public static String getClusterId(ModelElement clusterElem) {
        String clusterId = clusterElem.stringAttribute("id");
        return clusterId != null ? clusterId : "content";
    }
Returns of memory reserved on a host. Memory is reserved for the jvm if the cluster is combined
config-model/src/main/java/com/yahoo/vespa/model/content/cluster/ContentCluster.java