[rrd-users] [GRAYMAIL] RE: RE: Question about trend line
rseiwert
rob at vcaglobal.com
Thu Aug 13 00:03:52 CEST 2015
Those initial values are not ignored and are used in the least squares calculation. The problem is that all the data is taken into consideration, the initial few values near zero and the final values near 3000. To get to the 3000 the slope is great enough that the line needs to start in the basement. You can put the limit back on and it will stop at zero. Unless your slope is zero every line will eventually have a negative y. When your trend is "downhill" you will have a negative slope and the values will go negative at some point in the future. Bogus data, such as when you first start monitoring, can also skew your trend. There are graphs that show the same trend line but common sense will tell you LSL is only accurate on the first.
[http://reference.wolfram.com/applications/eda/NBMLImages/FittingDataToLinearModelsByLeast-SquaresTechniques/FittingDataToLinearModelsByLeast-SquaresTechniques_54.gif]
FYI with your date range you are using in the graph of 30 days on graph 1900 pixels large I'm guessing the step used in your calculations is more like 1 hour.
As your data looks like it is in flux right now. I would change your date range to trend only the last 48 hours and you will get a much different trend line. On some of my graphs I use three trend lines that are hardcoded for immediate (1 day), longer (2 days), and even longer (a week) and they rarely agree. The link below shows how to hardcode this.
http://hints.jeb.be/2009/12/04/trend-prediction-with-rrdtool/
BTW, if this is a WAN port shouldn't the label be megabits/sec or some other rate and not total megabytes.
The graph below you can see that the bogus data from last week is skewing the trend down
[cid:image001.png at 01D0D527.06260460]
The one week trend here is bogus due to an equipment change.
From: Ondrej [via RRD Mailinglists] [mailto:ml-node+s937164n7583085h86 at n2.nabble.com]
Sent: Wednesday, August 12, 2015 3:52 PM
To: Robert C. Seiwert <rob at vcaglobal.com>
Subject: [GRAYMAIL] RE: RE: Question about trend line
OK, this is clear how LSL should work, but since graph very beginning there as positive values (i double checked now using rrd dump) and there are never any negative values, so i'd still assume LSL will take these first two days into consideration. Now it looks to me that there are just ignored. In other words between 4-6 of august there are 5 minutes step always positive values.
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