Do not code DISP=MOD if the data control block (DCB) specifies RECFM=FBS and the data set is processed by QSAM. If you do and a block is shorter than the specified block size, QSAM assumes that the short block is the last block and starts end-of-file processing. By this action, QSAM can embed short blocks in your data set and so affect the number of records per track.