[rrd-users] How to graph delta from last

A Darren Dunham ddunham at taos.com
Tue Aug 4 18:19:44 CEST 2009

On Mon, Aug 03, 2009 at 10:13:28PM -0700, Niobe wrote:
> My aim in multiplying by the polling interval was to UNDO the division derive
> must do to convert the value to a per second rate. That is, I want to graph
> 'dx', not 'dx/dy'.

This is tricky to get right because the actual thing stored in the
database is the rate.  

> Say the file grew by 12MB to a total size of 500MB since the last poll, I
> want to see the graph display 12M. My understanding is DERIVE would display
> 40 (12000/300) if the polling interval was 300. Is my understanding
> correct?

Yes.  Using a rate makes things like missed polls easier to work with.
If a poll is missed and you have the following data:

delta   growth
300     12M
300     12M
600     24M
300     12M

How do you want to display that?  With a gap and a spike?  Also, are
your polls exactly on the top of the 5 minute periods?  If they occur at
other times (like 3:51:24, 3:56:24, 4:01:24), then you're going to have
to fudge the entry into the database or your numbers won't come out on
the graph in the way you expect.

If you don't need exact, I'd probably try DERIVE and then multiply by
the expected poll interval.


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