[rrd-users] newbie rrd question - probably a very old issue

Dan Gahlinger dgahling at hotmail.com
Sat Sep 1 17:26:28 CEST 2007


yes, we are just getting rather frustrated with why this is happening.
I hope you and Alex can understand.

and I have said in every post it is likely something we're doing, we just 
don't know what.
we can't see why with what we've done - how it's doing this.

it's going to be some small stupid thing we missed, that's my gut feeling.

we've eliminated the graph, which is good.
we are trying to understand how the raw data shows one value and query gives 
us another - using the command line functions.

I used RRD years ago, and became quite adept at it, I seem to recall we had 
this exact same issue and fixed it, but then I didn't use RRD for many 
years, and I forgot these details.

so hopefully Alex can understand the frustration, and I'll definitely post 
the details when I can

i appreciate the help, in the meantime i'll look into recreating this on my 
local system, if I can do that with a smaller data set we have a much better 
chance of finding the issue.

>From: Tobias Oetiker <tobi at oetiker.ch>
>To: Dan Gahlinger <dgahling at hotmail.com>
>CC: alex at ergens.op.het.net, rrd-users at lists.oetiker.ch
>Subject: Re: [rrd-users] newbie rrd question - probably a very old issue
>Date: Sat, 1 Sep 2007 17:14:33 +0200 (CEST)
>Dear Dan,
> > I think you're missing the point.
>just take a look at the number of posts alex makes. he does
>understand how rrdtool works, he know the code that makes it tick
>... so if you tell him he is missing the point, chances are that he
>will not bother anymore help you understand how rrdtool works ...
>to your question.
>RRDtool tries hard to work correctly independant of the number of
>points it has to work with. So your graph, should turn out the same
>results, regardles of the number of samples that are available
>(withing reason).
>This means for example, that if you input data from an RRA that is
>using AVERAGE consolidation, that the graph will also use AVERAGE
>consolidation to reduce the data until it fits in the graph. If you
>use a MAX function over this, the MAX will return what you see in
>the graph.
>If you are interested in keeping the MAX of the data you have
>originally fed into RRDtool, you must store it in an RRA that has a
>MAX function attached.
>A little example:
>You have to 2 points in your RRA
>a: 10
>b: 90
>On the graph they map onto the same pixel. Depending on the
>consolidation function of the RRA, rrdtool will draw the pixle at
>the following positions
>avg: 50
>min: 10
>max: 90
>on top of this you can use VDEF to calculate, but obviously the
>results will depend on the first round of consolidation that has
>already happened.
>From a performance point of view, given your box has enough memory,
>you should be able to get about
>  * 200-400 RRA updates a second per spindle
>Note that you can cheat this by increassing the amount of dirty
>buffers the OS tolerates as well as the ammount of time the dirty
>buffers can stay dirty.
>Especially if you run frequent updates this can be a neat strategy
>to increasse throughput dramatically (in cache updates, rrdtool 1.3
>can do about 20k per second on a current box.)
>Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten
>http://it.oetiker.ch tobi at oetiker.ch ++41 62 213 9902

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