95th Percentile

I am happy to announce that Jeff Gehlbach, over the weekend, managed to close an outstanding feature request that has been around for several years now.

Many moons ago we added support for JRobin to OpenNMS. JRobin is a Java port of RRDtool 1.0 and was developed by Sasa Markovic. When Sasa decided to no longer work on the library, he donated it to the OpenNMS Project.

Unfortunately, we haven’t had the resources to improve it much, so a number of features in future RRDtool releases have not been backported into JRobin. One useful one was the VDEF directive. In RRDtool, VDEFs allow one to work on the entire dataset instead of each value in the array.

The VDEF command allows for the calculation of such things as 95th Percentile. This is a method often used to calculate a value for billing burstable resources, such as network bandwidth, where the system can support short bursts above a committed rate without impacting the system. Basically, it is a number that says “95% of the time, the usage is at or below this amount“.

With Jeff’s change, this is now possible to do with JRobin. Since it is an operation on existing data, no change is required to the RRD files themselves, and it can be added simply by changing the snmp-graph.properties file.

This is a graph of the traffic from www.opennms.org. As you can see, the average outbound traffic is 3.40 Mbps with a peak of 33.53 Mbps. The 95th percentile, however, is only 10.70 Mbps, which is less than a third of the max. This calculation gets rid of the spiky bits while providing a more useful value for utilization.

The change will be in the next stable and unstable releases, and can be used now with the nightly snapshots.

Note: Here is the code used to generate the graph above.

report.mib2.HCbits.name=Bits In/Out
report.mib2.HCbits.command=--title="Bits In/Out"
GPRINT:totBits:AVERAGE:" Avg \: %8.2lf %s\n"
LINE2:rawbitsIn#00ff00:"Bits In"
GPRINT:rawbitsIn:AVERAGE:" Avg \: %8.2lf %s"
GPRINT:rawbitsIn:MIN:"Min \: %8.2lf %s"
GPRINT:rawbitsIn:MAX:"Max \: %8.2lf %s\n"
LINE2:rawbitsOut#0000ff:"Bits Out"
GPRINT:rawbitsOut:AVERAGE:"Avg \: %8.2lf %s"
GPRINT:rawbitsOut:MIN:"Min \: %8.2lf %s"
GPRINT:rawbitsOut:MAX:"Max \: %8.2lf %s\n"
LINE2:outpct#000077:"95 pct out"
GPRINT:outpct:" \: %8.2lf %s\n"
LINE2:inpct#007700:"95 pct in"
GPRINT:inpct:" \: %8.2lf %s\n"
GPRINT:inSum:AVERAGE:" Tot In \: %8.2lf %s"
GPRINT:outSum:AVERAGE:" Tot Out \: %8.2lf %s"
GPRINT:totSum:AVERAGE:" Tot \: %8.2lf %s\n"

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