I wanted to change default Xmx value for child containers (seems to be set to 512m by default, don't find where does this value come from. Not from ZooKeeper afaik) but the JVM options (I entered -Xmx256m or Xmx256m) are not taken into account. The instance.properties file in instance folder always contains the following options: item.1.opts=-server -Dcom.sun.management.jmxremote -Dzookeeper.url\="P198829\:2181" -Xmx512m.
Same thing regarding the Maven Proxy URL: no matter the URL I provide, I cannot find it in the container settings.
Yes, so the JVM arguments not being added to the script is a known issue in Fuse Fabric and will be fixed in a future release.
The maven proxy URL setting is used by the script that is pushed out to a remote machine to download a copy of the Fuse Fabric distribution. It should then also be set in the fabric registry so that agent can download artifacts according to it's profile, this wouldn't show up in the etc/ directory. If you go to the command line of your FMC console you can do "features:install fabric-zookeeper-commands", then do "zk:list -r -d | more" and it should be that output, can't recall the exact key that's stored under.
So unfortunately I think the bit of metadata that we're looking for is currently stored as a binary blob in the fabric registry, so would need to be unserialized using the fabric API. I'll log a ticket to store this in a more user-friendly format, in which case you could use fabric:export, edit the file and then use fabric:import to update it.