[rrd-users] VDEF, 95th percentile and graph width

[Marius Karthaus]

Hello list,

While searching the web on how to calculate the 95th percentile from an 
RRD, most results return something like:

rrdtool graph graph.png \
DEF:in=foo.rrd:in:AVERAGE \
DEF:out=foo.rrd:out:AVERAGE \
CDEF:inb=in,8,* \
CDEF:outb=out,8,* \
CDEF:allbits=inb,outb,MAX \
VDEF:pct=allbits,95,PERCENT \

However I find that the result produced this way is highly incorrect. 
Not only that, but the result is greatly influenced by the width (-w) 
option for generating the graph. On my test dataset changing -w from 400 
to 1000 pixels (while keeping all other things the same) the 95Th 
percentile swings back and forth between 17 and 38 Mbit. Am I doing 
something wrong, or is this behaviour to be expected?

My create command, the measurements I'm inserting are 'bytes transferred 
since last measurement'

# rrdtool create foo.rrd --start $start --step 300 
DS:in:ABSOLUTE:600:0:U DS:out:ABSOLUTE:600:0:U RRA:AVERAGE:0.5:1:100000

Regards,
Marius