For channel-to-channel connections, the MAXBFRU operand defines a constant sized buffer in increments of 4K pages. The value does not fluctuate throughout the life of the connection. This buffer must be defined as large as the largest PIU to cross the channel-to-channel connection. Also, if a nonzero DELAY value is chosen and you want to transfer multiple RUs in a single I/O operation, the MAXBFRU value must also be large enough to handle multiple RUs.

After the data transfer operation is complete, the PIUs are deblocked into VTAM® buffers as defined in the IOBUF start option. The VTAM buffers are allocated in 4K increments, so you might have to allocate more I/O buffers. If you are using dynamic buffer expansion, you should monitor the I/O buffers pool to ensure that there is not excessive buffer pool expansion and contraction because of these channel operations.

The maximum PIU size across a connection is defined by MAXBFRU times IOBUF size. For channel-attached cluster controllers, the maximum PIU size is defined by MAXBFRU times (IOBUF size minus 16).

If all LU-LU sessions end in a physical unit that provides segment assembly, path information unit (PIU) segmentation occurs across the link. If any LU-LU sessions end in a physical unit that does not provide segment assembly, the maximum PIU size must be large enough to handle RU sizes for these sessions. If the LU-LU session cannot have segments, either increase the value of the MAXBFRU operand or the I/O buffer sizes, or adjust the RU size in the MODEENT macroinstruction for the logical unit.