2. PERFORMANCE REPORTING ANALYSIS › 2.4 I/O Configuration Analysis › 2.4.3 Shared DASD Analysis Inquiry › 2.4.3.3 Case Study › 2.4.3.3.1 Problem Description

2.4.3.3.1 Problem Description

Shared DASD allows multiple systems to be connected to a
single volume.  This feature is highly desirable for some
applications' volumes, since critical applications may be
moved to another system in the event of a processor failure.
Also, shared DASD allows a single copy of critical data to be
shared by multiple systems.

However, shared DASD can introduce significant performance
problems by allowing workloads on one system to adversely
affect activities on other systems.  Since RMF reports on the
I/O activities of individual systems, rather than the I/O
activity of an installation as a whole, it is necessary to
summarize all of the HARDVA data for every system that shares
a volume with the specified system to obtain an accurate view
of each device.

For this case study, we used the Shared DASD Analysis Report
to analyze a configuration consisting of three IBM 3081s.
The objectives of the study were to determine the degree to
which the shared volumes were utilized by the processors and
the effect of the multisystem contention on the I/O response,
service, and queue times (CA MICS data elements DVAAVRTM,
DVAAVSTM, and DVAAVRTM-DVAAVSTM) of each of the systems.

The inquiry panels used to conduct the study and the output
reports are discussed in the following two sections.