[rrd-users] Transfer rate for rrd graph command

Karl Fischer rrd-users at ficos.de
Tue May 18 15:15:30 CEST 2010

> Hello all.
> I would like to ask a question regarding to the transfer rate for rrd graph command.
> I was checking the following page to find out why the y-axis value seems to decrease
> when the range of date changes.
> http://oss.oetiker.ch/rrdtool-trac/wiki/RrdFaq
> However, I couldn't quite understand the "transfer rate 120" meant in the paragraph.
> Could someone explain following items?
> 1. What does value 120 means?
> 2. What does transfer rate 120 means?

Hello Yu,

let's phrase it differently:

Assume the following:

* you have recorded data with 1 value per second
* you are displaying a graph for 10 minutes (600s)
* the graph width is 600px
=> that means 1px (horizontal) equals 1s
=> that will give you the exact values that were measured.

Now assume:
* you have recorded an average transfer rate of 10 MB/s
* for 3 seconds (peak) the recording is 20 MB/s
=> the above graph will show you exactly that.

Now assume you 'zoom out' to 1 hour (3600s) on the same graph
width of 600px - so 1px represents (the average of) 6s now.
=> for the 1px that includes the 3s 20 MB/s and 3s 10 MB/s
   the average is (3x10 + 3x20) / 6 => 15MB/s
   => so now your peak dropped from 20 to 15 MB/s (avg)

The above get's more complicated if the 3s period mentioned
doesn't fit into a 6s period, but spans two instead ...

Similar things happen with aggregation in larger scales ...

does that help?

- Karl

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