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

[Tobias Oetiker]

Today you sent me mail regarding [rrd-developers] RRDtool algorithms and...:

*> 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

you should define the range of data you expect ... do NOT put U into the DS
arguments ...

*> 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?
build the difference from the last recorded value and devide by the time
passed ... rrdtool integrates the space below the curve optained by looking
at the counters ...

*> 1b)    Presuming previous raw data values (in this case, counter values)
*> are used in the above calculations, are these somehow accessible to the
*> user?
no they do not get stored ...only the rates get stored ... rrdtool dump
shows you waht is in the database ..

*> 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)?

once it gets a new counter value it stores it and when the next one comes in
it builds the difference ...

*> 3a)    How does RRDtool determine that a counter has rolled over?
the difference is negative and by adding 2^32-1 the value becomes one which
is in the range defined when the database was set up (dont use U in the DS
argument)

*> 3b)    What does it use for a maximum counter value?  (and wouldn't this
*> be application specific?)
it tries 2^32-1 and 2^64-1 as these are the values to be expected from sane
snmp counters at this point in time ...

*> 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.

for bug reports please suply some sample code demonstrating the 'bug'
*> 
*> 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)?
yes, NaN is NOT NOT NOT equal to NaN this is a basic property of NaN ...
check how to get the IEEE math right on your system ... Its not that rrdtool
must fail if this is not given, but I would be carefull as this test works
fine on most systems so there might be hidden issues with this when it does
not work on your system ... 

*> 4b)    If so, what are some possible ways around this?
make it work ... what sort of a 'system' is it ?

cheers
tobi
*> 
*> 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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