You can use the ISMF Data Set Application to estimate how much space is being wasted because of inefficient block size usage, by comparing the data set's block size to the optimal block size that DFSMSdfp calculates and ISMF displays. The ISMF column, blocks unused, shows you how much space you can reclaim by using a system-determined block size. ISMF also displays the reblockable indicator for the data set.

Another way is to run the DCOLLECT access method services function.

Before using system-determined block size for your data sets, evaluate the effect on your applications. Applications that specify block size or data sets with very large logical record sizes might present implementation problems. Also, additional virtual storage might be required in the application's address space to store the larger buffers.