The data class of a volume is determined when a scratch tape volume is allocated. If the allocation is steered to an ATLDS or a MTL, the data class subparameter on the SETOAM statement of the CBROAMxx PARMLIB member for the Object or Object Backup storage group is used, if it is specified.

If the SETOAM statement does not specify the data class subparameter at the storage group level, use the DATACLASS parameter of the SETOAM statement for the OAM global level to specify the values for the tape volume. The DATACLASS parameter of the SETOAM statement at the global level applies to all tape volumes that belong to storage groups not having their own DATACLASS assigned. If you do not specify DATACLASS at the storage group or at the OAM global level, OAM uses the DEVSUP parameter default to determine tape compaction or no tape compaction for the tape volume. The ACS routines can also supplement or override the data class values that you specify for the tape volume from either the SETOAM statement or the DEVSUP parameter.

Recommendation for data class and ACS routines: Do not allow the ACS routines to assign or change the data class assignment of an OAM tape volume. Ensure that the ACS routines do not change DATACLASS specifications for OAM object tape data sets, including OAM.PRIMARY.DATA.*, OAM.BACKUP.DATA.*, or OAM.BACKUP2.DATA.* for any storage group. The data class for OAM tape volumes is determined by the SETOAM statement of the CBROAMxx PARMLIB member at MVS scratch tape allocation. The SETOAM statement provides this information either at the Object or Object Backup storage group level or at the OAM global level—whichever best suits the requirements for the tape volume that is being allocated. Allowing the ACS routines to alter this specification could create unexpected consequences (for example, no compaction of the data when the SETOAM statement specifies compaction). When you write the ACS routines, ensure that the data class construct for OAM tape volumes is kept intact.

See Figure 1 for a diagram of the process of storing objects to tape.