[rrd-users] Newby questions

Simon Hobson linux at thehobsons.co.uk
Tue Oct 30 08:51:33 CET 2012

Pablo Chacin wrote:

>The point is: is the way unknown values handle
>coherent with the calculation of totals?
>As I see it, there's no fundamental reason not to
>be able to calculate totals in a RRD.

You can compute totals in RRD - the problem is that the way it does 
it isn't the way you want to do it *in this application*. Which ever 
way it's done, there will be a lot of people for whom that is not the 
right way. Even in one setup the requirements may change - for 
example ...

At work I use RRD to log traffic on our network. We sell on some of 
our bandwidth to other tenants in the building, but we don't charge 
by volume of data or impose monthly caps.
Now consider these scenarios :

1) We have a failure in data collection, but the network is still up.
In this case, traffic is still flowing, but we aren't recording it. 
The most sensible option would be to assume that the traffic in the 
gap is roughly the average of the rates either side of the gap.

2) We have an actual outage (extended power failure so we lose our 
monitoring as well as the line).
In this case the sensible option is to assume the data was zero during the gap.

3) As in 1 - we have a collection failure but the network is still 
running. If we were imposing caps or charging by the gigabyte then 
we'd probably have to assume zero for charging/capping purposes as we 
wouldn't have the data to back up any other assumption. But for our 
own traffic management/planning we'd want to assume data was still 

So there you are - which way should RRD do it ? Either way is both 
right and wrong. As it is, it's possible to work around it either way 
if you use the right calculations when extracting the data.

If you want to assume zero, then this RPN will do the conversion for 
you, then work with the result :
which means: if a is unknown then substitute 0, else use a.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
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