[collectd] What is GaugeMax of tail plugin?

Florian Forster octo at collectd.org
Sat Sep 11 10:45:37 CEST 2010

Good morning,

On Fri, Sep 10, 2010 at 01:33:05PM +0400, Denis Melnikov wrote:
> Why collectd cannot update RRD for fixed points in time?

imho the problem is that the CF is not taken into account at the PDP
level. I imagine this is due to the root of RRDtool in MRTG: When you
read interface counters (or any other COUNTER or DERIVE data source) you
already have implicit averaging over the interval in which you read the
counter. And for the AVERAGE CF the PDP calculation does *exactly* what
you'd expect, i.e. it calculates the average of the time-shifted
interval to the highest possible precision.

> There will be some time shift, that may be done optional for those who
> take care of it.

What you could do is write a "target" for collectd to do something like:

  vl->time -= (vl->time % vl->interval);

If you use such a match in the *PreCache* chain, collectd won't mind the
changed time and RRDtool will most likely produce a value much closer to
what you expect. See the [[Chains]] wiki page for an in-depth
explanation of the filter subsystem of collectd.

Florian octo Forster
Hacker in training
GnuPG: 0x0C705A15
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