OpenNMS ships with the thresholds for some events already defined. For example, there is a memory threshold defined as:
<group name="netsnmp-memory-nonlinux" rrdRepository="/opt/opennms/share/rrd/snmp/">
<expression type="low" expression="memAvailReal / memTotalReal * 100.0" ds-type="node" ds-label="" value="5.0" rearm="10.0" trigger="2"/>
ie. if free memory drops below 5% then an event will be created. The alert will be cancelled automatically if free memory subsequently rises above 10%
I wanted to configure some specific nodes with a different threshold, eg. generate an event when free memory drops below 2.5%.
Here's what I did.
I had a utility server running RHEL 6.2 (I installed it as part of a RHEV evaluation process). However, I have no RHEL entitlements so am not able to get updates.
So, I converted it to CentOS 6.2, with a little help from this post:
yum clean all
rpm --import RPM-GPG-KEY-CentOS-6
rpm -e --nodeps redhat-release-server
rpm -e yum-rhn-plugin rhn-check rhnsd rhn-setup
rpm -Uhv --force *.rpm