[rrd-users] Graphing inconsistencies graphing different time slices

Munroe Sollog mus3 at lehigh.edu
Mon Jun 8 16:10:27 CEST 2015

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Very interesting.  Thank you for clarifying it.

My problem is, as you can see from the graphing function, I am arbitrarily taking numerous rrds
and adding them together to get a cumulative graph.  While some of these summation graphs are
predictable, many are not; thus I can't store a cumulative rrd in all situations.

My goal is to minimize this disparity that is caused by taking the MAX of a consolidated function.
 I am going to see if using the AVG instead of the MAX of the consolidated function will help, but
if I really want to fix this, my only recourse is to not consolidate? I'd be storing a 1years
worth of 1min slices for ~5000 devices.

On 06/08/2015 10:01 AM, Simon Hobson wrote:
> I wrote:
>> So when you draw a graph over a longer time period, depending on the consolidation, you may
>> see your maximum value go up (with these made up figures, by 1/3) when you add max values
>> together.
> To add ...
> I had something similar for some of my servers - I wanted a maximum mail queue size across
> several servers. Simply adding the max queue sizes doesn't work for the reasons I gave before,
> eg if 3 servers all have a queue size of 1 but at different times, the maximum cumulative queue
> size is 1, not 3. I wrote a script* that would periodically query the stats for each server,
> get the latest figures, add them, and put them into another RRD file for the totals. So as well
> as RRD files for each server, I have one for the combined values. As long as the script keeps
> the combined file up to date enough, it can use unconsolidated values from the individual
> servers.
> This combined RRD can then be consolidated and return the correct max values.
> * Which turns out to be more complicated than you might think ! It needs to find out the last
> complete "bucket" of data for each of the source files and see if the oldest of these is newer
> than that for the combined file - and if so, then update the combined file. You need to allow
> for delayed data from one or more servers (I'm using RRD cached which can delay updates unless
> you keep flushing it which defeats the object of the cacheing). But then, you need to allow for
> one (or more) servers being offline (as far as stats collection goes), so you need to decide
> how long to wait before taking an individual value as NaN.
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- -- 
Munroe Sollog
LTS - Network Analyst
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