Optimizing Microsoft SNA gateway with BARR/CHANNEL link service

BARR/NJE

VTAM uses pools of buffers to manage its data traffic. The most important of these buffer pools is the I/O Buffer (IOBUF) pool, which is defined by the IOBUF command in the VTAM startup member, SYS1.VTAMLST (ATCSTR00). The IOBUF pool provides the VTAM input and output buffers for all channel-attached SNA devices that this copy of VTAM has control over, including any FEPs Acronym for Front-End Processors. (1) Generally, computers or processing units that produce and manipulate data before other processors receive it. (2) In communications, computers that are located between communications lines and a main (host) computer and are used to relieve the host of housekeeping chores related to communications. running NCP Acronym for Network Control Protocol. The protocol responsible for negotiating network-layer details related to the transmission of TCP/IP packets over dial-up telephone connections., local 3174s (cluster controllers), and SNA gateways using BARR/CHANNEL. The typical IBM default IOBUF pool consists of 51 buffers each with a size of 256 bytes. Each channel-attached SNA device has an associated MAXBFRU setting which tells VTAM how many buffers from the IOBUF pool VTAM will set aside each time it reads in data from that device.

MAXBFRU is a parameter on the PU level in VTAM that is used for local channel-attached SNA devices. Assuming the IOBUF buffer size (bufsize) is set, the next task is to determine the correct MAXBFRU value for each channel-attached SNA device. The MAXBFRU tells VTAM how many buffers to set aside each time it reads in data from that device. As a general rule, the MAXBFRU times the IOBUF bufsize needs to be greater than or equal to the largest possible packet of information that might be received from this device. This will ensure VTAM will not have to issue two separate read operations to read in one packet of information. This can be summarized in the following formula:

For Ethernet One of the LAN physical standards. It allows multiple stations to access the transmission medium. connections, the largest information packet size must be kept to under 1493 bytes, unless segmentation is acceptable to all of the PUs in the system.

Using the formula above MAXBFRU can be solved as:

Using these system defaults, optimal in most cases, MAXBFRU:

Use this value for the MAXBFRU parameter in the VTAM PU definition. The default value for MAXBTU in Microsoft SNA Manager is 4105. This is also the largest value supported by the BARR/CHANNEL for SNA Server link service device driver.

Format of the IOBUF command

The other parameters relate to how VTAM can expand the pool if it starts to run out of available buffers.

General guidelines and rules for setting the values for IOBUF

Performance Tuning: Tuning the IOBUF pool is not recommended. Because the performance of your entire system can be dramatically affected by the IOBUF settings, careful consideration must be taken before making any changes to these settings. Unfortunately, there is no simple formula for determining the optimum settings for the IOBUF parameter. The recommendations from IBM and other third-party experts generally start by using VTAM monitoring tools such as TNSTATS and the D NET, BFRUSE command to gather performance statistics for your system. Then, by observing various patterns or unwanted events, you can adjust settings like the IOBUF buffer size, pool size, and pool expansion settings.

• IBM requires that your IOBUF pool has a buffer size of at least 182 when there are local SNA devices such as BARR/CHANNEL for SNA Server.

• If you have any local 3174s cluster controllers attached, make sure your IOBUF buffer size is an even number.

• If you decide to change your IOBUF buffer size, be aware that you will have to adjust the MAXBFRU settings for all of your channel-attached SNA devices.

• A larger buffer size is not necessarily better. A larger buffer size might help increase the performance of certain types of applications, but can reduce the performance or response time of other applications. The wrong settings might also result in wasted resources.

See also:

• Sample VTAM PU definition for a BARR/CHANNEL for SNA device