Blockset is DFSORT's most efficient technique. It supports many features not supported by DFSORT's less efficient Peerage/Vale and Conventional techniques (see ICE189A in z/OS DFSORT Messages, Codes and Diagnosis Guide for a list of these features). DFSORT always selects Blockset for a copy application. DFSORT selects Blockset for a sort or merge application unless something prevents it from doing so (see ICE800I in z/OS DFSORT Messages, Codes and Diagnosis Guide for a list of reasons Blockset cannot be used).

Message ICE143I indicates whether Blockset or a less efficient technique was selected for a particular run. If Blockset was not selected for a sort or merge application, check the reason code in message ICE800I, which indicates the reason Blockset could not be used. If you did not get message ICE800I, add the following DD statements to your application and rerun it:

//SORTDIAG DD DUMMY
//SYSOUT DD SYSOUT=*

If possible and appropriate, remove the obstacle that is causing Blockset not to be selected.