You can determine steady state by issuing the DISPLAY BFRUSE operator command. CURR TOTAL minus CURR AVAIL gives the number of buffers in use. Repeat this procedure several times, and take the average to determine the steady state value. Be sure to include in your calculation displays taken during times of peak network usage.

Any additional buffers needed can be obtained using dynamic expansion.

To do this, define xpanno to be the number of buffers that fit on one page. To calculate xpanno for I/O buffers, use Table 2, and do the following steps:

1. Find the bufsize range in the table that includes the bufsize value that was specified for your system.
2. Round up the current bufsize to the largest even number that appears in that range. (Some channel-attached SNA devices require that the I/O bufsize be an even number of bytes. Therefore, VTAM® rounds up bufsize to an even number if an odd number is specified.)
3. For one-page expansion, specify an xpanno value that is equal to the number in the left column of this row (the number of I/O buffers for each page).

• Expansion and contraction of buffer pools increase the amount of real storage that VTAM requires. If the buffer pools are continually expanding and contracting, you should change the values specified for xpanno and xpanpt. You can determine what values to specify by monitoring the output of the DISPLAY BFRUSE operator command or the SMS buffer trace and by modifying the values for xpanno and xpanpt accordingly.
• If the expansion point is set too small, VTAM might not be able to expand the buffer pool fast enough if there is a rapid demand for buffers. Setting the expansion point too small also causes the buffer pool to contract frequently.

  Message IST561I might indicate that storage is temporarily unavailable because of this rapid demand. Adjust the xpanpt and xpanno values to eliminate this problem.

  The expansion point for I/O buffers should be larger than the largest single request for buffers. Following are examples of the largest single request:

  • MAXBFRU allocation
  • NetView® session monitor trace buffer size
  • JES/NJE TPBFSIZ
• If excessive expansion and contraction are a problem (especially for LPBUF and CRPLBUF), define these buffers to expand two pages at a time.

  The buffer pool can expand and contract too frequently if the expansion increment and expansion point are not large enough to handle normal fluctuation in the pool. The I/O buffer pool is prone to this type of thrashing from application programs that send very large messages. VTAM always rounds up an expansion increment to fill a multiple of a full page.

Any elements (where an element consists of a buffer, whose size is rounded to a multiple of 8 bytes, plus a 16-byte header) that are required over a page should be controlled by dynamic expansion.

In environments where many sessions come up simultaneously because of either the VARY LOGON operator command or the LOGAPPL operand on a definition statement, you might need to preallocate enough CRPLBUF buffers to handle the influx of requests.

LFBUF is used only for EAS values (on APPL definition statements) less than 30. Because this pool is not used often, a baseno value of 2 is recommended. If you are using TSO, a buffer from the LFBUF buffer pool is used for each TSO user who is logged on. The pool should be expanded one page at a time.

xpanlim is useful in constraining the I/O buffer pool from excessive growth, particularly when a logical unit attempts to flood VTAM with requests (HOT I/O). Typically, the logical unit is malfunctioning; however, the logical unit may be functioning correctly but may not have had proper session parameters specified. Specifying xpanlim allows VTAM to detect such sessions before complete CSA exhaustion and, if the HOTIOTRM Start Option is also specified, terminate such sessions without the system being impacted.

To set xpanlim, issue DISPLAY BFRUSE,BUFFER=IO find the peak usage, and set the limit somewhat above that peak. Because xpanlim is specified in units of 1024 (K) bytes, you will need to convert buffers to KB. Use Table 2 to determine the number of I/O buffers of a given size that fit on a 4K page. Use the equation K=(number of buffers/number of buffers per page)*4.

Also, bear in mind that in order for the expansion limit to be meaningful, CSA must be available were the expansion limit to be reached and VTAM must not reach any CSA limit value that has been specified. In other words, ensure that you have adequate CSA available in the system and your CSA limits allow VTAM to actually allocate buffers up to your expansion limit.