Tuning I/O operations for multipath channel connections using channels

With MPC channel connectivity attachment, VTAM® performs I/O operations across multiple single-direction channel links. By analyzing statistics provided by VTAM, you can select I/O buffer sizes, data transfer delay, and other options that can improve performance of these I/O operations.

VTAM uses channel programs or Direct Memory Access (DMA) to transmit and receive data. The amount of data that VTAM can read in one operation depends on the size of the read buffer.

Each tuning statistics record contains information about the state of data-transfer operations across the multipath channel connection. Each record contains statistics that cover the time period since the last tuning statistics record was written for that multipath channel connection.

Note: Non-HPDT multipath channel uses a special data block called a sweep that is exchanged with the adjacent host to verify that data has not been lost. A host initiating a sweep request holds all outbound multipath channel transmissions until it receives a sweep reply from the adjacent host.

A sweep is initiated when either of the following occurs:

A timer expires in the host with the higher subarea number.
Receive queue depth in either host is excessive.

The host initiating the sweep sends the sequence number of the last output transmit block. The adjacent host compares this number with its last input transmit block sequence number. The adjacent host then sends a response to the initiating host that includes the adjacent host last output transmit block sequence number. The initiating host makes the same comparison. If the numbers do not match, or the sweep does not complete within a time limit, the multipath channel group will be deactivated. Otherwise, normal flow continues. The tuning statistics contain a count of how many sweeps are initiated by an expired timer and how many are initiated by excessive receive queue depth.