The data set reblock exit (ARCCDEXT) can reblock recalled or recovered data sets as they move from a source DASD to a target DASD. You can use the ARCCDEXT exit to allow the system either to specify a block size for these data sets using a DFSMSdss mover or to specify the block size yourself using a DFSMShsm mover. You can only use the ARCCDEXT exit to respond with a yes or no answer to the question: "Should this data set be reblocked?" In most cases, system-determined block size can block data sets and there is no reason to reblock them. You can use the ARCCDEXT exit, however, to prevent exception, block-size dependent data sets from accidentally being reblocked. Reblocking can allow more data to be stored on each DASD track or cause better performance with any device.