[rrd-users] Re: data resampling Q

[Tobias Oetiker]

Today you sent me mail regarding [rrd-users] data resampling Q:

*> 
*> Hello All,
*> 
*> 	I've noticed some unanticipated behavior in the way rrdtool does
*> data resampling and wanted to make sure it is working as designed.
*> 
*> 	I am collecting 5 minute Load Average samples every five minutes.
*> 
*> this is the command i used to create the rrd:
*> 
*> rrdtool create load.rrd DS:la:GAUGE:600:0:U RRA:AVERAGE:0.5:1:384
*> RRA:AVERAGE:0.5:6:384 RRA:MAX:0.5:6:384 RRA:AVERAGE:0.5:24:384 RRA:MAX:0.5:24:384
*> RRA:AVERAGE:0.5:288:384 RRA:MAX:0.5:288:384
*> 
*> 
*> So I want to average the averages but always keep the MAX.  Now I have
*> noticed that the true value for the 5 min Load Average is *not* being
*> recorded in the RRA:AVERAGE:0.5:1:384.  
*> 
*> e.g. i put a load average of 111 (it's really LAx100) into the rrd, but it
*> never got recorded:
*> 
*>  939831900:     12.00
*>  939832200:     41.70
*> # rrtdtool update load.rrd N:111
*>  939832500:     81.60
*>  939832800:     12.41
*> 
*> now, if i think to myself the area of the plot 12,41,81,12 vs 12,111,12,12
*> i think they are probably the same.  so it seems the single sample is
*> being *averaged* into the RRA somehow, although I just want the 'real'
*> value of the Load average. 
*> 
*> I've even tried RRA:LAST:0.5:1:384. but that seemed to exhibit the same
*> behavior.
*> 
*> I understaned the value of the behavior in certain situations, but i would
*> really just like the data in RRA:AVERAGE:0.5:1:384 to be raw data as
*> inputed by 'rrdtool update' without any averaging occuring.

the averaging has to occur because you are not inputting your updates at the
exact 300 second intervals but somewhere along the way ... therefore rrdtool
integrates the space below the curve and stores the values appropriate for
the points in time where the 300 second intervals occur thesse points are
when the number of seconds since 1970 modulo 300 equals 0 

cheers
tobi

*> TIA
*> 
*> 

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