Graphs in Collectd: understanding "1 hour" and "1 day"

Hello ! I recently installed collectd, I am mainly interested in bandwidth usage over some time. I am somewhat confused as to how to understand the graphs: when I look at "1 hour", and say, I see that it is hitting 3.0 MBytes/s. Now I switch over to "1 day", then I see that it is hitting around 850 KBytes/s about the same time. I suppose it is because the bandwidth/s is averaged over a longer time of period for "1 day" than for "1 hour", but I cannot find what the rules are. I would rather want to see the graph for 1 day or a week for bandwidth usage calculated over the same period of time as for "1 hour". But the only place I found was "Data collection interval" in the Collectd Settings, and this seems to be applied universally. Then I don't understand why the graphs over 1 day and 1 hour are showing different values.

I would appreciate very much if someone could please explain that to me !

The graphs are created by rrdtool.


The use of RRAs guarantees that the RRD does not grow over time and that old data is automatically eliminated. By using the consolidation feature, you can still keep data for a very long time, while gradually reducing the resolution of the data along the time axis.

In order to have a database that does not grow indefinetely (important if you have only little memory available), RRAs are used. Normally you have a high data resolution "now", and lower data resolutions the older the data gets.

You can read in detail about the working principle of rrdtool on the site linked above.
It's a lot to read, hard to understand at first, but once you get used to it, you can do quite neat stuff with rrdtool.

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Thank you very much for the link ! I read it a bit and kind of understood what's going on.
I think I should be able to get the graph as I want by replacing "average" by "max", instead of changing the length of RRD step interval (which can be risky because I don't know how large the file is going to be !) I want to check if the bandwidth I ordered from the ISP meets the need of the users, so max would be more useful than average over a certain period of time, if it's better to leave the length of this period intact for the sake of not inflating the file.
But when I click "show max instead of average" on luci, I don't get any graph anymore....
There should be also a script file containing phrases like "RRA:AVERAGE:...." somewhere, which contains parameters I would like to see , but I cannot find it !

I would appreciate your info very much !