Improved Status Transmission Procedure for FICON
Original Publication Date: 2001-Jun-02
Included in the Prior Art Database: 2003-Jun-18
BACKGROUND When status for an I/O operation is sent at a time when the connection is no longer needed, the status transmission protocol in the previous FICON specification required the sending of three IUs to present status: The control sends the status IU and leaves the inbound exchange open; the channel sends a status-accepted IU, closing the outbound exchange; and the control unit sends the device-level acknowledgement (DACK) IU, closing the inbound exchange. This protocol requries the control unit to retain resources dedicated to the I/O operation until it receives the status-accepted IU so that the control unit can retry the I/O operation if it does not receive the status-accepted IU. One of the functions of the DACK IU in this protocol is to alert the channel that the control unit has received the status-accepted IU and will therefore never request for a retry of the I/O operation. This protocol guarantees that when the channel receives the DACK IU, both the channel and the control unit view the I/O operation as complete. PROBLEM The above protocol, while guaranteeing that both the channel and the control unit are in synchronization with regard to the presentation of status, is costly in terms of the IUs sent. It also requires resources both at the channel and the control unit to be retained during the sending of the three IUs. Especially for short channel programs consisting of a small number of IUs, the sending of three IUs to present status significantly impacts performance. A method of eliminating one or more of the IUs, and allowing resources to be freed sooner, is desired. DESCRIPTION OF NEW SCHEME FOR STATUS PRESENTATION In the new status presentation scheme, the DACK IU is eliminated when status is presented during a connection for an I/O operation and the connection is no longer needed. This decreases the number of IUs required to present status from 3 to 2, or by 33%. In addition, resources at both the channel and the control unit are released earlier than in the previous FICON scheme. Details regarding the new status presentation scheme are described below.