Calculating VTAM I/O buffer pool (IOBUF) storage

You can estimate VTAM® buffer pool storage when you use the distributed data facility of DB2®. Every path information unit (PIU) that enters or leaves VTAM resides in one or more IOBUF buffers.

To calculate the maximum number of buffers that are required for the local DB2 subsystem:

1. Calculate the number of buffers that each PIU occupies, and call it PIUBUF.

   PIUBUF = CEILING(( 29 + RUSIZE ) / BUFSIZE)

   RUSIZE is the length of the RU in bytes. It is assumed to be the same for both session directions. BUFSIZE is the value you specified in the IOBUF pool definition.

   Assume you have a buffer size of 441 bytes, and an RUSIZE of 4096. With these values, PIUBUF would be 10 ((29+4096) / 441, rounded up).

   For channel-to-channel (CTC) and NCP connections, you need to be concerned with the VTAM MAXBFRU value. For CTC connections, MAXBFRU is the number of 4KB buffers allocated to hold the PIUs sent over the channel. If your RU size is 4096 and you allow 29 bytes for the header, then you need to allocate at least 2 4KB buffers. Thus, you need a MAXBFRU value of at least 2.

   When you route data through NCP, MAXBFRU is the number of VTAM IOBUF buffers allocated to hold the PIUs sent to the NCP, which means MAXBFRU must be at least as large as PIUBUF.

2. Calculate the maximum number of IOBUF buffers used by a session, and call it SESSBUF.

   SESSBUF = PACECNT 
   ×
    PIUBUF

   PACECNT stands for pacing count. For this example, assume that pacing is the same in both directions, and it is the same for all modes.

   If pacing is set to 2, then SESSBUF is 20.

3. Calculate the maximum number of sessions that can be active for all modes to all systems and call it SESCNT. Calculate the maximum for each mode, then add those results to get SESCNT.
4. Calculate the maximum number of VTAM buffers used by DB2, and call this DB2BUF. The formula for DB2BUF is based on a worst case scenario, because it assumes that all sessions are used by concurrent conversations.

   DB2BUF = SESCNT 
   ×
    SESSBUF

   If you assume that the maximum number of sessions that can be active is 50 (SESCNT), then 1000 is the number of IOBUF entries required by DB2 in a worst case scenario.

5. Calculate actual VTAM buffer storage consumption used by DB2, and call it STORAGE.

   STORAGE = DB2BUF 
   ×
    (BUFSIZE + 71)

   Each buffer includes 71 bytes for VTAM internal headers.

   So, to continue the above example, an upper value of real storage is estimated as follows:

   1000 
   ×
    (441 + 71) = 500KB