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"/>
</group>

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.

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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
mkdir ~/centos
cd ~/centos
wget http://mirror.centos.org/centos/6.2/os/x86_64/RPM-GPG-KEY-CentOS-6
wget http://mirror.centos.org/centos/6.2/os/x86_64/Packages/centos-release-6-2.el6.centos.7.x86_64.rpm
wget http://mirror.centos.org/centos/6.2/os/x86_64/Packages/yum-3.2.29-22.el6.centos.noarch.rpm
wget http://mirror.centos.org/centos/6.2/os/x86_64/Packages/yum-utils-1.1.30-10.el6.noarch.rpm
wget http://mirror.centos.org/centos/6.2/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.30-10.el6.noarch.rpm
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
yum upgrade
reboot

Nice!

When you install an update to a package on an RPM-based system, any configuration files which were originally distributed by the package and that have been changed are not replaced when the package is updated. Instead, rpm creates a new file with the extention ".rpmnew". It is then necessary to manually compare the existing file and the new one and decide how to deal with the new file.

So, the pattern I often need to use is "list all .rpmnew files and the corresponding original file". This is not a particularly complex issue, but is one that I don't use often enough to have at my fingertips. The trick is bash variable substitution; specifically pattern matching: ${variable%pattern}. This deletes the shortest possible match for pattern from the right of the contents of $variable.

An example using .rpmnew files created when upgading OpenNMS:

# ls -1 *.rpmnew| while read f ; do ls -l ${f%\.rpmnew}* ; done
-rw-rw-r-- 1 root root 7285 Jun  3 12:43 database-reports.xml
-rw-rw-r-- 1 root root 8232 Aug 10 19:01 database-reports.xml.rpmnew
-rw-rw-r-- 1 root root 333092 Aug 25 09:25 datacollection-config.xml
-rw-rw-r-- 1 root root 314141 May 11 01:31 datacollection-config.xml.rpmnew
-rw-rw-r-- 1 root root 3395 Jun  3 12:43 jasper-reports.xml
-rw-rw-r-- 1 root root 3664 Aug 10 19:01 jasper-reports.xml.rpmnew
-rw-rw-r-- 1 root root 25887 Aug 10 19:01 log4j.properties
-rw-rw-r-- 1 root root 25887 May 11 01:31 log4j.properties.rpmnew
-rw-rw-r-- 1 root root 19319 Apr 22 16:56 opennms.properties
-rw-rw-r-- 1 root root 19635 Aug 10 19:01 opennms.properties.rpmnew
-rw-rw-r-- 1 root root 9252 Apr 22 16:59 rrd-configuration.properties
-rw-rw-r-- 1 root root 9604 May 11 01:31 rrd-configuration.properties.rpmnew
-rw-r--r-- 1 root root 669444 Jun  3 13:51 snmp-graph.properties
-rw-rw-r-- 1 root root 604457 Aug 10 19:01 snmp-graph.properties.rpmnew