REFT4 control statement

This optional control statement is used as a reference threshold value. The REFT4 threshold value helps determine whether chained sequential I/O or direct I/O should be performed.

When the number of referred-to OSAM blocks, OSAM LDS Control Intervals, or ESDS Control Intervals (CIs) is below REFT4, CAB usually performs direct I/O. When the number of referred-to OSAM blocks, OSAM LDS CIs, or ESDS CIs has reached REFT4, CAB usually performs chained sequential I/O.

The REFT4 setting helps determine how often direct I/O and chained sequential I/O are performed. If the REFT4 setting is too low, CAB may perform chained sequential I/O in cases where direct I/O is superior. Some of the OSAM blocks, OSAM LDS CIs, or ESDS CIs read in chained sequential mode are not referred to at this time. If the REFT4 setting is too high, CAB may perform direct I/O in cases where chained sequential I/O is superior. CAB reads consecutive OSAM blocks, OSAM LDS CIs, or ESDS CIs individually in direct mode, instead of reading them together in chained sequential mode.

Usually, you do not need to code the REFT4 control statement. Consider coding the control statement only when the default value is not satisfactory. The recommended range of values for REFT4 is between 100% and 200% of RANSIZE. For example, when RANSIZE=8, the recommended range of values for REFT4 is from 8 to 16 inclusive.

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REFT4 n

Position: Description
1: Code the REFT4 keyword to change the REFT4 threshold.
7: Code the numeric value n to be left-aligned.
The default value is equal to the RANSIZE value.