When DFSMShsm recalls or recovers a data set, it requests MVS™ to reallocate the data set. DFSMShsm requests enough space in the allocation request so it can recall or recover the data set in one extent. If enough contiguous space is available on the target volume, DFSMShsm automatically reduces the number of extents as it migrates and recalls or backs up and recovers the data set. If you specified secondary extents when you allocated the data set, DFSMShsm releases any unused space during recall or recovery. This process makes a larger contiguous space available for allocation of larger data sets. DFSMShsm can migrate, then recall, a data set specifically to reduce extents.