[rrd-users] Help with creating an rrd-db

[Koenraad Lelong]

Hi,
I'm having troubles creating an rrd-db. I hope someone can point me in
the right direction.
I have a photovoltaic system I'm monitoring. At the moment I can make
nice graphs of the instantanious production, this means the power the
station is generating at a given moment. I poll the station about every
15 seconds.
Another value is the total produced energy. This is where my problem is.
I created this rrd :
rrdtool create etot.rrd --start 1206187000 --step 15 \
DS:ETOT:COUNTER:30:0:10000000 \
RRA:MAX:0.5:1:5760 \
RRA:MAX:0.5:5760:7 \
RRA:MAX:0.5:5760:31 \
RRA:MAX:0.5:5760:366

What I would like to get is graphs of the energy-production per day, so
the value "last measurement of the day" - "first measurement of the
day", displayed per week, month, year.

When I enter the values (also at 15 seconds interval), I get unexpected
results in the graph. I get results mostly is the milli-range, while all
the values I enter are by now 4.5 million (Wh produced).

What am I missing here ? When I dump the rrd, I also get values in the
milli-range. Could it be that the values in the rrd are the difference
between the input values ? The values that are entered don't change much
in 15 seconds, many times, the difference is zero.
How can I solve this ?

Thanks for any pointers.

Regards,
Koenraad Lelong.

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[Yashodhan deshpande]

Hi Koenraad,
The DS that you have chosen is COUNTER, which essentially means that it
is supposed to be an ever increasing number and RRD would store the rate per
second in the database. You mentioned that the values hardly change between
two updates, that would explain the small values getting logged in the DB.
Try using GAUGE instead as the typr of the DS.

Regards,
Yashodhan

[Alex van den Bogaerdt]

> Hi Yashodhan,
> Although the data is a counter, I tried the GAUGE, and now I get more
> acceptable results (see attachment). But, how do I get those dayly
> values, i.e. "max of day" - "min of day" ?

The graph you've shown shows that your counter increments from approximately
1.2M to 1.3M in 7 days. That is 100,000 per 7*86400 seconds, is 100,000 per
604800 seconds, is approximately 0.17 per second. If the graph shows
somewhere in the range of 170m, then that's not only acceptable but also
accurate.

If you store Joules per second, aka Watt, then you can compute Joules per
day by multiplying the average amount of Joules per second by the number of
seconds per day: J/s * s/day = (J*s)/(day*s) = J/day.

Just multiply 0.17 by 86400 to get numbers in the order of 14286 per day.
Does that sound about right?

HTH
Alex