[mrtg] Re: How much is too much?

[alex at ergens.op.Het.Net]

Bradford Woodcock wrote:
> 
> The main concern is ensuring the job runs for less than 5 minutes to
> complete. If it starts to go beyond 5 minutes (or even close) you'll want
> to break it up and stagger the start times. I've got over 58,000 lines of
> cfg split into 4 files each to start execution on a different minute. This
> works for us so far.

There may be another issue to worry about:

Suppose the box is rather busy (and it probably will be ...) and the
load on the server isn't constant either:

MRTG takes a sample of a router.  Doing so, the load on the server
varies.  The time at which the sample is actally taken will differ from
the timestamp that MRTG records.

Next sample: the time will be have another offset from reality.

Result: MRTG will calculate a difference in time which is, for example,
five minutes.  The actual samples were taken 4.5 minutes apart in real
life.  Assume a full 10mbps load, during 4.5 minutes: 270 seconds times
10,000,000 / 8 == 337500000 octets.  MRTG will calculate a rate of:
337500000 * 8 / 300 == 9.00 mbps ...
Next sample:  In real life 5.5 minutes apart, MRTG thinks they are 5
minutes apart again.
Actual transfer: 10,000,000 * 330 / 8 == 412500000
MRTG calculates: 412500000 * 8 / 300 == 11.00 mbps

That 10% error in bandwidth measurement is in direct relation to the
10% error in timestamp calculation.  I don't know what the *real* error
is but I know for sure that it won't be zero.

If your config file is too big and MRTG runs for more than five minutes,
this is not a big problem anymore (for recent versions of MRTG) as the
lock file takes care of this.  It will catch up in the next run.

The problem I described is, IMHO, a bigger problem.  Comment welcome!

regards,
-- 
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