MVS and DASD: performance and tuning
Tuning CICS® for virtual storage under MVS™ depends on several elements: z/OS® systems tuning, z/OS Communications Server SNA tuning, CICS tuning, and VSAM tuning. Because tuning is a top-down activity, ensure that you have already made a vigorous effort to tune z/OS before tuning CICS.
- The installation of a faster processor can cause the current instructions to be executed faster and, therefore, reduce task life (internal response time), because more transactions can be processed in the same period.
- Additional real storage, if page-ins are frequently occurring (if there are more than 5 to 10 page-ins per second, CICS performance is affected), can reduce waits for the paging subsystem.
- Installing faster DASD can reduce the time spent waiting for I/O completion, and this shorter wait time for paging operations, data set index retrieval, or data set buffer retrieval can also reduce task life in the processor.
- Service level objectives are missed.
- Users complain about response times.
- I/O indicators show signs of stress, or you see high DEV DLY or USG for an important workload directly in Monitor III reports.
MVS provides storage isolation for an MVS performance group, which allows you to reserve a specific range of real storage for the CICS address space and to control the page-rates for that address space based on the task control block (TCB) time absorbed by the CICS address space during execution.
You can isolate CICS data on DASD drives, strings, and channels to minimize the I/O contention suffered by CICS from other DASD activity in the system. Few CICS online systems generate enough I/O activity to affect the performance of CICS seriously if DASD is isolated in this manner.
- CICS and a large number (100 or more) of TSO users
- CICS and a large IMS system
- CICS and 5000 - 7500 SNA LU.
- Multiregion option (MRO)
- Intersystem communication (ISC)
- Multiple independent address spaces.
- Poor response times
- Short-on-storage
- Program compression
- Heavy paging activity
- Many well-tested applications suddenly abending with new symptoms
- S80A and S40D abends
- S822 abends
- Dramatic increase in I/O activity on the DFHRPL concatenation or dynamic LIBRARY concatenation.
The rest of this section describes techniques that you can use to improve the performance of CICS under MVS.