> I have a cacti graph which is a consolidation of 34 graphs of online user counts. > A primary purpose of this graph is to identify peaks in user counts. > My problem is that the SUM of the MAX values is incorrect unless I constrain > the timeline of the graph to the minute containing the peak. I have manually > calculated the MAX values from the RRD files and know that the total of a > recent peak is 242,409. Below is a screen capture of the one-minute sample > in the graph which shows a matching total.
I can't tell for certain without having the actual RRDTool commandline used to generate these graphs and the structure of the underlying RRD files, but it is probably because of one of these things -- 1. MAX( a+b ) <= MAX( a ) + MAX( b ) 2. MAX( a ) >= MAX( AVG ( a ) ) The first can occur if you are adding the components before they are stored into the RRD. This is probably not the case here as you state that the result is correct at small granularity (1min interval) but low at large granularity (5min). The second is more likely, and is caused when the wrong RRA is used to generate the graph. If your RRD files do not contain a MAX CF RRA, then this is approximated by RRDTool at graph creation time using the available data. When you zoom out, RRDTool will move from using the 1pdp-AVG RRA to using the 5pdp-AVG RRA, and so you will actually be graphing the maximum 5min average rather than the maximum sample. The same problem hit me a long time back with Routers2, and the solution was to explicitly use the MAX CF RRA in the RRDTool commandline -- RRD1.4 also allows you to enforce the 'reduce' method to be MAX and enforce the granularity where necessary to avoid this problem. Calculations like 95th Percentile are particularly prone to this. Unfortunately, your RRD file and the commandline are being generated internally by Cacti, so it is possible that it is not requesting MAX values correctly - or even that there are no MAX RRAs available in the RRD file itself. I would suggest that you use 'rrdtool info' to verify that you have MAX CF RRAs at the appropriate sizes and if not, create them. Steve Steve Shipway T: +64 9 3737 599 ext 86487 E: s.ship...@auckland.ac.nz
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