[rrd-developers] RRDtool algorithms and IEEE math compiler question

[Y. Brett Sauer]

Hello.

I am using RRDtool to store utilization data (bits in and bits out
across a tunnel "interface") at 5-minute intervals and need to be able
to verify data accuracy and answer a few questions about how the tool
handles certain situations.  I would appreciate it very much if anyone
can enlighten me on any of the below questions.

I am using the following statement to set up the RRD database and
archive:

rrdtool create ${main::rrd_data_path}/${host}.rrd --step 300
DS:bitsin:COUNTER:600:U:U DS:bitsout:COUNTER:600:U:U
RRA:AVERAGE:0.5:1:2304

Questions:

1a)    Given that counter values have been successfully obtained via
SNMP and fed to RRDtool's update function for several sampling periods,
how does RRDtool calculate average bits per second for the current
period?
1b)    Presuming previous raw data values (in this case, counter values)
are used in the above calculations, are these somehow accessible to the
user?

2)    If data is missing for several sampling periods (more than two, in
this case), how does RRDtool calculate the next couple of averages
(i.e., bits per second)?

3a)    How does RRDtool determine that a counter has rolled over?
3b)    What does it use for a maximum counter value?  (and wouldn't this
be application specific?)
According to my information, the maximum value for the counter in this
case is 2*32-1.  I did an RRDtool update using the value "0" for input,
and when I did an RRDtool fetch for this period the value for average
bits per second was an unduly high number, *much* greater than 2*32
minus the previous counter value.

On a completely different note (or so I hope) ...

4a)    In order to get RRDtool to compile on our system, I commented out
a line in the "configure" file containing the test "if (nan == nan)".
Is this likely to cause any real problem(s)?
4b)    If so, what are some possible ways around this?

Thanks very much.

Brett

--
Brett Sauer                     Network Management Engineer
cell/pgr:(612)889-6397          Central Operations
phone:(512)252-8590             WAM!NET Inc.
vmail:(651)256-5467             14437 Robert I. Walker Blvd
brett at wamnet.com                Austin, TX 78728




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