time component of a VDEF

Hi

I see this in the rrdtool_rpn man page under the VDEF section:
MAXIMUM, MINIMUM, AVERAGE <>
Return the corresponding value, MAXIMUM and MINIMUM also return the first occurrence of that value in the time component.
Example: VDEF:avg=mydata,AVERAGE

What is the “time component” referred to here? Is there a way to reference it?

The reason I’m asking is because I’m trying to determine during which time of day traffic is the busiest - the so-called “busy hour”. My approach was to:

1. Force step=3600 to make it hourly averages
2. Get the hourly MAX values from the DEF
3. Determine the LTIME of the first occurence of MAX

So this is what I tried (paraphrasing a bit to simplify the rpn):

DEF:bytes_in_max=/some/path:bytes_in:MAX:step=3600:reduce=AVERAGE
DEF:bytes_out_max=/some/path:bytes_out:MAX:step=3600:reduce=AVERAGE
CDEF:max_util=bytes_in_max,bytes_out_max,MAX,8,*
VDEF:busy_time=max_util,MAXIMUM,LTIME

But seems that the VDEF is not valid with the LTIME added… I get “Unknown function MAXIMUM,LTIME” error. :-/

Any idea how I can find the time value for the hour at which MAXIMUM was for the VDEF? This is rrdtool-1.4.8, I haven’t tried with the latest rrdtool yet, but just want to know if I’m on the right track of if there is a better way...

Thanks in advance. :)

Cheers,
  Jacques

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