Controlling buffer storage

VTAM® uses buffer pools for control blocks, network traffic data, and channel programs. A shortage of buffer pools can have an adverse effect on VTAM CPU time, storage consumption, and the ability to serve DB2® requests.

Procedure

To control buffer storage:

  • Monitor VTAM buffer pools by using one of the following methods:
    • The VTAM command DISPLAY NET,BFRUSE
    • A VTAM trace, obtained by entering the following z/OS® MODIFY command: 
      F procname,TRACE,TYPE=SMS,ID=VTAMBUF

      Procname in the command is the VTAM start procedure name. The data is collected by the generalized trace facility (GTF).

  • Use any of the following methods to prevent shortages of these VTAM buffers (IOBUFs):
    Increase the number of IOBUF buffers

    The IOBUF pool definition is one of the VTAM start options. You can enter the IOBUF option from the z/OS console, or you can include it at VTAM startup in SYS1.VTAMLST in member ATCSTRxx.

    Tuning the IOBUF pool encompasses both base allocation and dynamic expansion values. At installation, you can specify a base allocation for the IOBUF pool (in number of buffers) and a dynamic expansion (in number of buffers). When storage runs short in the buffer pool, VTAM temporarily expands the IOBUF pool by the dynamic expansion value, based on a trigger which you can also specify in VTAM definitions. Recommendation: Set a maximum size for the IOBUF pool size using the xpanlim start option for the buffer pool. If you turn off pacing accidentally, xpanlim prevents DB2 from causing VTAM to grab unlimited amounts of storage.

    Decrease the session level pacing count

    Pacing is vital for controlling the potentially large amounts of data that are transferred around the network.

    Decrease the number of concurrent conversations

    You can reduce the number of concurrent conversations by reducing the number of sessions.

    Decrease the request unit (RU) size

    The RUSIZES option is part of the mode entry statement.

    Because reducing the number of sessions and the RUSIZES value can adversely affect performance, you should first consider increasing IOBUF buffers and decreasing the session pacing count.