[mrtg] log and graph rates not equal

[Rubens Carlos de Souza Gomes]

I am running mrtg-2.9.25 on a Sun SPARC Solaris 7 to collect input/output
utilization rates
of Cisco routers.  And I see differences between the results for rates
displayed in the GD graphs
and their corresponding MRTG logs.  I am using gd-2.0.12, libpng-1.2.5, and
zlib-1.1.4.  My 
Options is:
Options[_]: bits, growright, noinfo, nopercent, integer

Here is the MRTG log for a given reading:
...
1054943909 485 590 485 590

that is,
 input rate:  485 Bytes/s  --> 485*8 = 3880 b/s
 output rate: 590 Bytes/s --> 590*8 = 4720 b/s

But the graph for the same reading plotted by GD show:
input rate:  3872 b/s
output rate: 4712 b/s

Why are the graphs rates different than the logs rates?

I also have another issue.  I have a perl program that takes the log results
from MRTG, 
converts into b/s (multiplying by 8), as follows:
...
    $utilization_ref->[2] = $current_in_rate * 8;  # convert to bits/sec.
    $utilization_ref->[3] = $current_out_rate * 8; # convert to bits/sec.
...
and store the result in an Oracle.  After reading the results stored in
Oracle I see for the 
above rates:

(Oracle stored) input rate: 3872 b/s
(Oracle stored) output rate: 4704 b/s

Bottom line:  Graphs rates different than log, different than calculated,
and different than what 
is stored in Oracle by my perl program.

Why?

Rubens.


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