Recommended size for IDMS journal fragment interval

Products

• IDMS
• IDMS - Database

Issue/Introduction

What is the recommended size of the IDMS JOURNAL FRAGMENT INTERVAL parameter in the system generation SYSTEM statement?

Currently it is set to 0. How to calculate the optimal size?

Environment

Release: 19.0

Resolution

The  journal fragment interval in the SYSTEM statement of the SYSGEN specifies the maximum number of journal blocks to write to the journal file before the system writes a dummy segment (DSEG) record to the journal file.
The DSEG records are used during warmstart. So this would only be of benefit if the CV abends and needs to perform warmstart. 
There is an overhead in writing the DSEG records so it is recommenced to set the journal fragment interval to 0.
This is documented in section Tuning for Improved Performance and Lower TCO:

Parameter	Recommended Setting	Effect	Benefit
JOURNAL FRAGMENT INTERVAL	0	Writes status information to journals	Reduced I/O to journal, saves CPU, reduces response time, increases throughput

 

However if your journal files are large, a journal fragment interval can reduce the amount of time it takes to perform warmstart.
This is discussed in section Influencing Journaling Performance. It should be  between 100 and half the number of blocks in your journal file.

Considerations

If your journal files are large (in terms of the number of pages), a journal fragment interval can significantly reduce the amount of time it takes to warmstart a DC/UCF system. The warmstart logic goes to the most recently accessed journal fragment and starts its recovery processing from that point. However, because there is overhead required to write dummy segment headers, your journal fragment interval should be at least 100. Choose an interval that is between 100 and half the number of blocks in your journal file.