Apparatus and associated methodology contemplating a data storage system having a memory that includes a first addressable storage space and a second differently addressable storage space. A controller selectively accesses stored instructions that when executed transfer some user data sets with the first addressable storage space in accordance with a dense allocation of the first addressable storage space, and transfer other user data sets with the second addressable storage space in accordance with a sparse allocation of the second addressable storage space that allocates a predefined reserved storage capacity between adjacent stored non-sequential user data sets.