Large format data sets are physical sequential data sets, with generally the same characteristics as other non-extended format sequential data sets but with the capability to grow beyond the basic format size limit of 65␠535 tracks on each volume. (This is about 3␠500␠000␠000 bytes, depending on the block size.) Large format data sets reduce the need to use multiple volumes for single data sets, especially very large ones like spool data sets, dumps, logs, and traces. Unlike extended-format data sets, which also support greater than 65␠535 tracks per volume, large format data sets are compatible with EXCP and don't need to be SMS-managed.

Data sets defined as large format must be accessed using QSAM, BSAM, or EXCP.