Polling Bandwidth as an Aggregate

Chris Adams cmadams at hiwaay.net
Fri Jan 20 09:53:44 CST 2012


Once upon a time, Leo Bicknell <bicknell at ufp.org> said:
> To suggest Netflow is more accurate than rrdtool seems rather strange
> to me.   It can be as accurate, but is not the way most people
> deploy it.

Comparing Netflow to RRDTool is comparing apples to cabinets; one is a
source of information and one is a way of storing information.

> RRDTool pulls the SNMP counters from an interface and records them to a
> file.

No, RRDTool stores data given to it by a front end such as MRTG,
Cricket, Cacti, etc.  That front end can fetch data from any number of
sources, including (but not limited to) SNMP.  RRDTool then stores
information in its database.

> With no aggregation, and assuming your device has accurate SNMP,
> this should be 100% accurate.  While you are right that the defaults for
> RRDTOOL aggregate data (after a day, week, and month, approximately)
> those aggregates can be disabled keeping the raw data.

RRDTool does not store the raw data.  Even for 5-minute intervals, it
adjusts the data vs. the timestamp to fit the desired interval.  Since
you don't read every counter at the exact time of your interval, RRDTool
is always manipulating the numbers to fit.  The only numbers that are
not changed before storing are the timestamp and value for the most
recent update (which get overwritten at each update); everything else is
adjusted to fit.

-- 
Chris Adams <cmadams at hiwaay.net>
Systems and Network Administrator - HiWAAY Internet Services
I don't speak for anybody but myself - that's enough trouble.



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