[rrd-users] icmp measurements with rrd?

itris at technologist.com itris at technologist.com
Wed Nov 1 02:01:27 MET 2000


I've been using RRD for a while now and am quite happy with it :)
Recently I received a request from my employer to graph the latency and
packetloss in our network. My first choice would be MRTG, but due to
it's limitation to be able to graph/measure only 2 things at a time
(leaving out max/min and any other things I'd like to graph/measure) and
some problems with scaling it we probably won't use it.

So then the question becomes, RRD or build your own.
I'd like to try RRD.

But I have a few questions first that I'd hope to get an answer to:

Let's say I have an RRD with a 5 second measurement interval. The
measurement will take some time, but it'll be less than a second. So
we'd be putting data in the RRD at time+0+measurement_time,
time+5+measurement_time, etc..
Would then, a measurement_time of 0.3 (and hence insertion time of for
instance time+5.3) be subjected to resampling, resulting in a
measurement of (if the measurement was 0.1) (0.1/5.3)*5 = 0.94 ?
Or, would the time+5.3 insertion time be too late for the step, and be
taken as time+6, resulting in (0.1/6)*5=0.83 as measured value?

I suspect both cases will be answered with "no".
But then, how about this:
The time needed to measure the latency is dependent on the latency
itself.. So if for some weird reason the latency would rise above 1
second, the measurement would take more than 1 second.
Therefore, the insertion time would be time+6.3 (for instance) with a
measured value of 1.2
The RRD will therefore resample and show a value of (1.2/6.3)*5 = 0.95
If this is correct, is there any way around this ?
(other than measuring every second, eliminating any resampling, but no
longer allowing latency to be measured over 1 second and creating so
much network traffic that it will influence the measurement)
Otherwise I think RRD will be useless for measuring latency..

Thank you for all your answers,


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