[rrd-users] Bits/sec and GB/month

Simon Hobson linux at thehobsons.co.uk
Mon Oct 20 17:02:42 CEST 2008

Eric wrote:

>As far as I understand, RRDTool never saves real data but only data from
>points in time. So, if somebody is making graphs with 'Bits/seconds' and
>stores this data in an rrdtool database, you cannot calculate back to totals,
>unless assuming the average for the default time of 5 minutes, was the same
>those 5 minutes and multiply this average by 5*60. By this, bandwidth bursts
>are heavily punished.

You misunderstand. The input data is normalised and (if neccessary) 
converted to a rate. If you feed in a counter from an interface (eg 
octets in/out) then it will convert these values to a rate. Although 
the rate stored will be 'slightly out' if you don't feed it numbers 
exactly on a step boundary, over a period of time it WILL work out 

Eg, suppose you have a step period of 300s (ie 5 minutes), and your 
rate is 300bytes/sec. The stored rate will be 1. Lets suppose that 
for 10 minutes you double your rate, but your updates are out of sync 
with the rrd database. For the 10 mins, your rate is now 2, but it 
would be stored as 1.5 for 5mins, 2 for 5 mins, and 1.5 for 5mins. 
Over an individual period you have errors, but over a longer term, 
the average is still correct.

Over a period of days/months it will work out correct and you can 
simply multiply average rate * time to get total - and apply a factor 
of 8 if required to convert between bits and bytes.

>So, since they are telling me their totals are coming from their rrdtool
>database, I think they provide me impossible values.

As long as the data going in is accurate, then the totals coming out 
should be correct.

Simon Hobson

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