# [rrd-users] Questions of new user

Simon Hobson linux at thehobsons.co.uk
Sun Dec 6 17:25:10 CET 2009

Pavel Rubtsov wrote:

>But there is some misunderstanding:
>
>C:\rrd\rrdtool.exe create test.rrd --start N --step 300 ^
>DS:test:GAUGE:600:0:100 ^
>RRA:AVERAGE:0.5:12:1
>
>C:\rrd\rrdupdate.exe test.rrd ^
>1260113100:10 ^
>1260113400:20 ^
>1260113700:30 ^
>1260114000:40 ^
>1260114300:50 ^
>1260114600:60 ^
>1260114900:70 ^
>1260115200:80 ^
>1260115500:70 ^
>1260115800:60 ^
>1260116100:50 ^
>1260116400:40
>
>Result:
>1260111600: nan
>1260115200: 4.1111111111e+01
>1260118800: nan
>
>But (10+20+...+50+40)/12 = 48,3! Why?

period is at 1260115200 (Sun  6 Dec 2009 16:00:00 GMT) and started at
1260111600 (Sun  6 Dec 2009 15:00:00 GMT). So you only have 9
complete step periods for which you have data. Remember that all
intervals are "n * s" from unix epoch - ie a whole multiple of the
step size (or in this case, step size times steps per consolidated
data point). Put simply, if the step (or ocnsolidation) is one hour,
then all intervals start on the hour - nn:00:00.

You have the following values in the consolidated interval :

Unknown * 3
10
10
20
30
40
50
60
70
80
So for the average rate for this period is (10+10+20+..+70+80)/9 = 41.1111

Or really, the calculation is :
(10* 300 + 10*300 + 20*300 + 30*300 + ... + 70*300 + 80*300) / 2700
Remember that rrdtool only ever works in rates, so a gauge value of
30 for 300 seconds is really a rate of 0.1. 2700 is simply 9x300 - ie
you've got data for 9 step times of 300 seconds.

Why are there two entries of 10 in that calculation ? Your first
update comes after all the data is unknown. You've configured your DS
to allow up to 600 seconds between samples before the data is unknown
- therefore your first update of 10 actually works out to "the data
for the previous 2 sample periods (600/300) is "10". If you didn't
want that then you'd need to set an initial value (eg update with
1260112800:0) to explicitly set a start boundary to your data.

That last bit isn't a statement based on profound knowledge or
examination of the source code - it's what the numbers work out to
and is logically consistent with the documentation !

--
Simon Hobson

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