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

[Steve Shipway]

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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