Use the input/output problem determination (IOPD) facility to detect pending I/O requests when VTAM® sends a request to another part of the network and no response is received after a certain period of time. The IOINT start option determines the length of time during which a response must be received.

To enable the IOPD facility, issue the MODIFY IOPD command or the MODIFY VTAMOPTS command, or set the IOINT start option.

During the initialization of a large VTAM network, you might see more pending I/O operations than usual. If you use the IOPD facility to track I/O problems during initialization, the number of message groups issued can degrade your network's performance.

To instruct the IOPD facility to write just one message group for each type of pending I/O operation, rather than one group for each operation, use the IOMSGLIM start option. The resulting reduction in the number of messages issued can improve your network's performance during initialization.

The IOPD facility issues messages IST530I or IST1278I, IST1051I, and IST1062I for each operation that is pending longer than the specified time interval. See z/OS Communications Server: SNA Messages for a description of these messages. For more information about event codes and event IDs, see z/OS Communications Server: IP and SNA Codes.

These messages are only an indication that a problem might exist. The longer an operation remains pending (for example, the more messages issued for the same request unit), the more likely it is that a problem exists. See Wait for more information about identifying pending I/O problems.