[rrd-users] RRA AVERAGE:0.5:1 undesirable result

[kssgill]

Hi,  I've setup a test RRA as shown below and create a perl script to popuate
it with test data. I'm see rounding off of data which is not what I was
expecting.

This is how I created an rrd file:

rrdtool create testfile.rrd  -s 20 DS:await:GAUGE:40:0:10000000
RRA:AVERAGE:0.5:1:1598400

Attached is the perl script I use to update the rrd file. 

#!/opt/perl2exe/perl5/bin/perl

my $i=2;
while ( true ) {

print "updating: $i\n";

my $rrdUpd = `rrdupdate testfile.rrd -t await N:$i`; chomp($rrdUpd);
$i=$i+2;
sleep(20);


}


I let it run for a few minutes while it updates the numbers:
updating: 2
updating: 4
updating: 6
updating: 8
updating: 10
updating: 12
updating: 14
updating: 16
updating: 18
updating: 20
updating: 22
updating: 24
updating: 26

I then did a dump of the rrd in a txt readable format as :

rrdtool dump testfile.rrd > testfile1.dump

I then look for my updates.

grep -v 'NaN' testfile2.dump


<!-- Round Robin Database Dump --><rrd> <version> 0003 </version>
        <step> 20 </step> <!-- Seconds -->
        <lastupdate> 1255382402 </lastupdate> <!-- 2009-10-12 17:20:02 EDT
-->

        <ds>
                <name> await </name>
                <type> GAUGE </type>
                <minimal_heartbeat> 40 </minimal_heartbeat>
                <min> 0.0000000000e+00 </min>
                <max> 1.0000000000e+07 </max>

                <!-- PDP Status -->
                <last_ds> 8 </last_ds>
                <value> 2.3504256000e+01 </value>
                <unknown_sec> 0 </unknown_sec>
        </ds>

<!-- Round Robin Archives -->   <rra>
                <cf> AVERAGE </cf>
                <pdp_per_row> 1 </pdp_per_row> <!-- 20 seconds -->

                <params>
                <xff> 5.0000000000e-01 </xff>
                </params>
                <cdp_prep>
                        <ds>
                        <primary_value> 7.7067147000e+00 </primary_value>
                        <secondary_value> 0.0000000000e+00
</secondary_value>
                        <unknown_datapoints> 0 </unknown_datapoints>
                        </ds>
                </cdp_prep>
                <database>
                        <!-- 2009-10-12 17:19:00 EDT / 1255382340 -->
<row><v> 2.0000000000e+00 </v></row>
                        <!-- 2009-10-12 17:19:20 EDT / 1255382360 -->
<row><v> 3.7085260000e+00 </v></row>
                        <!-- 2009-10-12 17:19:40 EDT / 1255382380 -->
<row><v> 5.7077767000e+00 </v></row>
                        <!-- 2009-10-12 17:20:00 EDT / 1255382400 -->
<row><v> 7.7067147000e+00 </v></row>
                </database>
        </rra>
</rrd>


And I'm surprised to see that instead of see 2,4,6,8. The numbers are
2,3.7,5.7,7.7

2.0000000000e+00 
3.7085260000e+00
5.7077767000e+00 
7.7067147000e+00

Not able to understand how I lost percision on the number.  From reading
online Average:0.5:1 should be the number it self.


rrdtool version is:

 rrdtool -v
RRDtool 1.2.27  Copyright 1997-2008 by Tobias Oetiker <tobi at oetiker.ch>
               Compiled Oct 24 2008 03:40:57

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