Browse Prior Art Database

Improved Status Transmission Procedure for FICON

IP.com Disclosure Number: IPCOM000013650D
Original Publication Date: 2001-Jun-02
Included in the Prior Art Database: 2003-Jun-18
Document File: 4 page(s) / 42K

Publishing Venue

IBM

Abstract

BACKGROUND

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This is the abbreviated version, containing approximately 38% of the total text.

Page 1 of 4

Improved Status Transmission Procedure for FICON

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.

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In the new status presentation scheme, the
control unit closes the inbound exchange with
the sending of the status IU, and the channel
closes the outbound exchange with the sending
of the status-accepted IU. The control unit
does not retain resources for the I/O
operation after it sends the status IU, and
therefore can not request a retry of the I/O
operation after sending the status IU even if
a status-accepted IU is not received. When the
channel receives the status IU, it can be sure
that the control unit will never request a
retry for the I/O operation, and it is
guaranteed that both the channel and the
control unit view the I/O operation as
complete. The purpose of the status-accepted
IU is simply to end the outbound exchange.
This protocol eliminates one of the three IUs
which were previously required for the sending
of status, it allows the control unit to fr...