[rrd-users] Re: Compromised value after NaN

Alex van den Bogaerdt alex at ergens.op.het.net
Wed Nov 29 14:59:41 MET 2006

On Wed, Nov 29, 2006 at 12:35:01PM +0100, Markus Wiget wrote:
> Hi rrd-users
> I'm using RRDtool for measuring availability. The values 0, 100 or U are
> added every 5 minutes, except at 2:30 when the job is ommited. The job
> is a Perl script which uses the RRDs module.

Are you sure, very very sure, only one update is missing ?

> My RRD files are created as follows (but with Perl, using RRDs module):
>     rrdtool create slac.rrd --start=<starttime> --step=300 \
>         DS:REAL:GAUGE:600:0:100 \
>         DS:SLA:GAUGE:600:0:100 \

step 300, heartbeat 600.  This means that if the updates would be
exactly five minutes apart, with the exception of that single one,
you should see no gap, at all.

>         RRA:AVERAGE:0.5:1:210816 \
>         RRA:AVERAGE:0.5:1:576
> The job does this (but with Perl, using RRDs module):
>     rrdtool update <file> --template=REAL:SLA N:100:100
>     (According to the log file, the RRD update is done somewhere between
>     first and third second of minute.)

How should people reproduce a problem when you say "somewhere" ?

Why don't you create a script that shows the problem?  I mean a
complete script, creating a (small!) database, doing (a few!) updates
and showing the resulting data with rrdtool fetch (!)
Use timestamps, not "N".

> So around 2:30 when the cron job is ommitted, I have two NaN's in the
> RRD file; this I understand. 

I don't.  Please explain why this would be.

>                              But what I do not understand: the first
> value after the NaN's is 9.99...+01, but I would expect 1.00...+02.

It looks as if your NaN is somehow being mangled into zero.  If this
is indeed the case, I can see why your data would be corrupted.

Alex van den Bogaerdt

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