Browse Prior Art Database

Prevention of Channel Lockout in Serial Input/Output Subsystems

IP.com Disclosure Number: IPCOM000105528D
Original Publication Date: 1993-Aug-01
Included in the Prior Art Database: 2005-Mar-20
Document File: 2 page(s) / 80K

Publishing Venue

IBM

Related People

Beardsley, BC: AUTHOR [+8]

Abstract

In a configuration where multiple systems are sharing data through a DASD control unit it is possible for some systems to be "locked out" from the device by other systems. This "lockout" means that the system is not able to get access to the device due to the device constantly being accessed by the other systems.

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This is the abbreviated version, containing approximately 52% of the total text.

Prevention of Channel Lockout in Serial Input/Output Subsystems

      In a configuration where multiple systems are sharing data
through a DASD control unit it is possible for some systems to be
"locked out" from the device by other systems.  This "lockout" means
that the system is not able to get access to the device due to the
device constantly being accessed by the other systems.

      The reason for the "lockout" is that some systems appear to be
faster than others and always respond to the control unit's request
for status presentation (Request Connect/Request In) "faster".  When
this status is in an Owed Device End (ODE) status, the system will
respond by driving a Start Input/Output (I/O) to the control unit,
thereby getting an I/O operation started.  The "slower" system also
responds to the control unit's request for status presentation,
receives the ODE status and drives a Start I/O to the control unit.
However, since it is "slower" than the "faster" system, the device is
already being accessed by the "faster" system and therefore Device
Busy status is presented to the "slower" system.

      In a serial I/O system, the serial channels will always seem to
be slower than the parallel channels due to the additional overheads
involved in serial I/O.  The channels with the longer distances will
always seem slower than the channels at short distances.  Parallel
channels will always appear to be the fastest.  Therefore, the
channel lockout problem occurs in serial and mixed serial/parallel
configurations.

      The solution to the channel lockout problem is to provide a
"round-robin" priority mechanism where each channel gets a sufficient
head start in the ODE status presentation race so that the subsequent
Start I/O has a high probability of success, i.e., it is accepted by
the control unit.  For example, the first ODE presentation would give
a head start to channel A by signalling the Request In/Request
Connect to this channel "x" microseconds before signalling it to the
other channels (which have ODEs).  The second ODE presentation would
give a head start to channel B, and so on.  The value of "x" is...