DFSMShsm makes sure that space is available on your DASD volumes so that you can extend your old data sets and allocate new ones. DFSMShsm automatically deletes expired or temporary data on your DASD volumes and lets you reuse that space for new data. DFSMSdss eliminates or reduces the fragmentation of free space. Your installation can choose to have DFSMShsm automatically reclaim your excess requested space once a day, or choose to have DFSMSdfp reclaim excess requested space each time a data set is closed.