migration. This may be accomplished by assigning the data set to one management class upon data set creation, and then transitioning that data set to another management class after 180 days from creation.
If the ACS routines indicate that only the management class should be transitioned, then no actual data movement is performed. DFSMShsm just assigns the data set to the new management class.
Storage Group: When a new storage group is assigned, DFSMShsm will attempt to move the data set to the appropriate storage group. Up to fifteen storage groups may be specified. DFSMShsm will attempt to allocate the data set on one of the fifteen storage groups, excluding the storage group to which the data set is already assigned. It is up to the storage administrator to write the ACS routines such that meaningful transitions occur.

Note: The new SMS parameters for class transition are set as follows, and in this specific order:

The new storage class parameter value is set according to the existing management class parameter and the existing storage group parameter.
The new management class parameter value is set according to the existing management class parameter and the new storage class parameter.
The new storage group parameter value is set according to the new management class parameter and the new storage class parameter.

Figure 1 shows sample ACS routines for assigning new classes and groups during a class transition.

Figure 1. Sample ACS routines for new classes and groups during a class transition

Storage Class:
IF &ACSENVIR = 'SPMGCLTR' THEN
   SELECT (&STORCLAS)
      WHEN ('SSD')      SET &STORCLAS = 'EASYTIER'
      WHEN ('EASYTIER') SET &STORCLAS = 'SCSATA'
      OTHERWISE         SET &STORCLAS = &STORCLAS
   END
ELSE ...

Management Class:
IF &ACSENVIR = 'SPMGCLTR' THEN
/* SPACE MANAGEMENT CLASS TRANSITION */
   SELECT (&MGMTCLAS) 
      WHEN ('NOML2')  SET &MGMTCLAS = 'ML2OK'
      WHEN ('DB2NEW') SET &MGMTCLAS = 'DB2AGED'
      OTHERWISE       SET &MGMTCLAS = &MGMTCLAS
   END
ELSE ...  

Storage Group:
IF &ACSENVIR = 'SPMGCLTR' THEN
   SELECT (&STORCLAS)
      WHEN ('EASYTIER') SET &STORGRP = 'EASYTIER'
      WHEN ('SCSATA')   SET &STORGRP = 'SGSATA'
      OTHERWISE         SET &STORGRP = &STORGRP
   END
ELSE ...