[rrd-users] Re: CDEF - creating a increasing series

Alex van den Bogaerdt alex at ergens.op.het.net
Tue Sep 7 10:13:07 MEST 2004

On Tue, Sep 07, 2004 at 09:28:32AM +0200, ciaran deignan wrote:

> The first time the expression is encountered, PREV is undef.
> But a value and UNDEF combined make... undef!
> So second time around PREV is *still* UNDEF, and so it continues ;)


> Personally I'm trying to calculate total traffic volume by summing
> a counter, but its difficult since you need to multiply the "counter
> is a rate" by the effective step,


> but the effective step is decided automatically by RRD depending on the 
> size of the graph, and RRD doesn't appear to make this value available
> to CDEF operations :(

With step you probably try to describe the amount of time in each column.
This is something different than "step", which is the amount of time per
PDP.  PDPs are not used when graphing (or otherwise reporting) data. They
don't exist anymore at that time.

The size is decided by you, the user.  You are giving commands to the
program.  You are in control.

If you want to know in advance how much time each column will contain,
do not use "now" (not written by you, nor implicit).  Use an end-time
that is a whole number times the step size times steps per CDP.
Do the same for the start-time.
Make sure both start and end are inside the RRA that will be used.

If "step" is 300, and "steps" is 6, use an end time that is "n*1800".
If the amount of time included in the RRA is one week, do not exceed
that amount.

Do NOT use "--end now --start end-24h" because end is probably not n*1800.
Do NOT use "--start end-24h" because that contains an implicit "end now".
Do NOT use "--end 00:00 --start end-8d" because that is more time than
RRDtool can find in the RRA and thus will select another RRA.

I ask you to respect any "Reply-To" and "Mail-Follow-Up" headers.  If
you reply to me off-list, you'd better tell me you're doing so.  If
you don't, and if I reply to the list, that's your problem, not mine.

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