[collectd] What is GaugeMax of tail plugin?

[Denis Melnikov]

Florian,
thank you for the detailed answer.
Why collectd cannot update RRD for fixed points in time?
There will be some time shift, that may be done optional
for those who take care of it.

Denis

> -----Original Message-----
> From: Florian Forster [mailto:octo at collectd.org] 
> Sent: Thursday, September 09, 2010 8:34 PM
> 
> Hi Denis,
> 
> On Thu, Sep 09, 2010 at 07:52:07PM +0400, Denis Melnikov wrote:
> > Now if I see for example 783.3 on my graph I believe I can see " 200
> > resp=783.3" in the logfile. But I don't see it! Instead, I 
> find " 200
> > resp=1537.387".
> 
> I believe this is due to the way RRDtool builds the "Primary 
> Data Point"
> (PDP). It basically comes down to this: RRDtool uses fixed 
> intervals. If
> you use a ten second step, for example, these intervals would start at
>   t=0, t=10, ..., t=1284049090, t=1284049100, ...
> and last for ten seconds each. If you add an update to RRDtool at an
> arbitrary point in time, say
>   value[t=17]=200
>   value[t=27]=1000
>   value[t=37]=100
> then the value for PDP[t=20] will be calculated from the 
> values t=17 and
> t=27 as
>   PDP[20] = 7/10 * value[17] + 3/10 * value[27] = 140 + 300 = 440
>   PDP[30] = 7/10 * value[27] + 3/10 * value[37] = 700 +  30 = 730
> 
> So the problem is that the "Consolidation Function" (CF) is not taken
> into account when building a PDP -- only to create "Consolidated Data
> Points" (CDP).
> 
> > `rrdtool fetch` lists as follows:
> > 
> > 1284033470: 2.4634500000e+01
> > 1284033480: 7.8328150000e+02
> > 1284033490: 7.7944550000e+02
> > 1284033500: 1.9102000000e+01
> 
> This output supports the argument given above: The value 
> (1537.387) was
> apparently distributed over two PDPs:
>   7.8328150000e+02 + 7.7944550000e+02 = 1562.727
> 
> > Is it possible?
> 
> With RRDtool, no. Not that I'm aware, anyway. You could use the "CSV"
> plugin, for example. That doesn't do any of that magic and will store
> 1537.387 as you'd expect.
> 
> > Otherwise, what is a benefit of GaugeMax?
> 
> If within one interval *multiple* lines are matched by the regular
> expression, only that value will be passed on to the write plugins. As
> long as the value is within collectd, it *is* 1537.387. The unintended
> averaging happens after the value has left collectd's control.
> 
> Hope this helps.. Regards,
> --octo
> -- 
> Florian octo Forster
> Hacker in training
> GnuPG: 0x0C705A15
> http://octo.it/