[rrd-users] How to find a grafics error?

Simon Hobson linux at thehobsons.co.uk
Thu Jun 11 16:48:46 CEST 2015

Peter A. Menzel <prot_from_spam at famenzel.at> wrote:

> Here is the setup script:
> rrdtool create AgroFire.rrd --step 1 \
>  DS:KesselTemp:GAUGE:2:U:U \
>  ...
>  RRA:AVERAGE:0.5:1:864000 \

Can you be certain that you provide an update *every* second ?
You have a step size of 1, and a heartbeat value of 2. That means that you can only have data 2 seconds old before it is considered unknown. What isn't clear is which side of the point the decision is taken as the manual says :
> heartbeat defines the maximum number of seconds that may pass between two updates of this data source before the value of the data source is assumed to be *UNKNOWN*.

Eg, if you miss an update at time t=1 having done one at time t=0, does the value become unknown at time t=2 or *after* time t=2. The difference is that if you miss just one update, then in the former condition, when you come to do the update at time t=2, the data from time t=0 to time t=2 will be marked as unknown; but if it's the latter then the data wouldn't become unknown unless you also miss an update at time t=2.

So firstly I would be checking to see that you are reliably doing an update every second and not creating a few unknown values. You might consider increasing the heartbeat value.

You have specified an xff value of 0.5, so you would regard a consolidation as unknown if 50% of the source values are unknown. Since missing one update could be marking 2 seconds worth (ie 2 primary steps) as unknown, then missing an average of one update every 4 seconds would result in missing consolidated data.

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