[rrd-users] Questions of new user

[rrd-users at ficos.de]

Pavel Rubtsov schrieb:
> Hi All!
> 
> Im new user of rrdtool, i have many questions (and i search for web,
> but some thin questions is still no ansver...; and sorry for my english... :)
> 
> 1. How rrdtool does updating in time interval?
> 
> After creating of database and strict by step?
> Or it countdown timer after last adding data?
> 
> Example:
> 
> i have --step 300
> i update *.rrd base at 16:00:00 (h:m:s)
> i update *.rrd base at 16:02:00
> NOW next step begin in 16:00 or in 16:07?

Hi Pavel,

rrdtool strictly updates based on (unix) time of epoch (1970-01-01 00:00:00)
modulo your step, so a step 300 means xx:05:00, xx:10:00, xx:15:00 ...


> 2. Example:
> 
> i have --step 300
> If i will update often, then 300 sec (for example every 10 seconds)
> what will be? Which data is stored?

if you update more often than required for you step size, the data will be
aggregated using the aggregation function you've chosen (MIN, MAX, AVERAGE etc)
so either the smallest or the highest or the average of the values will be stored.


> 3. I create rrd with:
> --step 60
> 
> DS:test:GAUGE:120:0:100
> RRA:MIN:0.5:1:30
> 
> i update every 60 sec with values:
> 10, then 20, then 30, etc.
> But when i create graph i see NOT Minimum value, its looks like averaged... why?

see above ...
first, if you don't update on the 'top' of the interval, eg. xx:xx:00 for a 60s,
the data will be interpolated, so for example you updated at

08:22:10 value 10
08:23:10 value 20

then the database will need the value for 08:23:00 which you didn't provide, so
it will be interpolated. At 08:23:00 it was 5/6 on the way from 10 to 20, so it
will write 10 + (20 - 10) * 5 / 6 = 18.333


hth

- Karl