Types of buffer pools

Table 1 lists the types of buffer pools.

Fixed buffer pools are allocated in extended CSA subpool 227 with KEY=6. Pageable buffer pools are allocated in extended CSA subpool 231 with KEY=6.

Table 1. VTAM buffer pools
Buffer pool	Use	Default storage type	Recommendations and requirements
APBUF	Used to provide fixed common storage if VTAM® fails to obtain other storage from the operating system. For buffers not related to I/O.	Fixed	IBM-supplied values are appropriate for most systems.
BSBUF	Used to maintain session information for peripheral nodes for which VTAM performs boundary function. One buffer is required for each boundary LU session (SSCP-PU, SSCP-LU, LU-LU).	Fixed	Set base number to the average number of concurrent boundary LU sessions.
CRA4	Used for scheduling and error recovery.	Pageable	IBM-supplied values are appropriate for most systems.
CRA8	Used for scheduling and error recovery.	Pageable	IBM-supplied values are appropriate for most systems.
CRPLBUF	RPL-copy pool. In general, one buffer is required for each VTAM application program request until the operation is complete.	Pageable	Monitor output from the DISPLAY BFRUSE command to ensure that the number of buffers specified is adequate for your system.
IOBUF	Used for input/output data. Every PIU that enters or leaves VTAM resides in an I/O buffer. This pool is 31-bit addressable.	Fixed	Monitor output from the DISPLAY BFRUSE command to ensure that the number of buffers specified is adequate for your system. Ensure that the largest MAXBFRU value is less than xpanpt minus slowpt. Also ensure that bufsize is greater than or equal to the UNITSZ operand on the HOST definition statement in the NCP. Note: See the z/OS Communications Server: SNA Resource Definition Reference for additional requirements and restrictions concerning the IOBUF pool.
LFBUF	One buffer is required for each active application program with an EAS value (on APPL definition statement) less than 30. If the EAS value is greater than 30, this information is contained in SFBUF. One buffer is required for each TSO user who is logged on.	Fixed	IBM-supplied values are appropriate for most systems.
LPBUF	Used for scheduling and audit trail (error recovery). One buffer is required for each active VTAM process.	Pageable	Set baseno to 9, and set xpanno in the range 4–6 to force 2-page expansion.
SFBUF	Used to contain application program information and LU blocks. One buffer is required for each active application program with an EAS value (on APPL definition statement) greater than or equal to 30.	Fixed	IBM-supplied values are appropriate for most systems.
SPBUF	Used for large message (LMPEO) requests. One buffer is required per concurrent LMPEO send request.	Pageable	IBM-supplied values are appropriate for most systems.
TIBUF	Used to perform input/output operations for CSM-capable protocols. This pool is in 31-bit storage.	Fixed	IBM-supplied values are appropriate for most systems. The default value for this pool is set at a conservative value in case the functions that use this pool are not used. If using the functions that use this pool, use the DISPLAY net,BFRUSE command to monitor usage and then set the BASENO for the pool at the normal high period usage. For HPR and IP - Defining more initial buffers and larger buffer extents may help increase throughput. Defining too few may cause an IP retransmit.
T1BUF	Similar to the TIBUF but larger. Used as a packing buffer by HiperSockets™ accelerator and QDIO. Also used to contain the HPR headers and the media, IP, and UDP headers for an Enterprise Extender connection.	Fixed	IBM-supplied values are appropriate for most systems. The default value for this pool is set at a conservative value in case the functions that use this pool are not used. If using the functions that use this pool, use the DISPLAY net,BFRUSE command to monitor usage and then set the BASENO for the pool at the normal high period usage. For HPR/IP - Defining more initial buffers and larger buffer extents might help increase throughput. Defining too few might cause an IP retransmit.
T2BUF	Similar to the T1BUF but larger. T2BUFs are used exclusively for HiperSockets accelerator and QDIO when the T1BUF is perceived to be of insufficient size to pack all the headers and data. Also used by HPR for all retransmissions over HiperSockets and QDIO.	Fixed	IBM-supplied values are appropriate for most systems. The default value for this pool is set at a conservative value in case the functions that use this pool are not used. If using the functions that use this pool, use the DISPLAY net,BFRUSE command to monitor usage and then set the BASENO for the pool at the normal high period usage. For Enterprise Extender connections using QDIO/iQDIO device drivers, defining more initial buffers and larger buffer extents might help increase throughput. Defining too few might cause an IP retransmit.
XDBUF	Used for physical I/O to VTAM peripheral nodes for connection activation (for example, XID exchange processing). This pool is 31-bit addressable.	Fixed	IBM-supplied values are appropriate for most systems. Because this buffer pool handles the concurrent activation of physical units, you should specify a small size and enable dynamic buffering. The buffer pool can then expand to accommodate the concurrent activation and can shrink after the resources are active.