[rrd-users] Re: percentile calculations

Dan Cech dcech at phpwerx.net
Wed Sep 6 05:10:00 MEST 2006

Alex van den Bogaerdt wrote:
> On Tue, Sep 05, 2006 at 06:04:38PM -0400, Dan Cech wrote:
>> I did manage to make this work for a single RRD, by using code like:
>> DEF:rawin=$rrdfile.rrd:in:LAST:end=$now
>> VDEF:raw95in=rawin,95,PERCENT
>> CDEF:raw295in=myin,POP,raw95in,8,*
>> VDEF:95in=raw295in,MAXIMUM
> take one value out of range rawin, place it in raw95in
> raw95in is not a range, it is a number.
> create raw295in, a time series of data, rate 8 times raw95in.
> Each entry is the same rate.
> Take the maximum of raw295in (average, minimum or last should
> also work, no?)
> Wouldn't this ...
> DEF:rawin=$rrdfile.rrd:in:LAST:end=$now
> CDEF:bytes=rawin,8,*
> VDEF:95in=bytes,95,PERCENT
> ... produce the same number?  Am I missing something?

Thanks for the pointer, I had tried this initially and it didn't seem to
work, but I gave it another shot and it seems to be successful.

I quickly ran it over the aggregate graph and it does seem that setting
the end time for the aggregated data sources will have the same effect,
so it seems that this has done the trick.

Thanks again for bearing with me as I come to grips with how the whole
thing works.  I'm still a little fuzzy on what happens to the dependent
CDEF after the end time of the various data sources (I had assumed that
it would be populated with NaN which doesn't seem to be the case), but
that's a question for another day.


>> While this does appear to work, because it relies on access to the raw
>> DEF it cannot be used with data that is aggregated across multiple rrds.
> VDEF can also work on CDEF data.

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