Increasing Number of CHPIDs (Channel Path Identifiers) for LCUs (Logical Control Units)

The channel subsystem (CSS) and the IBM z/OS operating system need to know what hardware resources are available in the computer system and how these resources are connected. This information is called hardware configuration. Hardware Configuration Definition (HCD) provides an interactive interface that allows you to define the hardware configuration for both a processor’s channel subsystems and the operating system running on the processor (1).

A channel is the piece of hardware with logic in the CPC (Central Processing Complex) and to which you connect a cable in order to communicate with an outboard device. FICON channels are used in recent configurations. Channel path identifiers (CHPIDs) are the 2-byte identifiers for channels. You use a CHPID to identify a channel to the hardware and software in the HCD. Although the two terms are often used interchangeably, we refer to attaching a control unit to a channel and using the CHPID in a z/OS CONFIG command to identify the channel you want to bring online or offline (2).

256 devices are supported for CU (Control units). LCU definitions are used to circumvent this limitation. Multiple CUs are defined for each block of 256 devices. Most zOS shops use a set of 8 channels for all LCUs. As long as channel utilization is below 90% percent, there is no problem and these definitions can be used forever. If channel utilization closes maximum steadily, then some channel definitions should be added in hardware configuration.

Following is the view of control unit definition panel in Hardware Configuration Definition:

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As we cannot define more than 8 channels per LCU, we define channels in a circular way. Suppose we have channels A1, A2, A3, A4, A5, A6, A7 and A8. Further suppose that existing LCU definitions are as follows:

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We will add 4 more channels like B1, B2, B3 and B4. We will add CHPIDs in a circular fashion to have channels distributed evenly. New definition would be as follows:

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Since number of LCUs may not be the multiple of channels, usage of some channels would be one less then others. In our case use counts of channels A5, A6, A7, A8, B1, B2, B3 and B4 will be one less than others.

After performing modifications in a new work file, create a new production I/O definition file. You can write I/O configuration to one of IOCDS locations in the processor complex.

The problem may start when you try to activate I/O configuration dynamically. In a complex I/O configuration environment, you could receive system abend 878 with reason code 10:

IEA705I ERROR DURING GETMAIN SYS CODE = 878-10 IBMUSER IKJACCNT IKJACCNT

You increase TSO address space region to a point where you may not get more virtual storage and you may not activate I/O configuration dynamically:

IEF085I REGION UNAVAILABLE, ERROR CODE=20

IEF187I IBMUSER FAILED – SYSTEM ERROR IN INITIATOR

IEF450I IBMUSER IKJACCNT IKJACCNT – ABEND=S822 U0000 REASON=00000014

Even if you use ACTIVATE console command to test or activate configuration dynamically, you may not do so due to below messages:

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But these messages reminds you channel path and device associations not used any more. You have to VARY following paths OFFLINE using console commands similar to VARY PATH(C100-C1FF,A5),OFFLINE command:

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After varying unused paths, you test and activate new configuration successfully. Next thing is to keep an eye on RMF (Resource Management Facility) or BMC Mainview CMF Monitor reports (whichever you use) to observe a better-balanced I/O distribution.

___

Notes:

(1) z/OS Hardware Configuration Definition User’s Guide

(2) I/O Configuration Using z/OS HCD and HCM

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