[rrd-users] clear values from RRA

Alex van den Bogaerdt alex at vandenbogaerdt.nl
Wed Jan 14 03:36:17 CET 2009

----- Original Message ----- 
From: "Levente Kovacs" <leventelist at gmail.com>
To: <rrd-users at lists.oetiker.ch>
Sent: Wednesday, January 14, 2009 12:13 AM
Subject: [rrd-users] clear values from RRA

> Hi,
> http://logonex.eu/tc/
> I have another question. If you see my temperature sensor test site, you 
> may
> realize the very first values of the blue and green lines to be -60. It is
> because my hardware has 3 ports, but only one (red) had an actual sensor
> connected to it. The unconnected ports read out the minimum measurable 
> value,
> that is -60deg.

Will it ever be near to -60?  Can you live with e.g. -50 as the actual 

If so: use "rrdtool tune" to set a sane minimum value, below which rrdtool 
should consider its input as invalid (resulting in NaN, unknown).

This helps for the future, and you will be able to filter out those extreme 
values, see next.

> Anyways... I'd like to clear those values from my RRAs. How can I do it?
> Shlould I write a perl script which modifys the XML dump? If I do so, and 
> when
> I restore the data... will the newly aquired data lost which were recorded
> during the process of the XML?

You already know how to create an xml file using rrdtool dump. You also know 
about rrdtool restore to read that xml file back. But are you aware of 
the --range-check parameter?

If you've set an apropriate minimum value, such as -50, then restore should 
ignore those -60 values.

How much time is spent doing rrdtool dump immediately followed by rrdtool 
restore?  I bet you can start this sequence just after an update occured, 
and then it will finish well before the next update occurs.

Make a copy of your rrdfile so you can practice on it, and once you're 
confident enough you can make one more copy (as a backup, just in case) and 
start dump,restore.

> Or is there any way inside the RRD system to do say "do this and that with
> values smaler and/or larger then X".

As the other reply suggests: CDEF. There's an example in my tutorial.


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