Browse Prior Art Database

Command for Selectively Resetting Allegiance of I/O Device Only if Data Will Not be Lost

IP.com Disclosure Number: IPCOM000105316D
Original Publication Date: 1993-Jul-01
Included in the Prior Art Database: 2005-Mar-19
Document File: 2 page(s) / 84K

Publishing Venue

IBM

Related People

Beardsley, BC: AUTHOR [+3]

Abstract

A new command called Reset Allegiance is executed by an I/O subsystem to attempt to reset the allegiance of an I/O device when a fault prevents resetting allegiance in the normal way. The command resets allegiance only under certain conditions where data will not be lost. The control unit that receives the command returns a byte that tells WHAT allegiance existed at the device, if any, and the result of the attempt to reset the allegiance. If an allegiance was not reset because data might have been lost, the data processing system can then select another recovery procedure.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Command for Selectively Resetting Allegiance of I/O Device Only if Data Will Not be Lost

      A new command called Reset Allegiance is executed by an I/O
subsystem to attempt to reset the allegiance of an I/O device when a
fault prevents resetting allegiance in the normal way.  The command
resets allegiance only under certain conditions where data will not
be lost.  The control unit that receives the command returns a byte
that tells WHAT allegiance existed at the device, if any, and the
result of the attempt to reset the allegiance.  If an allegiance was
not reset because data might have been lost, the data processing
system can then select another recovery procedure.

      Allegiance is needed in a data processing system in which
independent processors share the same device, a magnetic disk storage
device for example.  While a disk is performing an I/O operation, it
returns a busy signal if it gets a request to perform another
operation:  the operation is said to be atomic at the device level.
Some operations must be atomic over a series of steps where the
device would otherwise become not-busy and thus generally accessible.
In a common example, a central processor reads a record on a magnetic
disk, performs some processing operation that changes the record, and
then writes the changed record back on the disk.  To prevent other
users of the record from accessing the outdated copy on the disk
while the new copy exists in the processor, the processor first
performs an operation to reserve the device, then updates the record,
and later performs an operation to reset the allegiance of the
device.

      The new command is useful with devices that set an implicit
allegiance to the path that is selected to perform one or more
related (i.e., chained) I/O operations when the first operation is
initiated by the path.  If the control unit receives a request on a
different path (e.g. from an independent processor), it returns a
busy signal until the operation or operations are completed when the
implicit allegiance is then normally reset at the device.

      The new command is also useful in resetting a contingent
allegiance that is typically set at the device when a fault is
detected at the...