The storage class attribute of GUARANTEED SPACE allows the volumes on which an SMS data set is to reside to be chosen explicitly when the data set is created. If GUARANTEED SPACE is used and if any of the volumes specified are not part of one of the storage groups to which the storage class maps, the data set creation fails. The use of GUARANTEED SPACE requires some special considerations if DFSMShsm is to manage data sets assigned to such a storage class.
DFSMShsm is used as the data mover on partitioned data sets, catalogs, and other data sets that are not supported by DFSMSdss. When DFSMShsm is the data mover, the Access Method Services functions of EXPORT and IMPORT back up and recover VSAM data sets. The IMPORT function redefines a VSAM data set by using a copy of the catalog information saved in the EXPORT copy of the data set. This define request explicitly specifies the volumes from which the data set was exported. The explicit specification of the volume affects the way data sets are imported with the GUARANTEED SPACE attribute. Likewise, for non-VSAM data sets, VTOC Data Set Services uses the volume serial number passed by DFSMShsm if the storage class is GUARANTEED SPACE.