In the HIGHLVL.SOURCE (IDDCVSAM) the comments for HSTACCTA,HSTACCTB,HSTSTATA and HSTSTATB allocation states Secondary extents must not be specified. What is the reason for that?
You can specify a secondary allocation, but that will cause VSAM to continue to extend the data set until the maximum number of extents are reached (125 for non-extended VSAM) or the volume fills up. It won't switch to the alternate file until that happens. With no secondary space defined, the switching occurs as soon as the primary extent is filled. That is far more preferable in terms of both performance and space management.