The DASD Component of CPExpert

In its MVS Performance Notebook, IBM stated that "over 75% of the problems reported to the IBM Washington System Center can be traced to some kind of I/O contention. Channel loading, control unit or device contention, data set placement, paging configurations, and shared DASD are the major culprits..."

A large computer installation often has hundreds or even thousands of DASD devices which must be managed. Data Base Administrators or Performance Analysts have time to address only the most serious problems. Fortunately, not all devices experience serious performance problems.

The DASD Component of CPExpert is designed to identify the most serious DASD problems, and to provide the information necessary to solve the problems.

The DASD Component processes information from SMF (contained in your MXG or NeuMICS performance data base) to build a model of your I/O configuration. This configuration model is used by CPExpert to assess DASD performance problems.

Basic Analysis

The DASD Component uses the model of your configuration and SMF Type 70(series) information to perform a basic analysis of the most significant DASD problems.

In the basic analysis, the DASD Component identifies the volumes having the worst I/O problems, and suggests ways to correct the DASD problems. Additionally, the DASD Component "ranks" the devices, so you can see the severity of the problems identified, compared with other devices. The DASD Component analyzes the causes of performance problems:


Excessive I/O Supervisor queuing delay (PAV and non-PAV)

Excessive PEND delay (channel, director port, controller, device)

Excessive DISCONNECT delay (Cache, PPRC, etc.)

Excessive CONNECT delay (including FICON channel delay)

Interference from non-DASD I/O on channels

Excessive missed cache read miss or write miss

Overloaded channel paths or strings

Excessive seek delay (legacy systems)

Excessive missed RPS reconnect (legacy systems)

Shared DASD analysis

The DASD Component automatically detects when DASD problems might be caused by shared DASD. The DASD Component then analyzes data from all systems in your performance data base to identify the systems that share the DASD devices.

The channel and device utilization and performance characteristics of these systems are analyzed to determine which systems cause the shared DASD problems.

The DASD Component can identify specific applications causing shared DASD problems if an optional (but no added cost) modification for MXG or NeuMICS has been installed.

Application analysis

The DASD Component optionally can perform an analysis of the applications using your DASD configuration. The DASD Component uses a proprietary modification to MXG or NeuMICS software to relate the DASD problems to specific workload categories (such as CICS or TSO). With the modification to MXG or NeuMICS installed, the DASD Component can perform an analysis of DASD problems based on workload category. You define important workloads to the DASD Component (using service classes (for Goal Mode) or performance groups (for Compatibility Mode). The DASD Component will select RMF and SMF information pertaining to the defined workloads, to determine: 

Which DASD volumes are causing the most delay to the specific application systems.

Whether non-critical work is causing I/O delays to critical workloads, and what is the effect of the interference.

Which jobs and job steps are responsible for the problems.

VSAM analysis

VSAM data sets typically account for a large percent of I/O activity (more than 70% at some sites). Adjusting a few parameters or correcting common problems often can result in significantly improved performance (IBM benchmarks show as much as 90% performance improvement resulting from some simple changes).

The DASD Component optionally analyzes problems with VSAM data sets. This analysis is a partial automation of the analysis and guidance given in IBM's VSAM Demystified Redbook.

Data set response analysis

The DASD Component optionally can identify periods in which specific data sets experienced DASD response problems. This feature is particularly important in a System Managed Storage (SMS) environment, since SMS manages where data sets are placed.


With the data set response analysis, you specify important data sets and the DASD response objective for each data set.

The DASD Component analyzes information contained in SMF Type 42 (Data Set Statistics) to identify data sets that exceed the DASD response objective. Alternatively, the DASD Component analyzes information contained in SMF Type 14, Type 15, and Type 30 records to identify the devices on which the data sets are located and which applications reference the data sets.

The device response of these devices (contained in the SMF Type 74 records) is compared with the response objective for the data sets.

When the device response time exceeds the response objective for any important data set on the device, the DASD Component performs a detailed analysis of the device.


Sample "rule" description from the DASD Component User Manual.

Return to main page

Last updated by Don Deese on 10/28/04.