Browse Prior Art Database

IBM 3990 Microcode Static Switch

IP.com Disclosure Number: IPCOM000121086D
Original Publication Date: 1991-Jul-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 4 page(s) / 179K

Publishing Venue

IBM

Related People

Beardsley, BC: AUTHOR [+5]

Abstract

The Transaction Processing Facility (TPF) operating system does not support alternate path addressing to a given DASD, but both the IBM 3880 and the IBM 3990 control units provide this capability. The IBM 3990 assumes that the host operating system supports alternate paths. As a result TPF would only be able to utilize half of the paths of the IBM 3990 without modification to the IBM 3990 itself. If the TPF operating system would access an unmodified 3990 and it had paths to all of the subsystem paths, it would not be able to successfully initialize itself because it would find a given device on multiple interfaces. If this occurred, either initialization would fail or TPF would not recognize the device as the same device on an alternate channel.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 48% of the total text.

IBM 3990 Microcode Static Switch

      The Transaction Processing Facility (TPF) operating
system does not support alternate path addressing to a given DASD,
but both the IBM 3880 and the IBM 3990 control units provide this
capability.  The IBM 3990 assumes that the host operating system
supports alternate paths.  As a result TPF would only be able to
utilize half of the paths of the IBM 3990 without modification to the
IBM 3990 itself.  If the TPF operating system would access an
unmodified 3990 and it had paths to all of the subsystem paths, it
would not be able to successfully initialize itself because it would
find a given device on multiple interfaces.  If this occurred, either
initialization would fail or TPF would not recognize the device as
the same device on an alternate channel.

      For the IBM 3880 the problem could be avoided by using the
Static Switch RPQ which is a modification of the DASD itself.  This
RPQ is not supported on the most current versions of the IBM 3880
DASD or the IBM 3390 DASD.

      The modifications to TPF to use alternate pathing are rather
complex and include changes to their data sharing support.  The TPF
data sharing support uses the Limited Lock RPQ which assumes no
alternate paths to the DASD.

      The Microcode Static Switch allows TPF to operate with the IBM
3990 without the changes to TPF required to support alternate paths
or to change their data sharing mechanism. Further, the solution
provides the multi-path reconnection within the system while it
provides a user interface to the device that is simpler to use and
understand than the previous Static Switch RPQ.  The multi-path
reconnection within the subsystem improves both the performance and
RAS characteristics of the subsystem in comparison to previous static
switch solutions.

      The IBM 3990 is installed as either one 4-way subsystem or two
2-way subsystems.  In either case channel access to a device is
allowed through two storage directors.  In the 4-way case, each
storage director consists of two storage paths, thus allowing four
paths to each device.  In the 2-way case each storage director
consists of a single storage path, allowing 2 paths to each device.

      In this type of configuration all devices are addressable
through Paths 0, 1, 2, and 3 (see Fig. 1). Provided that a given host
has multiple channels which connect to the subsystem, each of those
channels can access any attached DASD.  In an environment in which
alternate paths (channels) are not supported the performance
capabilities of the box are constrained.  The IBM 3990 can also be
configured with 2-way attachment of DASD, in which case a device is
addressable through Paths 0 and 2 or Paths 1 and 3.

      The Microcode Static Switch involves a modification to the
microcode to limit the channel access of a given device to a given
storage director while still allowing the addressing of the DASD via
all storage paths in the subs...