You should check the real rrd data -- or widen that graph to avoid pixel
averaging. rrdtool will take the data in the time frame and divide by the
pixel width which gives X datapoints per pixel. Perhaps it's averaging that
pixel down due to missing datapoints in the pixel.
________________________________
From: Aleksandr Pasechnik <aleksandr.pasech...@gmail.com>
To: rrd-users@lists.oetiker.ch
Sent: Wednesday, April 27, 2011 5:56 AM
Subject: Re: [rrd-users] Graph discontinuity or jump
But what could possibly cause rrdtool to interpolate wildly down? Is it somehow
taking a U reading, considering it a zero, and interpolating with that? I would
understand it if it took the old 1015.8 and the new 1012.8, and got something
in between... a 1014 or something. But it doesn't do that! The "interpolated"
number is outside the range of *any* previously seen datapoints, actually. Is
this a bug? Or an undocumented feature? What is going on?
(a question of etiquette: should I be including all of the previous text from
the ongoing thread, or just the bits I am responding to?)
Al
On Apr 27, 2011, at 4:11 AM, Jaime Lozano wrote:
> Hello Aleksandr,
> What you tell is very detailed. I don't know what is happening to your
> graph. I don't know if this will help you. But remember that rrdtool
> don't stores real data. I mean that maybe rrdtool is interpolating and
> that's why the graph is not as exactly as you think it should be.
> bye
>
> 2011/4/27 Aleksandr Pasechnik:
>> Hello all!
>>
>> I have an rrd collecting meteorological data. It has a step of 300 seconds
>> and data is fairly reliably entered every 150 seconds. There are times when
>> my computer is off and no data is entered. Explicit U's would be entered if
>> there was a problem getting the data, but I haven't encountered that yet.
>> The strange thing is that I have been getting these big, unexplained jumps
>> in the graphs (with corresponding strange numbers in the rrd dump) after
>> blank periods. The parameter in question (it happens to the others
>> sometimes, but for some reason this last time it only manifested in this
>> one) is the atmospheric pressure. It is stored as a GAUGE with a heartbeat
>> of 600, a min of 0 and a max of U. I've got some RRA's, namely Average, Min,
>> and Max at xff 0.1, 1 data point, and 7340 rows (30 days of data i think);
>> and xff 0.5, 12 data points, and 44040 rows (5 years of data). I'm currently
>> plotting the last 24 hours of data with a resolution of one pixel per
data point. Before the b
lan
>> k occurred, my script was writing 1015.8 into the pressure. Once the
>>computer came back up, it was entering 1012.8 and 1012.4 (a bit of sensor
>>noise between measurements i suppose). At no point did i write a 1008.9 or
>>whatever its plotting there. Here is the offending graph output:
>>
>> http://russkey.tumblr.com/post/4971101013/jumping-rrdtool
>>
>> Any thoughts why this is happening? Anything I can do to stop it? I thought
>> it might have something to do with the xff, so i changed it from 0.5 to 0.1
>> for the 1 data point per row RRA's but that doesn't seem to have helped.
>> Though now that I think about it, perhaps it would make more sense to have
>> just a single RRA for the 1 data point per row section, and have it a Last
>> instead of an Average? Any help at all would be appreciated!
>>
>> Al
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>
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