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Direct And Indirect Addressing of Peripheral Devices

IP.com Disclosure Number: IPCOM000100934D
Original Publication Date: 1990-Jun-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 3 page(s) / 132K

Publishing Venue

IBM

Related People

Beardsley, BC: AUTHOR [+3]

Abstract

In direct-access storage device (DASD) peripheral data storage subsystems, the devices can be paired, with one device in each pair being a primary device and a second device in each pair being a secondary device. A connected host processor stores data and accesses data only from the primary device. The peripheral subsystem automatically makes a second or dual copy of the data in the secondary device. In the event of the failure of the primary device, the subsystem accesses the data storage area on the secondary device. On occasion, it is desired for the host to access the secondary device while not accessing the primary device. An example of such access is for diagnostic and identification purposes.

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Direct And Indirect Addressing of Peripheral Devices

       In direct-access storage device (DASD) peripheral data
storage subsystems, the devices can be paired, with one device in
each pair being a primary device and a second device in each pair
being a secondary device.  A connected host processor stores data and
accesses data only from the primary device.  The peripheral subsystem
automatically makes a second or dual copy of the data in the
secondary device.  In the event of the failure of the primary device,
the subsystem accesses the data storage area on the secondary device.
On occasion, it is desired for the host to access the secondary
device while not accessing the primary device.  An example of such
access is for diagnostic and identification purposes.  The subsystem
includes a joint array structure (JAS) which includes the
identification of the devices in each group or pair of devices.  A
perform subsystem function (PSF) command includes subsystem
designators which either select a primary device for normal
operations, a secondary device for specific operations, or command a
secondary device to broadcast through all of the host processors a
device state transition signal.  Such broadcast identifies the
secondary device to all of the host processors.  The subsystem does
the indirect addressing to the secondary device, relieving the host
processor of addressing responsibilities.

      When two DASD devices are grouped into a duplex pair, one
becomes a primary device (DP) and the other one becomes a secondary
device (DS).  The secondary device address is not available to the
host processor for DASD access commands.  All write operations to the
primary address automatically update the secondary, all within the
peripheral subsystem by programming in the controllers.  If one of
the devices in a group develops a media problem, a perform subsystem
command PSF is used to allow operation through the primary address to
be routed to either the primary or secondary device without affecting
the other device in the pair.

      If the Directed I/O includes a write operator, the duplex pair
must be in failed duplex.  If the write is to the secondary, the
updated cylinder will be marked "out of sync".

      In a DASD peripheral subsystem, dual copy provides high data
availability and reliability by maintaining two copies of data to
which this attribute is to apply.  This is accomplished by supporting
the definition of a set of physical volumes as a group called duplex
pair.  Once the duplex pair is established, an image of the duplex
pair is a single, extremely reliable "volume".  Both volumes of the
pair are updated as a result of any write.  Reads to dual-copy
volumes use normal cache algorithms.  If the data to be read is not
in cache, it is read from the primary device.  Reads which fail on
the primary device of the duplex pair are attempted on the secondary
device of the duplex pair.  Since the volume of a...