1. OVERVIEW › 1.3 I/O Analysis and Tape Drive Availability

1.3 I/O Analysis and Tape Drive Availability

The configuration and utilization of input/output devices is
a critical aspect of system performance management.  To
support this part of system performance management, the
CA MICS Performance Manager Option enables configuration
analysis, including channel/path/device utilization analysis,
DASD data set placement contention reporting, and tape drive
availability analysis.

The I/O Analysis software for DASD implements four different
algorithms:  the Shared DASD Analysis Algorithm, the I/O
Component Analysis Algorithm, the DASD Skew Analysis
Algorithm, and the Multiple Volume Mapping Algorithm.

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

Shared DASD often introduces significant performance
problems, however, by allowing workloads on one system to
adversely affect activities on other systems.  Because 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 RMF data from multiple systems to
evaluate the performance of a shared device.

The I/O service time is the most significant part of response
time for all but the most CPU bound applications.  As CPU
internal processing speeds increase, performance problems
that result from I/O delays increase.

DASD skew is a condition that existed prior to the
introduction of controllers with RPS avoidance, such as EMC
and 3990-6 controllers.  It stems from the nature of
rotational position sensing (RPS) devices and RPS miss
probabilities.  If you are not using a controller with RPS
avoidance, DASD skew occurs when one or two of the devices on
a string are heavily used and cause performance problems for
the remaining devices on the string.

Dual density DASD devices or devices that offer adequate
storage capacities offer cost and floor space reductions.
The conversion to this type of device must be well planned.
Devices that are to be mapped together to form a dual density
volume should not be experiencing high I/O activity during
the same time periods.  If such volumes are combined, severe
arm contention (that is, delays) will result.

The Tape Drive Analysis facility allows you to analyze time
periods that have significant tape drive availability
problems, and to model the effects of moving existing work to
different time periods, as well as the effects of increasing
tape usage.  You can investigate both the effects on
performance of rescheduling existing jobs and the capacity
implications of increased tape drive usage.

See Section 2.4 I/O Configuration Analysis, for details on
all these algorithms and the reports that are produced.