Protect your IMS Local Buffers with Sysplex Manager

The IBM IMS TM Shared Queue environ­ment resolved issues with buffers in shared queues, but some buffer issues still remain, because some messages are kept in local buffers and not on the shared queue. Mes­sages that are kept in local buffers include serial transactions, command output and master terminal messages, and the first segment of a multi-segment message.

IMS will expand and contract the local buf­fer pool when needed. The following IMS parameters control the buffer pool expan­sions and contractions:

• QBUF= The initial number of queue buffers allocated. Min=3 and max=9999.
• QBUFMAX= The maximum number of queue buffers allocated. Min=200 and max=9999. If not specified, an unlimited number of buffers can be allocated.
• QBUFHITH= The high threshold percentage. When the buffer reaches the high threshold, it is dynamically expanded until it reaches the QBUF­MAX limit. The default is 80%.
• QBUFLWTH= The low threshold percentage. When the buffer reaches the low threshold, it is compressed (compression occurs only when the number of buffers exceeds the QBUF number). The default is 50%.
• QBUFPCTX= The percentage of the initial queue buffers that are dynami­cally expanded when the QBUFHITH (high threshold) is reached. The default is 20%.
• QBUFSZ= The buffer size in bytes. It must be greater than or equal to the value for LGMSGSZ=. The maximum size can be 30 632 bytes. If not specified, the value for LGMSGSZ= is used.

The problem that IMS administrators are facing is that when they do not specify QBUFMAX, or when they specify QBUF­MAX=0, there is no limit to the number of buffers that IMS will use, which can lead to memory usage problems. If they specify QBUFMAX, what value should it be? When IMS reaches the limit they set, IMS will cause an abend or freeze because local buffers are needed for IMS messages. How can this be prevented?

The IMS Sysplex Manager product from IMSTOOLS has a new feature for Shared Queue Local Buffer Protection. This new feature is built on top of the IMS exit routine DF­SQSSP0. The new feature tracks when local buffers are allocated and when local buffers are freed. It can report on buffer usage, and it can control buffer allocations by applica­tions. It is designed to run under the Generic QSN (Queue Space Notification) Exit Facility that allows the feature to be dynamically enabled and disabled.

This new feature:

• Monitors the use of the local buffers
• Reports buffer usage information
• Identifies large buffer users
• Issues warning messages when user-defined buffer thresholds are crossed
• Protects IMS from running out of local buffers

The new feature can run in one of several modes. Users have full control over the opera­tion through new IMS Sysplex Manager parameters. Users can specify a run mode of either INACTIVE, REPORT, or ENFORCE.

INACTIVE mode: The feature is inactive, and no action is taken. This mode can be used to disable the feature.

REPORT mode: There is no buffer protection enforcement in this mode. The REPORT mode is still very useful because users can track actual buffer usage. The feature writes buffer usage information to the MVS console and to the IMS master terminal. Users have full control over the messages generated in REPORT mode. They can specify when the reporting should start and how frequently it should be done.

In REPORT and ENFORCE modes, the feature also collects information on large buffer users. This information is written periodically to the MVS console and to the IMS master terminal for review.

ENFORCE mode: Use this mode to protect an IMS system from running out of local buffers. The exit looks at the IMS local buffer pool as being in one of three user-defined states: WARNING, ACTION, or CRITICAL. These states are defined by the user as a percent of all buffers used.

When buffer usage reaches the WARNING level, the feature periodically reports usage in­formation but takes no action. Here is an example with the warning level set to 10%, and the reporting interval set to 3 minutes:

When buffer usage reaches the ACTION level, the feature periodically reports usage information and starts failing the allocate buffer calls for large buffer users. Large buffer user applications will get an error return on the next message insert:

When buffer usage reaches the CRITICAL level, the feature periodically reports usage information and fails any new buffer allocation by any application. Users should set this threshold high, 90% or higher. The feature prevents the IMS abend and freezing problem because it prevents applications from making buffer allocations. The feature does not interfere with buffer allocations needed by IMS for IMS messages:

In summary, the IMS Sysplex Manager provides you with a flexible tool to protect local IMS buffers in a shared queue environment. You can manage IMS memory usage better by limiting the amount of buffers that IMS can use when using QBUFMAX without the fear of causing an IMS abend or freezing IMS because it ran out of buffers.