[rrd-users] Re: DERIVE or COUNTER

[Alex van den Bogaerdt]

Michael Wells wrote:
> 
> I read the manual page and see your point. Would it
> be fair to say that *any* COUNTER that is zeroed on reboot would be
> better off being a DERIVE? I'm not sure that detecting the difference
> between an overflow and a zeroing is worth attempting, and that seems to
> mean this is the only 'correct' solution.

Not per se.  You can say that if the front end doesn't properly
address this issue and you should THUS make a work around in the
back end, you're better off with DERIVE and a minimum of 0.

Of course, the front end could simply remember the last value and
compare this to the current value.  If the current value is lower,
send an "U" value to the back end (rrdtool) in stead of the value.

There's more a front end could/should do.  For instance, it could
try and conpensate for time differences between sampling the value
and processing it.  Especially on an overloaded system this may
help.

Reasoning:
The time difference used to calculate the rate comes from the OS.
Any variance in, for instance, network delay is influencing the
calculated rate.  This could result in network rates that seem to
be higher than the theoretical maximum in a worst case scenario.
This happens when the actual time between samples (time on the
monitored device) is higher than the difference between the
timestamps as generated by the OS.  The smaller you make the
intervals, the larger this error will be.  A difference of 100ms
in a one-minute interval is already 0.17 %  Larger errors can
occur if the OS introduces delay.


> My graphs are looking great now, by the way. ;-)

great.

cheers,
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