Guidelines for setting UNITSZ

To choose the best UNITSZ value, first determine the average size of inbound and outbound messages. Then, compare them to each other, and set the UNITSZ equal to the larger of the inbound or outbound message sizes. For channel-attached cluster controllers add 16.

Note: The message size is not determined by VTAM®. You can determine the average message size by examining your application programs (for example, CICS® and IMS™). You can also use the NetView® Performance Monitor to analyze the average message size for your network.

Suppose the inbound size is much smaller than the outbound size. Choose a UNITSZ at least large enough to contain one complete inbound message. However, the UNITSZ should be larger than the inbound message so that relatively few buffers are needed for the outbound message. Because the extra buffers increase path length, it is better not to specify too many buffers.

As another example, consider the case where the inbound message size is much greater than the outbound size. The process is similar to the previous one, except that the outbound message is the guide for choosing UNITSZ. Choose a UNITSZ that is greater than or equal to the outbound message size so that relatively few buffers are needed to contain one inbound message.

Where practical, choose UNITSZ so that no more than five to seven buffers are needed for transferring one message, whether it is inbound or outbound.