[rrd-users] Problems with Last, Min and Max in rrd graph

[Steve Shipway]

I think what is happening is that you are using different RRAs in the two graphs, hence different values.

This may be because the large graph, having more pixels in width, wants more data points than the small graph (since one pixel in this graph represents a smaller time window than one pixel in the small graph).

So, assuming you have some very wide RRAs, there might be two RRAs capable of providing data for the graph.  The larger graph is using the finer granularity (EG: 1cdp=1hr), and the smaller graph uses the larger granularity (EG: 1cdp=1day).  Since you are using a VDEF 'max' over an 'Average' RRA, the 'max' on the smaller graph will in fact be a maximum of the average over the larger interval, and hence will be smaller (which it is).

Note this will affect the LAST and AVG as well, as a smaller-granularity RRA will be recalculating these more often than a large-granularity.

There are a couple of solutions.

1.  For your 'max' and 'min' values, make these calculated over MAX and MIN RRAs.  You may need to add these to your RRD file if they do not already exist in there.  In this case, the max of a max is always the same, whatever interval the first is consolodated over, so the MAX and MIN will work.
2. When you call rrdtool graph, also specify the step size to use, with --step .  This will allow you to make sure that both use the same RRAs even if a different RRA is more efficient or more accurate.

The Routers2 frontend had this problem once a long time back, but uses the second method to avoid it now, as I didn't have the option to add new RRAs to handle the larger graph image sizes.

HTH

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

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.oetiker.ch/pipermail/rrd-users/attachments/20130911/f45be1ce/attachment-0001.htm