[rrd-users] Odd mismatch in fetch results (via Perl/CLI)

[Tobias Oetiker]

Hi Marco,

note you may be using two different version of rrdtool ... there
were changes the in the number of entries returned from fetch ...

cheers
tobi




Today Marco Marongiu wrote:

> Hello *
>
> I am fetching data from RRD files via Perl modules. For all files but
> one I get the results I expect. For just one I get data as like I
> shifted the start/end interval one hour back. If I fetch from the
> command line using "rrdtool fetch", data comes out as expected.
>
> I am using:
> Ubuntu Linux 10.04.1 LTS
> rrdtool 1.3.8 (ubuntu package)
> RRDs version 1.3008 (perl -MRRDs -le 'print $RRDs::VERSION')
> RRDTool::OO 0.28 (perl -MRRDTool::OO -le 'print $RRDTool::OO::VERSION')
>
> If you think you can help, please keep reading. Detailed explanation
> follows.
>
> ------------------------------------------------------------------------
>
> I am creating a report using data from ~300 RRD files. All but one comes
> from cacti, while the last one is created and updated by a Perl script
> of mine. All the files come from the same machine, and the system clock
> is set to UTC.
>
> My script uses RRDTool::OO which, in turn, uses the RRDs module. It
> parses an XML file, fetches some numbers, and then calls:
>
> $rrd->update(values => $hostmetric{$metric}) ;
>
> which results in update being called with a timestamp of "N".
>
> When the report is due, I dump all the RRD files to XML, copy them over
> to a "reporting machine", where I rebuild the RRD files from XML,
> aggregate some of them in fewer RRD files, and then make the report. The
> "special" file mentioned above is also rebuilt and used unchanged.
>
> To aggregate data, for each RRD file I call:
>
>       my ($start,$step,$names,$data) =
>         RRDs::fetch($sourcefile,$cf,
>                     '--start',$tsstart,
>                     '--end',  $tsend) ;
>
> $tsstart and $tsend are always the same for all RRDs.
>
> For each RRD, I convert timestamps to a readable form using:
>
>         @$timestamps =
>           map { scalar gmtime( $start + $_ * $step ) } (0..$#$data) ;
>
> (note gmtime is used here).
>
> The strange stuff happens here. While for the aggregated files I get the
> data I expect: e.g.:
>
> Wed Dec  1 00:00:00 2010	10,61
> Wed Dec  1 01:00:00 2010	10,61
> Wed Dec  1 02:00:00 2010	10,34
> ...
>
> data from my RRD file shows up as like I called fetch with the start/end
> interval shifted one hour back:
>
> Tue Nov 30 23:05:00 2010
> Tue Nov 30 23:10:00 2010
> Tue Nov 30 23:15:00 2010
> ...
> Fri Dec 31 22:55:00 2010
> Fri Dec 31 23:00:00 2010
> Fri Dec 31 23:05:00 2010
>
> This means that I get one hour of NaNs at the top of the report, and one
> hour less data at the bottom.
>
> At first, I thought it depends on the way the files are updated (since
> the start/end timestamps don't change). While, as said, my "special"
> file is updated with an implicit timestamp, my aggregate RRDs are
> updated with explicit timestamps, as in:
>
>     $aggrrd->update( time   => $ts,
>                      values => $combined{$ts} ) ;
>
> which will call update with a timestamp of $ts.
>
> But this doesn't make sense: if I use "rrdtool fetch" directly from the
> command line and with the same parameters as the script, I get the
> values I expect:
>
> $ rrdtool fetch rrd/t*.rrd AVERAGE -s 1291158000 -e 1293836400 | awk
> '/:/ { print $1 }' | head -n 3
> 1291158300:
> 1291158600:
> 1291158900:
>
> Any ideas?
>
> Thanks in advance
>
> Ciao
> --bronto
>
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>

-- 
Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten, Switzerland
http://it.oetiker.ch tobi at oetiker.ch ++41 62 775 9902 / sb: -9900