Using QSAM files

When using QSAM files, use large block sizes whenever possible by using the BLOCK CONTAINS clause on your file definitions (the default with COBOL is to use unblocked files).

You can have the system determine the optimal block size for you by specifying the BLOCK CONTAINS 0 clause for any new files that you are creating and omitting the BLKSIZE parameter in your JCL for these files. You can also omit the BLOCK CONTAINS clause for the file and use the BLOCK0 compiler option to achieve the same effect. This should significantly improve the file processing time (both in CPU time and elapsed time).

Performance considerations using I/O buffers for a program that reads 14,000 records and wrote 28,000 records with no BLOCK CONTAINS clause and no BLKSIZE in the JCL:

Using BLOCK0 was 90% faster and used 98% fewer EXCPs than NOBLOCK0.

Additionally, increasing the number of I/O buffers for heavy I/O jobs can improve both the CPU and elapsed time performance, at the expense of using more storage. This can be accomplished by using the BUFNO subparameter of the DCB parameter in the JCL or by using the RESERVE clause of the SELECT statement in the FILE-CONTROL paragraph. Note that if you do not use either the BUFNO subparameter or the RESERVE clause, the system default will be used.

Performance considerations using I/O buffers for a program that reads 14,000 records and wrote 28,000 records with no blocking:

Using DCB=BUFNO=1 took 0.452 CPU seconds
Using DCB=BUFNO=5 took 0.129 CPU seconds
Using DCB=BUFNO=10 took 0.089 CPU seconds
Using DCB=BUFNO=25 took 0.067 CPU seconds

Refer to Chapter 4.1 for a discussion on the location of QSAM buffers.