You can either specify a data control interval size or default
to a system-calculated control-interval size. If you do not specify
a size, the system calculates a default value that best uses the space
on the track for the average record size of spanned records or the
maximum record size of nonspanned records.
If a CONTROLINTERVALSIZE value is specified on the cluster level,
this value propagates to the component level at which no CONTROLINTERVALSIZE
value has been specified.
Normally, a 4096-byte data control interval is reasonably good
regardless of the DASD device used, processing patterns, or the processor.
A linear data set requires a control interval size of 4096 to 32768
bytes in increments of 4096 bytes. However, there are some special
considerations that might affect this choice:
- If you have very large control intervals, more pages are required
to be fixed during I/O operations. This could adversely affect the
operation of the system.
- Small records in a data control interval can result in a large
amount of control information. Often free space cannot be used.
- The type of processing you use can also affect your choice of
control interval size:
- Direct processing. When direct processing
is predominant, a small control interval is preferable, because you
are only retrieving one record at a time. Select the smallest data
control interval that uses a reasonable amount of space.
- Sequential processing. When sequential
processing is predominant, larger data control intervals can be good
choices. For example, given a 16 KB data buffer space, it is better
to read two 8 KB control intervals with one I/O operation than four
4 KB control intervals with two I/O operations.
- Mixed processing. If the processing
is a mixture of direct and sequential, a small data control interval
with multiple buffers for sequential processing can be a good choice.
If you specify free space for a key-sequenced data set or variable-length
RRDS, the system determines the number of bytes to be reserved for
free space. For example, if control interval size is 4096, and the
percentage of free space in a control interval has been defined as
20%, 819 bytes are reserved. Free space calculations drop the fractional
value and use only the whole number.
To find out what values are actually set in a defined data set,
issue the access method services LISTCAT command.