This problem appears to have begun after an upgrade from AMQ Online 1.6.1 to 1.6.2.

On many occasions, broker pods are failing to start, with this error message:

Auto tuning journal ...
java.lang.RuntimeException: Cannot initialize queue:Resource temporarily unavailable
	at org.apache.activemq.artemis.nativo.jlibaio.LibaioContext.newContext(Native Method) 

The "resource" that is exhausted here is AIO IO contexts in the kernel. After a failure, sysctl shows the current settings, which are disturbingly close:

fs.aio-max-nr = 65536
sysctl: fs.aio-nr = 65024

fs.aio-max-nr = 65536
sysctl: fs.aio-nr = 65024

However, these values appear in all brokers, whether they are working or not. The value of 65536 is the default in an OpenShift pod, although it is low compared even to a desktop Linux system. Presumably it's low because all pods on a worker not share the same kernel resources.

The normal use of IO contexts in AMQ 7 is controlled by the setting <journal-max-io>, whose default recently increased from 500 to 4096. However, even the higher value is far smaller than the limit in the pod.

To be honest, I don't know if this is a problem with AMQ Online, or with the broker. The broker configuration generated by the AMQ Online operators seems perfectly sane but, at the same time, I've never seen this problem in a plain AMQ 7 installation.