[collectd] What is GaugeMax of tail plugin?

Florian Forster octo at collectd.org
Thu Sep 9 18:33:38 CEST 2010

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
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,
Florian octo Forster
Hacker in training
GnuPG: 0x0C705A15
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