Auxiliary temporary storage: monitoring and tuning

The performance of auxiliary temporary storage is influenced by the characteristics of the VSAM data set DFHTEMP that you set up for temporary storage. It is also affected by the number of VSAM buffers and strings that you specify for the CICS® region.

About this task

The CICS temporary storage statistics show information about the use of auxiliary temporary storage, the use of buffers and strings, and I/O activity. For additional information about data set performance, use RMF or the VSAM catalog.

The cost approximations for auxiliary TS queues do not include any VSAM I/O cost. A VSAM I/O costs approximately 11.5K instructions 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 a control interval from DASD with no available buffer space, if the least recently used buffer must first be written out.

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

Procedure

The following actions can influence the performance of auxiliary temporary storage:

  • You specify a control interval (CI) size when you set up the VSAM data set DFHTEMP. When the use of temporary storage by applications or by CICS changes in your CICS region, verify that the control interval size is still suitable.
    If you write items larger than the control interval size to a temporary storage queue in auxiliary storage, CICS processes the items, but performance might degrade.
    For information about the control interval size, see The control interval size.
  • For more efficient use of DASD space, you can specify secondary extents when you set up the VSAM data set DFHTEMP.
    CICS uses secondary extents if there are no control intervals remaining in DFHTEMP with sufficient space for new data. You can define a temporary storage data set with a primary extent large enough for normal activity, and with secondary extents for exceptional circumstances.
    For instructions to define additional extents, see Defining data sets with multiple extents and volumes.
  • To help ensure that space is not wasted in auxiliary temporary storage, specify expiry intervals in your temporary storage models for nonrecoverable queues.
    Expiry intervals make CICS automatically delete any temporary storage queues that might not be deleted by applications. When an unused queue is deleted from a DFHTEMP control interval, CICS can move the remaining records to the start of the control interval, and use the space for new data. Efficient deletion of old queues can reduce the time required to locate a control interval with free space, and reduce the need to use secondary extents.
    For more information about expiry intervals, see Automatic deletion of temporary storage queues.
  • If you specify the system initialization parameter SUBTSKS=1, CICS runs temporary storage VSAM requests on the concurrent (CO) mode TCB, which could increase throughput.
  • You use the TS system initialization parameter to specify the numbers of VSAM buffers and strings for auxiliary temporary storage in the CICS region. If auxiliary temporary storage is heavily used in the CICS region, you might want to experiment with adjusting these numbers.
    Increasing the numbers of buffers and strings can reduce task waits and VSAM I/O requests, but it also increases storage use in the CICS region.

What to do next

Look out for messages from CICS about high usage of auxiliary temporary storage:
  • CICS issues message DFHTS1316 when auxiliary temporary storage data set usage has reached 75% of the capacity, the initial warning threshold, and repeats this message each time the usage rate increases beyond further warning thresholds.
  • CICS issues message DFHTS1311 if an application attempted but failed to write data because the auxiliary temporary storage is full and cannot be extended. In this situation, applications cannot write to temporary storage queues in auxiliary temporary storage until storage becomes available.

If either of these messages are issued, try to delete old temporary storage queues to free up storage. CICS issues message DFHTS1317 and resets the warning threshold to the initial value of 75% when storage usage falls below 70% of the maximum auxiliary temporary storage.