[rrd-users] Debugging graph smoothing or bad data samples? interpolation bug?

Marc MERLIN marc_rrd at merlins.org
Mon Nov 16 23:54:24 CET 2009

On Mon, Nov 16, 2009 at 11:21:10PM +0100, Tobias Oetiker wrote:
> > The graph definitions are in the links and the rrd was created like so:
> > /usr/bin/rrdtool create /var/lib/cacti/rra/housepower_21.rrd --step 120  \
> > DS:HouseNoAC:DERIVE:600:-1000000000:1000000000 DS:House:DERIVE:600:-1000000000:1000000000 \
> > DS:PGE:DERIVE:600:-1000000000:1000000000 DS:AC:DERIVE:600:-1000000000:1000000000 \
> > DS:PV:DERIVE:600:-1000000000:1000000000 \
> > RRA:AVERAGE:0.5:1:2628000 RRA:AVERAGE:0.5:1440:10950  RRA:AVERAGE:0.5:5:3153600 \
> > RRA:AVERAGE:0.5:30:525600 RRA:AVERAGE:0.5:120:131400 \
> > RRA:MAX:0.5:1440:10950 RRA:MAX:0.5:30:525600 RRA:MAX:0.5:120:131400 \
> >
> > I'm just trying to understand what could make these data problems possible.
> > My original data on disk (before it was fed to the rrd) is correct.
> > Even if there was a time delay in when the data was given, rrdtool knows how to interpolate
> > data to make things right, so I don't get it.
> >
> > Any ideas?
> you are talking about 'original' data, are you feeding the data to
> rrdtool INCLUDING the collection time, or are you just using N: the
> graph looks as if there was some bad case of jitter at work ...
Sorry, I should have said that: the data is fed through cacti which does not
feed the sample time into rrdtool. So rrdtool is left with just the sample
and time at which it received it.
Samples are every 2mn so even if I had a few seconds of jitter, it should
not have made such a big difference.
> two samples being 'faked' into one slot ...

yes, that's pretty much what I'm seeing.

If cacti/cron/the system were overloaded though, I would have expected to
have an empty sample and then a double sample to catch up.
I'm very perplexed here since the graphs clearly show a double sample first
and then an empty sample, which is the wrong way around.

Does that make any sense to you?

"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
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                                      .... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/  

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