[rrd-users] Unexpected decimal values in RRA

[jan-terje.larsen at telenor.com]

Hello,

My RRD is set up like this:
create test.rrd \
         --start 0 \
         --step 1800 \
         DS:delay:GAUGE:3600:0:U \
         DS:D_delay:DERIVE:3600:0:U \
         RRA:LAST:0.5:1:17520

I'm logging real values with no decimal points to this RRD like this (from a perl script):
RRDs::update("test.rrd", "N:{value for datasource 1}:{value for datasource 2}")

When I fetch the values I see this:
1225359000: 3.8814344466e+03 -7.3078100609e-05
1225360800: 3.8815660814e+03 7.3156179499e-05
1225362600: 3.8814343401e+03 -7.3130882112e-05
1225364400: 3.8820000000e+03 3.1429841736e-04
1225366200: 3.8815225605e+03 -2.6548739493e-04
1225368000: 3.8810000000e+03 -2.9006816062e-04
1225369800: 3.8810000000e+03 0.0000000000e+00
1225371600: 3.8810000000e+03 0.0000000000e+00
1225373400: 3.8814840175e+03 2.6467914193e-04
1225375200: 3.8820000000e+03 2.9087641362e-04
1225377000: 3.8820000000e+03 0.0000000000e+00
1225378800: 3.8815161907e+03 -2.6880629013e-04
1225380600: 3.8814838961e+03 -1.7775989338e-05
1225382400: 3.8815155756e+03 1.7507452317e-05
1225384200: 3.8814842391e+03 -1.7862583975e-05
1225386000: 3.8815172634e+03 1.8332050402e-05
1225387800: 3.8810000000e+03 -2.8695019483e-04

It is the values from the first data source that bug me. I know the logged values range from 3881 to 3882 (for this short interval).

Why do I get the decimals from time to time ? It seems every time the polled value changes I get a "soft" change over three samples. The step is set to 1800 and from the timestamp (left column) it seems all values were logged in 1800 second intervalls. Are the left column from the fetch also a "calculated" value perhaps ?

I'm not able to google an answer from the maillinglist.

Best Regards,
Jan-Terje