[rrd-users] VDEF, 95th percentile and graph width

Marius Karthaus lists at karthaus.nl
Sat Sep 6 11:28:13 CEST 2014

Hello Steve,

The version I am using was 1.4.7 on ubuntu precise. I've now compiled 
the latest stable (1.4.8) but there are no differences between them. 
Also I've played around with the order of the commands (putting all the 
graph rendering in front/behind etc), but that also does not make a 

Marius Karthaus

On 06-09-14 01:54, Steve Shipway wrote:
> Looking at the two graphs, they cover the same data width but the 
> second is 2x the pixel width, and so has less on-the-fly consolidation 
> before graphing.  From this, as I would expect, the peaks in the first 
> graph are averaged out more, and so show up as lower (around 50M).The 
> second graph shows the peaks narrower but higher (around 70M) due to 
> their having less averaging applied.  So far, so good and as expected.
> However, the percentile calculation (as I understand RRDTool's data 
> handling) should not be affected by the graph-time consolidation, and 
> should apply to the extracted data set, which we have forced to the 
> same resolution in both graphs.  However, the lower value printed 
> would seem to indicate that this is not the case, and the graph 
> consolidation is taking place /before/ VDEF calculation.  This only 
> shows up because the data are extremely spiky in pattern and have a 
> fair amount of consolidation being applied.  If this is the case it 
> would seem to be contrary to the behaviour implied by the 
> documentation (that sounds like a long way of saying "i think its a 
> bug' :)
> I don't believe that you have told us which version of RRDtool you are 
> using?  If you're not on a recent version, it might be worth trying an 
> upgrade., just in case this is related to a recently-fixed bug.
> Steve
> *Steve Shipway*
> University of Auckland ITS
> /UNIX Systems Design Lead/
> s.shipway at auckland.ac.nz <mailto:s.shipway at auckland.ac.nz>
> Ph: +64 9 373 7599 ext 86487
> //

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