[rrd-users] Combine average data to one CDEF
Simon Hobson
linux at thehobsons.co.uk
Wed May 21 09:41:29 CEST 2008
Hans van Kilsdonk wrote:
>This works like a charm. However, when I want to add another DEF where
>the data in the RRD file starts later the whole graph starts at that
>time. Any data before the latest start time is not shown in the graph.
>
>Is there a way to avoid this?
That's because the data is unknown - and could apply to any of the
rrd should the collection routine stop for any reason.
x + unknown = unknown
Thus is ANY of your values is unknown, the the total will also be unknown.
The answer is to test each one, something like this :
0,x,x,unkn,if
Check the rrd rpn pages for more details, but x,unkn, evaluates to
true if x is unknown, then 0,x,<expr>,if says if expr is true then
give x, otherwise give 0.
Apply such a formula to each value, then add up the results.
One downside is that if ALL the values are unknown, then you still
get zero, not a gap in the graph.
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