[rrd-users] rrdtool getting incorrect values after update
s.shipway at auckland.ac.nz
Wed Mar 13 23:16:33 CET 2013
> 1363128441 - 5791
> 1363128741 - 6665
> 1363129041 - 7482
> And here are the results rrd has stored (this is from a xml dump):
> <row><v> 2.9050692504e+00 </v></row>
> <row><v> 2.8128932327e+00 </v></row>
> Now as per my understanding, the first value should be the result from:
> (6665 - 5791)/300 = 2,913333333333333 ... which doesnt match the first
> value. The same issue applies to the second value and any other value i
> tested. What am i doing wrong ?
Although you are updating every 300s, you are not updating exactly on the boundary. Therefore, since your rates are not constant, in the stored value you'll get a bit of the first update and a bit of the second.
Buckets are 12:00-12:05, 12:05-12:10, etc
Update happens at 12:01, 12:06, 12:11 etc
So, when the 12:06 update happens, the 12:00-12:05 bucket is filled, using 20% of the 11:56-12:01 rate and 80% of the 12:01-12:06 rate.
Thus, you don’t get exactly the values you expect to see, because they are time-normalised.
If you round your update time down to a multiple of 300 then you'll see what you expect, though the values will then not be accurate...
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