[rrd-users] Compromised value after NaN

[Markus Wiget]

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.

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 \
        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.)

So around 2:30 when the cron job is ommitted, I have two NaN's in the
RRD file; this I understand. But what I do not understand: the first
value after the NaN's is 9.99...+01, but I would expect 1.00...+02.

rrdtool dump <file>:
Time                                 REAL             SLA
2006-11-27 02:15:00 CET / 1164590100 1.0000000000e+02 1.0000000000e+02
2006-11-27 02:20:00 CET / 1164590400 1.0000000000e+02 1.0000000000e+02
2006-11-27 02:25:00 CET / 1164590700 1.0000000000e+02 1.0000000000e+02
2006-11-27 02:30:00 CET / 1164591000 NaN              NaN
2006-11-27 02:35:00 CET / 1164591300 NaN              NaN
2006-11-27 02:40:00 CET / 1164591600 9.9979329293e+01 9.9979329293e+01
2006-11-27 02:45:00 CET / 1164591900 1.0000000000e+02 1.0000000000e+02
2006-11-27 02:50:00 CET / 1164592200 1.0000000000e+02 1.0000000000e+02

When I do "rrdtool graph" for getting the availability over a specific
time frame, I don't get 100(%) but only 99.99x(%). Even though the job
always "updated" with value 100.

Hint: When doing the RRD file creation and update manually with explicit
      time, then it is as I would expect 1.00...+02 after NaN.

      rrdtool update <file> --template=REAL:SLA 1164590400:100:100

      Then the dump shows:

Time                                 REAL             SLA
2006-11-27 02:20:00 CET / 1164590400 1.0000000000e+02 1.0000000000e+02
2006-11-27 02:25:00 CET / 1164590700 1.0000000000e+02 1.0000000000e+02
2006-11-27 02:30:00 CET / 1164591000 NaN              NaN
2006-11-27 02:35:00 CET / 1164591300 NaN              NaN
2006-11-27 02:40:00 CET / 1164591600 1.0000000000e+02 1.0000000000e+02
2006-11-27 02:45:00 CET / 1164591900 1.0000000000e+02 1.0000000000e+02

I would be grateful for your support.

Thanks,
Markus Wiget
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