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PROVIDING MORE THAN 8 CHPIDS TO A CONTROL UNIT USING ADJUNCT CHANNEL SUBSYSTEMS

IP.com Disclosure Number: IPCOM000013281D
Original Publication Date: 2000-Nov-20
Included in the Prior Art Database: 2003-Jun-18

Publishing Venue

IBM

Abstract

The S/390 I/O architecture provides for up to 8 channel paths (CHPIDs) to be defined to a single device. However, today we see controllers being built with as many as 32 interfaces. The architecture currently requires at least one CHPID to be defined per physical interface on the controller. Thus, the limitation of 8 CHPIDs means that the device can only be accessed by a subset of interfaces provided on the controller. This requires manual partitioning by the customer, involving workload and performance analysis. By providing a means to utilize more than 8 CHPIDs to access a device, manual customer workload is reduced, which reduces cost of ownership and complexity; it also provides for the potential for improved efficiency and performance since more resources are available for use when accessing the device. A very straightforward way to provide more interfaces is to simply expand all the relevant fields in the S/390 I/O architecture; this would include the CHPID array in the subchannel and all the bit masks, currently 8 bits each. However, this is a very disruptive change. An alternative, which minimizes the disruption by leaving the subchannel format and fields unchanged, is to create Adjuct Channel Subsystems (ACS) . There are a number of ways to associate the subchannels in the ACS with the same device, represented by a subchannel in the normal Channel Subsystem (CSS). For example, the ACS could have subchannels which corresponded 1:1 with subchannels in the CSS by having the same subchannel number in all the channel subsystems; alternatively, matching device number could be used to associate a subchannel in the ACS with a subchannel in the CSS. More than 1 adjunct channel subsystem can be provided; each adjunct channel subsystem provides an additional 8 CHPIDs by which the device can be accessed. Adjunct channel subsystems thus provide additional CHPIDs by which a device may be accessed; e.g., the normal channel subsystem plus 3 adjunct channel subsystems would permit up to 32 CHPIDs to be used to access a single device. At the same time, all the subchannel fields remain the same size and maintain the same structure as they currently have. 1