CICS VSAM and file control: Performance and tuning
This section describes performance tuning issues related
to VSAM and file control.
Subtopics
VSAM tuning: General objectives
Tuning consists of providing a satisfactory level of service
from a system at an acceptable cost. A satisfactory service for VSAM
is likely to be obtained by providing adequate buffers to minimize
physical I/O, and allowing several operations concurrently on the
data sets.
Using VSAM subtasking
The optional concurrent (CO) mode TCB is used for processes
that can safely run in parallel with other CICS activity
such as VSAM requests. The SIT keyword SUBTSKS has
been defined to have numeric values (0 and 1) to specify whether there
is to be a CO TCB. The system initialization parameter, SUBTSKS=1,
defines that subtasking is to be used.
Using data tables
Data tables enable you to build, maintain, and have rapid
access to data records contained in tables held in virtual storage
above the 16 MB line. Therefore, they can provide a substantial performance
benefit by reducing DASD I/O and path length resources. The path length
to retrieve a record from a data table is shorter than the path length
to retrieve a record that is already in a VSAM buffer.
Using coupling facility data tables
The API used to store and retrieve the data from a coupling
facility data table (CFDT) is based on the file control API used for
user-maintained data tables.
Using VSAM record-level sharing
VSAM record-level sharing (RLS) is a VSAM data set access
mode, introduced in DFSMS, and supported by CICS. RLS enables VSAM data to be shared, with
full update capability, between many applications running in many CICS regions. With RLS, CICS regions that share VSAM data
sets can reside in one or more MVS images
within an MVS sysplex.
Threadsafe file control applications
By default, CICS forces file control commands issued by
threadsafe applications to run on the QR TCB. If you change the system
initialization parameter FCQRONLY to specify
NO, file control commands for local VSAM LSR or RLS files can run
on an L8 or L9 TCB.
File control API costs
For read operations, the VSAM I/O cost is not included
because the need to access DASD depends on the workload. For the read
operation to complete, both the index and data must be accessed. If
the index or data are not in a buffer, an I/O operation is required
for each level of index and one for the data.
Last updated: Thursday, 27 June 2019