Browse Prior Art Database

Allowing Cache Accesses on Secondary Address

IP.com Disclosure Number: IPCOM000106237D
Original Publication Date: 1993-Oct-01
Included in the Prior Art Database: 2005-Mar-20
Document File: 4 page(s) / 102K

Publishing Venue

IBM

Related People

Legvold, V: AUTHOR

Abstract

A method for accessing data that exists only in the cache while the physical device is being used by another channel is disclosed. This method has been implemented on the 3990 Control Unit but applies to all control units that support selecting the logical device without selecting the physical device.

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

Allowing Cache Accesses on Secondary Address

      A method for accessing data that exists only in the cache while
the physical device is being used by another channel is disclosed.
This method has been implemented on the 3990 Control Unit but applies
to all control units that support selecting the logical device
without selecting the physical device.

      Before read and write operations are allowed a secondary
address must be established that can be used to perform cache only
operations.  The secondary device must be a device that is not being
used for other activity.  The physical device for the secondary does
not need to exist.  The secondary device will assume all the
attributes of the primary device, e.g., device type.

      The Figure shows a 3990 with a single string (30) of attached
DASD connected to two channels (1) and (2).  The 3990 contains a
Logical Device Control Block (10) and a Physical Device Control Block
(20) for each possible device.  The 3990 supports two strings or 64
devices.  When the 3990 receives a request from a channel,
information is stored in the appropriate Logical Device Control Block
(10).  If the physical device (30) must be accessed, the Physical
Device Control Block (20) for the device is required.

      The secondary address can either be defined by convention or
could be established by the user using an adaptation of the Dual Copy
Establish Perform Subsystem Function order.  The initial prototype
was done on the 3990 Record Caching RPQ.  This RPQ limits the number
of devices to a single string of 32 devices (30) while control blocks
exist for 64 devices.  By convention, the RPQ will use the
corresponding device in the string that is not installed as the
secondary address.  The secondary structure for Device 0 (10) would
be Device 32 (11).  The Physical Device Control Blocks (21) for
devices that are not installed will never be used.

      When a device disconnects, the 3990 will use the control area
(11) of the alternate device to process caching operations for the
primary device (10).  If the host drives a chain that requires the
physical device...