[collectd] Strange SNMP collection glitches

Mirko Buffoni briareos at eswat.org
Wed Jan 13 12:36:20 CET 2010

At 12.07 13/01/2010 +0100, you wrote:
>On Wed, Jan 13, 2010 at 11:57:05AM +0100, Mirko Buffoni wrote:
> > If that is the case, how could I solve this behavior which is going to
> > cause the graphs to be unusable due to the oversized scale factor?
>One way is to replace the COUNTER data source with a DERIVE data source
>and set the minimum value to zero. DERIVE data sources are documented
>at [0]. Then, when the counter is reset to zero, the rate conversion
>((new value - old value) / interval) will result in a negative value
>(because "new value" is zero). This negative value is then ignored due
>to the minimum value being zero. The downside is that this will happen
>also when the counter overflows, so you will occasionally lose a
>legitimate value.
>You can change the DS type of existing RRD files using
>   rrdtool tune --data-source-type …
>   (see rrdtune(1))
>Another possibility is to set a correct maximum value. For example, if
>you have a 10 Mbps line you could set maximum to 1250000 (Byte/s).
>You can change the maximum value of a DS using
>   rrdtool tune --maximum …
>   (see rrdtune(1))

I set the correct maximum value to the RRD archives for both datasource
tx and rx.

rrdtool tune archive.rrd --maximum rx:2400000
rrdtool tune archive.rrd --maximum tx:2400000

with a dump I see that the change has been done correctly.
However the past daily/weekly/monthly graphs are unchanged and retain
those autoscaled values.  I wouldn't want to change collection.cgi, but
I'd like to readjust past rrd values to the new constraints.  Is this


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