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RE: file limits see the section on "Configuring the users who start IBM MQ" subsection "Maximum open files" https://www.ibm.com/docs/en/ibm-mq/9.1?topic=linux-configuring-tuning-operating-system
Note: Be sure to validate your ulimits against those in the example. You may obtain them using the command mqconfig. There is additional documentation http://www.bearintegration.com/2021/04/linux-systemd-for-integration-nodes-as.html but the same caveats exist.
Also be aware that if you instantiate mq/ace/iib as a systemd service, using the CLI(e.g. strmqm or mqsistart) to start/stop them rather than starting or stopping the service (systemctl start <service>) will change the service to inactive, even if in fact mq/ace/iib are running.
And if you happened to add the restart clauses
to the service unit file then stopping from the CLI will cause systemd to restart automagically based on the restart configuration.