[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
correct.
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
Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
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