BLOCK0

Default
NOBLOCK0
Recommended
BLOCK0
Reasoning
Blocked I/O can reduce the number of physical I/O transfers, resulting in fewer EXCPs. The BLOCK0 compiler option changes the default for QSAM files from unblocked to blocked (as if the BLOCK CONTAINS 0 clause were specified for the files), and thus gain the benefit of system-determined blocking for output files. BLOCK0 activates an implicit BLOCK CONTAINS 0 clause for each file in the program that meets all of the following criteria:
  • The FILE-CONTROL paragraph either specifies ORGANIZATION SEQUENTIAL or omits the ORGANIZATION clause.
  • The FD entry does not specify RECORDING MODE U.
  • The FD entry does not specify a BLOCK CONTAINS clause.

As a performance example, one test program using BLOCK0 that meets the above criteria was 90% faster than a corresponding one using NOBLOCK0, and used 98% fewer EXCPs.

related references
BLOCK0 (Enterprise COBOL for z/OS® Programming Guide)