[rrd-users] changing step time?
linux at thehobsons.co.uk
Sat Apr 10 15:54:00 CEST 2010
Gregory Guthrie wrote:
> > > "step" - specifically what happens if values are fed in more often?
>> Step is the size of each bin in the database - ie the length of each
>> time slot for which it can store data. Anything you feed it more
>> often that the step time is accumulated until the period is complete
>> and then it's all used to work out the single value stored for that
>> Eg, if you feed in values of 0,0,5,0,0 which all fit within one step
>> time, then the min, max, and average are all 1.
>This is a surprise! It would certainly appear at first glance that
>the max/min were 5 & 1.
RRD does not store anything but an accumulator within a step - so if
you feed in those 5 values, the accumulator will hold the values 0,
0, 5, 5, 5 immediately after each update. At the end of the step,
we've told it about 5 <somethings> in total, and if the step were 5
then the overall rate during the step is 1. Consolidate that with
min, max, or average, and you still get 1.
I think it will be clearer when you've read Alex's tutorial on normalisation.
Alex van den Bogaerdt wrote:
> >> I think you have to dump the data then do one of two things :
>>> 1) create a new database and feed the old data in
>>> 2) edit the data and re-import it
>> My data comes from a live feed source, is there any standard way to export
>> from rrd in a manner that allows re-import?
>> I suppose one could write something that would do fetches from the old,
>> and updates into the new.
>The biggest challenge is to get MIN and MAX correct.
In a situation like that, you would have to accept that the data
doesn't exist. In changing from a step of 1 (day, hour, whatever) to
half then the only sensible way I could see would be to simply
duplicate each set of values - so at time 0.5 you insert the values
you had at time 1, and insert them again at time 1. Or if the rrd
definition allows, just insert at time 1 and let rrdtool fill in the
same value at time 0.5.
The end effect is that the rrd database will hold the same
rates/consolidated values from time 0 to 1 as it did before. But
going forward, the new data will be processed at the higher
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