[rrd-users] 95th and rrd

[Eduardo M. Bragatto]

Tobias Oetiker wrote:

> * if you use the PERCENT VDEF function in rrdtool graph you get a
>   percentile of what is shown in the graph. Unless your graph is
>   very wide this will not be 5minutes but however wide a pixle, in
>   most cases this will be a good aproximation, but not the
>   'real-thing'.

Actually, we can calculate 95th percentile for a whole month even on a
daily graph, IF we have the DS in question being loaded twice, using
"start=end-30" on the one of the declarations. Then you use that DS to
calculate the 95th (it will be wide as we need, but we'll use the other
DS with a shorter range of time to plot the graph).

Example:

'DEF:monthin=$table:inbytes:AVERAGE:step=300:start=end-30'
'DEF:monthout=$table:outbytes:AVERAGE:step=300:start=end-30'
'DEF:bytesin=$table:inbytes:AVERAGE'
'DEF:bytesout=$table:outbytes:AVERAGE'

Then, use bytesin/bytesout to plot the graph, but use monthin/monthout
on the VDEF PERCENT to get the percentile from the whole month.

If your graph is empty, it will take up to 36 hours until 95th
percentile shows any value (you need more than 5% of 30 days to actually
have the first value).

You'll however, need a RRA large enough for those DSes to store at least
30day of 5min samples (it's not too large though, my files are about
600K each).

Regards,
Eduardo M. Bragatto.