VSAM dynamic access optional logic path
With the PTF for APAR PH56036 for AMODE 31 or PH56037 for AMODE 64 installed, an optional
alternate logic path is introduced for VSAM files that use the ACCESS IS DYNAMIC
mode. This alternate logic path does not alter your program logic, but instead changes the runtime
logic used to locate a record by key (read-by-key). Both the default and alternate logic path return
the same results.
The default COBOL runtime ACCESS IS DYNAMIC logic path gears toward those who
use more READ NEXT (read-next-record) after positioning to a key record (read-by-key). This logic
path works best in this case because VSAM does record pre-fetches in anticipation of programs that
do a read-next-record. The COBOL runtime would point to a record by key, then issue a sequential
read from that point priming the read-ahead buffers.
The alternate logic path introduced uses a direct read-by-key request instead of a point to a record by key. This approach takes advantage of IBM®'s hardware disk systems that feature zHyperLink. zHyperLink improves application response time, which cuts I/O-sensitive workload response times. Note that the CPU cost is not neutral for some environments. You are highly recommended to use this approach where your critical business demands depend on I/O sensitive workload response times. You might get varying performance results, so you should do your own performance measurements.
You can enable this alternate logic path by using the COBOL runtime option VSAMDYNAMICDIR. For more details, see VSAM dynamic access read option VSAMDYNAMICDIR in the Programming Guide.
More information about zHyperLink is available at zHyperLink I/O in the IBM z/OS® documentation and the Getting Started with IBM zHyperLink for z/OS on IBM Redbooks®.
VSAM dynamic access read option VSAMDYNAMICDIR (Enterprise COBOL for z/OS Programming Guide)
zHyperLink I/O