[rrd-users] Problem with Time series data entry

Simon Hobson linux at thehobsons.co.uk
Tue Jun 19 08:43:50 CEST 2007

Chris G. wrote:
>I have some historical time series data.  The data was collected approx.
>every 15 mins, I say approx. because the times are not exactly 15 mins to
>the second apart.  In some cases there could be 10-15 seconds of delay
>between each of the 15 min collections.  In some cases if a collection cycle
>was skipped a time entry less then 15 mins will occur.   For example, data
>collected at 10:00:01, 10:15:07, 10:30:03, 10:45:11, 10:48:13, 10:51:12,
>I have created the following:
>rrdtool create test.rrd --start 1179721292 --step 900
>DS:value:GAUGE:1830:-5:5000 RRA:LAST:0.5:1:672 RRA:AVERAGE:0.5:4:2160
>I expected this to allow me to keep the actual measured value and the time
>the value was collected for 7 days and then the hourly average for the
>previous 90 days.

>When I perform a rrdtool fetch the first data entry I get has a timestamp of
>1179721800 this is not the date/time I passed in with the update.

No it won't be the timestamp you passed in the update UNLESS that 
timestamp is a multiple of the step size you used when creating the 
rrd. Unless every update is timestamped exactly on a step boundary, 
then it will be normalised to fit those steps.

>Is it not possible to collect/store and then graph time series data using
>the exact time the measurement was taken.

No, this is not what rrd was designed for, if you want to do that 
then use a more appropriate tool. Rrd was designed to do one task, 
and it does it very well, to store historical data which is 
normalised and consolidated as it is entered so as to a) minimise the 
processing required for data extraction/graphing and b) to minimise 
the data storage requirements.

>Any help/guidance would be appreciated.

As well as the project web pages, there is some excellent information 
at Alex van den Bogaerdt's site http://www.vandenbogaerdt.nl/rrdtool/ 
In particular, see under the tutorial section where it says :

>>Rates, normalizing and consolidating explains how RRDtool processes 
>>its input data, normalization and consolidation are described there 
>>as well as some generic stuff about the properties of an RRA

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