You can let VSAM select the size of a control interval for a data
set, you can request a particular control interval size in the DEFINE
command, or you can specify data class in DEFINE and use the CISIZE
attribute assigned by your storage administrator. You can improve
VSAM's performance by specifying a control interval size in the DEFINE
command, depending on the particular storage and access requirements
for your data set. See Control Intervals for information about
the structure and contents of control intervals.
Control interval size affects record processing speed and storage
requirements in the following ways:
- Buffer space. Data sets with large control
interval sizes require more buffer space in virtual storage. For information
about how much buffer space is required, see Determining I/O Buffer Space for Nonshared Resource.
- I/O operations. Data sets with large control
interval sizes require fewer I/O operations to bring a given number
of records into virtual storage; fewer index records must be read.
It is best to use large control interval sizes for sequential and
skip-sequential access. Large control intervals are not beneficial
for keyed direct processing of a key-sequenced data set or variable-length
RRDS.
- Free space. Free space is used more efficiently
(fewer control interval splits and less wasted space) as control interval
size increases relative to data record size. For more information
about efficient use of free space, see Optimizing Free Space Distribution.