Browse Prior Art Database

Non-Specific Read Cache Sidefile

IP.com Disclosure Number: IPCOM000117155D
Original Publication Date: 1996-Jan-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Bittan, A: AUTHOR [+8]

Abstract

A technique is disclosed for efficiently supporting Non-Specific Read channel commands that address data in the Storage Subsystem Cache without either consuming more Cache space or complicating the existing Storage Subsystem Cache directory.

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Non-Specific Read Cache Sidefile

A technique is disclosed for efficiently supporting

Non-Specific Read channel commands that address data in the Storage Subsystem Cache without either consuming more Cache space or complicating the existing Storage Subsystem Cache directory.

A Non-Specific Read channel command (1) is a channel command that may address data that belongs to devices other than the selected device. Non-Specific Read channel commands execute in the I/O environment defined by the Extended Count Key Data (ECKD) Architecture (2). Non-Specific Read channel commands differ from other standard ECKD channel commands since standard ECKD channel commands only address data that belongs to the selected device. Non-Specific Read channel commands are used to support applications where the knowledge of which data the System needs to read resides in the Storage Subsystem and not the System. In such cases, it is desirable for the System channel to initiate reading from the Storage Subsystem via a Non-Specific Read channel command chain and to allow the Storage Subsystem to choose which data to transfer without regard to which device was selected by the System channel to initiate the Non-Specific Read channel command chain.

The Storage Subsystem's primary function is to provide the System channel access to data on outboard devices called Direct Access Storage Devices (DASD). In order to improve performance, a layer of Cache has been added to many modern Storage Subsystems, e.g., 3990-3 (3). When the System channel addresses data that happens to reside in the Storage Subsystem Cache this is referred to as a Cache hit. Clearly, I/O performance is improved when the frequency of Cache hits is high, and the frequency of Cache hits is obviously affected by the amount of Cache space available for System channel addressable data. Since Cache is a finite resource, it is desirable to use it efficiently and allocate as much space as possible for System channel addressable data.

In order to quickly address data in its Cache, a complex directory structure has been added to the Storage Subsystem. This directory structure is tailored to support fast access by standard ECKD channel commands to data that resides in Cache. Obviously, it is desirable not to add complexity to the existing directory structure.

The dis...