[mrtg] Re: Interval ramafacations?

[Alex van den Bogaerdt]

Jeff  Warnica wrote:
> 
> I am using the default measurement time of 5 minutes. It would be helpful
> for me to shorten that time so I can get a better indication of when the
> phone lines are busy.
> 
> Is there anything I should be aware of if I change the interval (and run the
> job more frequently in cron)?
> 

Two (related) things pop up right away:

1) The load on your machine will be higher.
2) The risc of starting one job while the previous one has not yet finished
   is higher.

These two problems will amplify each other.  Worst case scenario may be
that (2) happens, a mail is generated (as there is error output) and this
makes (1) worse.  Running jobs will slow down and thus (2) happens.  Repeat.
The system may crawl and become *very* slow, unless it is a windows box
which will just crash after a while.

Having said this:  If you have a box with no other tasks (so: no surprises)
and you monitor it closely, there should be no problem.  The actual sample
time and the calculated time in MRTG may differ for a few seconds but this
should not be a problem, especially for gauges.

Explanation of this last statement:
If you sample at, say, 12h06m10s and the system is slow, MRTG may "see"
the value as if it were received at 12h06m30s.  This is not nice when
doing octets per second.  It doesn't really matter for the number of
lines in use at 12h06.

***important***
Without other actions apart from increasing the sampling interval,
the data stored in the MRTG database will still be in five minute
intervals.  You will gain better averages and maxima however you
cannot show those maxima in the five-minute graphs.  MRTG has an
option to alter the storage interval but, as can be read from the
documentation, this is not tested very well.  Look for "Step" in the
file "config.txt".

If you have a lot of monitoring to do and are concerned about the
performance issues, consider monitoring with RRDtool.  This also allows
you to graph fewer or more lines.  You could, for instance, make one
graph that shows the logins on several port masters.  Using RRDtools
stack feature, you can show the separate portmasters as well as the
total.  Also, you can very easily create a database that stores the
values in other-than-five-minute intervals.

regards,
-- 
   __________________________________________________________________
 / alex at slot.hollandcasino.nl                  alex at ergens.op.het.net \
| work                                                         private |
| My employer is capable of speaking therefore I speak only for myself |
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