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OMVS HIGH CPU with SYSPLEX=YES

Question & Answer


Question

In a shared file system configuration, high CPU consumption for the OMVS address space and increased XCF traffic might result on the file system owner when an application running on a client system heavily accesses the remotely owned file system.

Cause

OMVS sysplex worker threads on the file system owner will be heavily used to service the client requests.

Answer

Shared file system configuration can be determined with a D OMVS,O command and verifying SYSPLEX=YES.
Periodically issuing the D OMVS,W (waiters) command on the system experiencing the high CPU consumption will give information about the cross-system requests in progress. The msgBPXO063I 'RECEIVED SYSPLEX MESSAGES:' section will show the ASID and TCB for the job originating the request on the client system, and the target file system for the request. The D OMVS,A=ALL command can be used to associate the ASID with a jobname on the client system.
Using this information, a decision can be made if it is reasonable to move ownership of file systems to the system where the applications are running to reduce the CPU consumption caused by sysplex worker threads handling cross-system requests.
There is an as-is tool named WJSIPUSE in the z/OS Tools and Toys repository which can also be used to identify file systems encountering heavy cross-system requests (function-shipping).   (This tool is not applicable if the cross-system requests are related to FIFOs or BRLM).

As a short-term solution, file system ownership can be moved via the TSO ISHELL File_systems pull-down menu.
- Select option 1. Mount table

- From the 'Work with Mounted File Systems' panel, select the file system to be moved by using the M=Modify option.

-Use option 2. 'Change Owning system from (sysname)' to move the file system ownership

-Optionally: use option 3. 'Change AUTOMOVE attribute' (via an INCLUDE list) to limit the systems where the file system ownership can be automoved due to a serving system going down. These changes can be made dynamically via the SETOMVS command and then changed permanently by updating the MOUNT statement parameters for the file system in the BPXPRMxx member. 

-Long-term consideration: Decide whether the file system should be configured as sysplex-aware / RWSHARE.  See the "Using zFS read/write sysplex-aware file systems" section of the zFS Administration Guide for details.

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Document Information

Modified date:
27 March 2025

UID

isg3T1010376

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