When
you specify a secondary allocation quantity of zero for nonstriped
VSAM data sets, an extend causes the allocation to occur on a new
volume (if there is one), using the primary-space amount. For striped
VSAM data sets, extensions occur by stripe, so if there is a secondary
quantity of zero, the primary space value is used for the extensions
across all stripes until space is exhausted on the primary and new
volumes, or until the maximum extents for the data set is reached. 