RRDtool Frequently asked Questions with Answers

Graphing

Peaks get smaller the larger the graph resolution gets

Q: I notice that the graph of day (--end now --start end-86400) is accurate but my other graphs are not looks like they are decreasing, say in the day graph u see it gets to 30000 point on the Y-axis but in the week (--end now --start end-604800) its not passing the 25000 point and in month (--end now --start end-2419200) its not passing the 23000 and year (--end now --start end-29030400) its not passing the 15000 point

A: If you are graphing an AVERAGE RRA then the data shown in the graph will be consolidated using the same method in order to fit the graph. In a high resolution graph, each pixel may represent 5 minutes worth of data. In this graph you would see a 5 minute transfer rate of 120 as a 1 pixel wide peak of 120. In a graph where 1 pixel represents 1 hour worth of data, the same incident would show up as a 1 pixel wide peak of 10.

Logging

RRDtool does not return what I put in

Q: I created an rrd file and then used rrdtool update to input a few values. I then used rrdtool fetch to get them back out. The values that come out of rrdtool are sort of similar to what I put in, but they do not match exactly.

A: Think of rrdtool as a 'sampling-device'. The data you provide through the update interface will be resampled to fit the bins you defined when setting the --step value at rrd creation time. If you want to get out exactly what you put in, you have to make sure that you provide your input exactly at the step boundaries (10:00:00, 10:05:00).