2. PERFORMANCE REPORTING ANALYSIS › 2.7 Graphic Analysis › 2.7.4 I/O Tracking Graphs

2.7.4 I/O Tracking Graphs

Analyzing and tracking the performance of disks or direct
access storage devices (DASD) is an important part of
performance management.  The relative speed of moving data to
and from DASD is orders of magnitude slower than the speed of
moving information in central or expanded storage.  Some of
the characteristics of DASD make the potential for causing
poor system responsiveness even more pronounced.

If two workloads are attempting to access data in central
storage, one may have to wait for the other, but once a
memory access cycle is made available to the second workload,
it will be able to access its data at full memory speed.  In
contrast, if two workloads are accessing data on a single
DASD volume, they may require that the device's read-write
heads move back and forth from one file to another.  The
existence of such heavy seeking activity on a disk can
contribute to poor overall system performance or to poor
performance of individual workloads.

Even a single TSO or online transaction may cause itself this
kind of delay, if it is accessing information at several
locations on one device.

This section discusses the following graphic analyses:

    1 - Device Response Components
    2 - Average MPL Waiting for Devices