Browse Prior Art Database

Allocating Paths to Queued I/O Requests

IP.com Disclosure Number: IPCOM000049968D
Original Publication Date: 1982-Aug-01
Included in the Prior Art Database: 2005-Feb-09
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Easton, MC: AUTHOR

Abstract

This invention relates to a method for allocating path availability to queued I/O requests accessing a common device to which a state variable (activity number) counterpart to each path is incremented when a Start I/O (SIO) issues over that path and is decremented when a channel program is completed. The method contemplates that if at least two paths are free, then the path having the lowest number is assigned. Further, in the event that only one path is free, this path is assigned only if its new activity number following the assignment will not exceed a threshold that depends on both the activity number on another channel and on a parameter THRESH.

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Allocating Paths to Queued I/O Requests

This invention relates to a method for allocating path availability to queued I/O requests accessing a common device to which a state variable (activity number) counterpart to each path is incremented when a Start I/O (SIO) issues over that path and is decremented when a channel program is completed. The method contemplates that if at least two paths are free, then the path having the lowest number is assigned. Further, in the event that only one path is free, this path is assigned only if its new activity number following the assignment will not exceed a threshold that depends on both the activity number on another channel and on a parameter THRESH.

Illustratively, given two IBM System/370 channels with one path from the CPU to a device through each channel, requests for SIO are queued in software until a path is chosen over which the SIO can be issued. The count of I/O's outstanding in each channel is maintained under this method. Whenever an SIO is issued to a channel, its count of outstanding I/O's is incremented. Whenever a completion occurs, the count is decremented.

A path to a device is free if the channel, control unit, and string are available for SIO. In this regard, a path may be free even though its channel has a positive count of outstanding I/O's since all of the outstanding I/O's may be performing disconnected operations such as the dispatch of an arm on a DASD. When an SIO is to be issued and there exists a...