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I'm trying to trace the status of a line, I have two values, the signal strength 0-5 and the connection status (connected = 3, disconnected = 1), the values are read every minute and I want to keep track of a year.

Currently when there is a change in one of the two values the change in the graph is not clear, I see that a half step is almost always added, instead I would like the graph to exactly reflect the state of the values, if the power goes from 4 to 5 I would like to see it in the graph with a vertical step, without added half steps.

I did several tests following the documentation and tutorials but obviously I am missing something.

Database creation

rrdtool create db.rrd \
--step 60 \
DS:rssi:GAUGE:120:0:5 \
DS:wan_state:GAUGE:120:0:10 \
RRA:MIN:0.5:1:525960

Update

# $TS is the timestamp, 4 the signal power, 3 the wan connected
rrdtool update db.rrd -t rssi:wan_state $TS:4:3

Graphs

rrdtool_graph() {
    secs=$1
    span=$2
    rrdtool graph graph-${span}.png \
            --start=now-${secs}s --end=now \
            "DEF:rssi=db.rrd:rssi:AVERAGE" \
            "DEF:wan_state=db.rrd:wan_state:AVERAGE" \
            "AREA:wan_state#0000FF:Internet" \
            "LINE1:rssi#00FF00:Signal" \
            >/dev/null
}

rrdtool_graph 3600 hour
rrdtool_graph 86400 day
rrdtool_graph 604800 week
rrdtool_graph 2629800 month
rrdtool_graph 31557600 year

enter image description here

Alex
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1 Answers1

2

You are being affected by Data Normalisation. The sample data you are storing are being adjusted by RRDTool in order to fit them into the precise time bands of the RRD. In addition, when you look at graphs over longer time periods, the samples will be averaged and rolled-up to match the pixels on your graph.

If you want to avoid this, you have a few options.

  • Save the data precisely on the time boundary. Since your step is 60s, then you need to store at precisely on the top of each minute.
  • Use MIN (or MAX) instead of AVERAGE when graphing as well as in the RRA definition
  • Reduce your step size and set the xff proportionally lower (this would potentially use a lot more storage)
  • Round the data off when graphing
  • Use a relational database instead - if you want precise historical data then RRDTool may not be the correct choice

You also have the problem that your RRD is defined with a single RRA of type 'min' but you use Average in your graph calls. You should probably change this to an Average RRA, and add more RRAs for the lower-granularity graphs (else it will take a long time to build your yearly graph on the fly!)

Finally, you might like to do some clever calculations on your wan_state DS and display it instead as a coloured background, maybe green for up and red for down, instead of a number.

Steve Shipway
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  • Thank you very much – Alex Jul 22 '19 at 05:52
  • Saving precisely to the minute and making the chart with MAX works very well for hours, days and weeks, with the month the resolution starts to be a problem because MAX is always chosen even if it is only a peak, I would need a function that counts the values examined and visualize the most present, is there something of the kind? – Alex Jul 25 '19 at 20:38
  • @Alex - you can use AVERAGE, MIN, MAX, and LAST. However the AVERAGE is a mean average, whereas you appear to want a modal one, which is not currently an option with RRDTool. Possibly you could use the AVERAGE and simply round off to the nearest integer to make the same effect? – Steve Shipway Jul 27 '19 at 07:23