On 27 October 2015 at 03:59, Pablo Chacin <pcha...@sensefields.com> wrote: > > From the answers by Steve and Donovan, it seams like there's something fundamental I'm not understanding about RRDTools. > Following is the explanation from Wikipedia [...] > My understanding was that when making the consolidation, al the primary data points were available to the Consolidation function. Therefore, it could be possible to calculate the percentile. > > However, from what you explain, it looks as RRDTools re-calculates the aggretate point with each arriving primary point. Is that correct?
Correct. The aggregation of PDP's into CDP's is done incrementally at each input sample. Note that even if this was not the case, calculating meaningful percentile values would require large "steps" values on the corresponding RRA's. To calculate a reasonable 95th percentile requires at least 20 points. This thread made me look at the RRD docs again and I discovered it now supports DS's of type COMPUTE. That means it can already support calculating value^2 as another DS, making it possible to do this; rrdtool create traffic.rrd \ --start now --step 1m \ DS:rate:COUNTER:2m:0:1000000 \ DS:rate2:COMPUTE:rate,rate,* RRA:AVERAGE:0.5:1m:8d \ RRA:AVERAGE:0.5:1h:64d \ RRA:AVERAGE:0.5:1d:2y \ and then get an approximate 95 percentile in your graphs by calculating 2*stddev like this; DEF:rate=/home/rrdtool/data/traffic.rrd:rate:AVERAGE DEF:rate2=/home/rrdtool/data/traffic.rrd:rate2:AVERAGE CDEF:variance=rate2,rate,rate,*,- CDEF:stddev=variance,SQRT CDEF:95ptile=stddev,2.0,* Unfortunately it doesn't look possible to calculate an approx 95 percentile assuming a log-normal distribution, because RRD doesn't seem to have the functions necessary to calculate lognorminv(), at least not easily. -- Donovan Baarda <a...@minkirri.apana.org.au>
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