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List:       cacti-user
Subject:    Re: [cacti-user] Debugging graph smoothing or bad data samples?
From:       Marc MERLIN <marc_news () merlins ! org>
Date:       2009-12-06 7:12:47
Message-ID: 20091206071247.GK3834 () merlins ! org
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On Mon, Nov 16, 2009 at 02:09:50PM -0800, Marc MERLIN wrote:
> http://gargamel.merlins.org/cacti/graph.php?action=properties&local_graph_id=39&rra_id=0&view_type=tree&graph_start=1258302354&graph_end=1258315217
>  
> and just in case my data samples were somehow bad (and I checked the source,
> they're not), the same thing happened at the same time on another data
> source at the same exact times:
> 
> http://gargamel.merlins.org/cacti/graph.php?action=properties&local_graph_id=40&rra_id=0&view_type=tree&graph_start=1258302354&graph_end=1258315217
>  
> There is just no way it's a concidence.
> Also I noticed that what shows up is a data sample with twice the value it
> should have and then the next sample is pretty much 0.
> 
> 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?

I should answer myself on this one for the benefit of the archives.

The problem was as such:
- cacti polled data from my file every 2mn, more or less
- my script got data from somewhere and wrote it to the file every 2mn or
  so.

There is of course a race condition here: sometimes my script had written
the next sample before cacti polled the previous one and cacti got 2
samples' for one time interval and then no data increase at the next
interval.

Fix was to disable rrd updates from cacti and just have my script update the
rrd each time it got data so that there aren't 2 processes that can be
slightly out of sync.

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

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