[rrd-developers] problem(bug?) with CDEF

[Ashok Mandala]

Hi All,

I am facing a graphing problem while using CDEF's.  I
have already tried this question once on the mailing
list, but maybe the question was not clear, so let me
try again.

I have an rrd(say rrd1) created with a start of t and
step 3600 and update it at t+3600. When I graph this
one rrd, I get a straight line between t and t+3600. 

I have another rrd (rrd2) with a start time of t+3000
and step of 30 and have three updates at t+3030,t+3060
and t+3090. When I graph this, I get the expected
graph starting from t+3000 and ending at t+3090.

Now,  I want to draw a combined graph with these two
lines. The graph should use data from rrd2 and if the
data is unknown at that time, it should use data from
rrd1.  I used this CDEF :

DEF:val1=rrd1:<blah>...
DEF:val2=rrd2:<blah>...
CDEF:val3=val2,UN,val1,val2,IF
LINE2:val3<blah>....

When I graph val3, I expect a line with rrd1 data from
t to t+3000, then some lines with rrd2 data till
t+3090 and then again rrd1 data till t+3600. But,
Instead I see rrd2 data from t+3000 till t+3090 and
then rrd1 data starting from t+3600 and ending at
t+7200. This seems to be an inconsistency in the rrd
graphing.  My previous mail with the rrd statements is
here :
http://www.ee.ethz.ch/~slist/rrd-users/msg04686.html

and the graph images are attached. I would appreciate
any info on this. If people think there is a bug, I
can go through the code to look at it. Else, please
let me know if I am doing something wrong in the CDEF.
This will save me from wading through the code.

Thanks,
Ashok

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