On Tue, Jan 22, 2013 at 1:57 AM, Ivan Tretyakov <itretya...@griddynamics.com
> wrote:
> Hello!
>
> We've some graphs which constantly increasing. It would be more convenient
> to see it's derivatives.
> Could ganglia pre-process graphs before displaying?
> Could ganglia view graphics derivatives?
>
Ganglia does this via a feature in rrdtool to do this but there are some
caveats. When a metric is initially created in ganglia (the first time it
is seen) the RRD is created for that metric and set to a particular type of
COUNTER, GAUGE, DERIVE, etc based on the "slope" that is passed in the
metadata. The type that is selected determines how rrdtool calculates
values over time. By default, if no slope is provided, it defaults to
GAUGE, which will display literal values - so if you have an increasing
metric it'll show that increasing slope you describe. If COUNTER or DERIVE
is selected then rrdtool will calculate the delta between each interval and
display that delta instead of the literal values - this sounds like what
you want.
You can either modify whatever is collecting the metrics and sending them
to gmond so that they provide the appropriate slope (slope = positive if
you want counter behavior) or you can use rrdtool to modify the rrd to be
the appropriate type. If you search around the web for examples of how to
convert a gauge to counter you'll find some guidance.
That said, I've found the counter/derive calculations to not be entirely
without problems and so I've been a bigger fan of pre-calculating those
deltas before I push metrics into ganglia. This isn't a ganglia problem, it
has to do with rrdtool. I find doing it this way produces more consistent
graphs and you can avoid spikes (positive and negative) by doing this. When
you do this all your metrics are gauges and you never worry about slope.
Aaron
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