Extrapartition transient data considerations

Extrapartition destinations are, in practice, sequential data sets where CICS® uses the QSAM PUT LOCATE or PUT MOVE commands.

Why CICS region waits occur and how to avoid them

The main performance factor to note is the possibility of operating system waits; that is, the complete CICS region waits for the I/O completion. A lengthy wait can occur for one of the following reasons:
  • No buffer space is available.
  • Secondary space is allocation.
  • Volume (extent) switching is available.
  • The data set has been opened or closed dynamically.
  • A forced end of the volume has been caused by the application.
  • The data set is defined on a physical printer and the printer has run out of paper.
  • A RESERVE command has been issued for another data set on the same volume.
Therefore, try to eliminate or minimize the occurrences of CICS region waits by:
  • Having sufficient buffering and blocking of the output data set
  • Avoiding volume switching by initially allocating sufficient space
  • Avoiding dynamic OPEN or CLOSE actions during peak periods.

Comparing extrapartition transient data with user journal

An alternative method of implementing sequential data sets is to employ a CICS user journal. Table 1 summarizes the differences between these two methods.

Table 1. Extrapartition transient data versus user journal
Extrapartition TD User Journal
Region (CICS) may wait Task waits
Buffer location: In MVS storage Buffer location: In DSA
Number of buffers: 1 - 32767 2 buffers
Input or output Both input and output, but tasks may wait
Accessible by multiple tasks
  • Accessible for output by multiple tasks
  • Accessible for input by single task under exclusive control

I/O cost in extrapartition TD queues

The approximate calculations for performance costs in extrapartition TD queues do not include any I/O cost. An I/O operation for a physically sequential file costs approximately 7 K and occurs in the following situations:
  • When attempting to write an item that does not fit in any buffer.
  • When reading an item that is not in the buffer.
  • When reading data from DASD and there is no available buffer space. If this situation occurs, the least recently used buffer must first be written out.

Therefore, under certain circumstances, a READQ could incur the cost of two I/O operations.

Extrapartition TD queues are nonrecoverable

WRITEQ READQ
1.2 1.0

Using indirect destinations for combining the output of several destinations

To avoid specifying extrapartition data sets for the CICS-required entries (such as CSMT and CSSL) in CSD definitions for TD queues, you are recommended to use indirect destinations for combining the output of several destinations to a single destination. This saves storage space and internal management overheads.

Long indirect chains can, however, cause significant paging to occur.