[rrd-users] Re: Max/current value is greater than the interface bandwith on

[Alex van den Bogaerdt]

anant at vsnl.com wrote:

> I am getting in and out bytes using snmpwalk through crontab every 5
> minutes and updating the rrd using 'N'.


> So what can be the reason for getting max value greater than actual
> interface bandwith. (i.e. if the int b/w 8Kbytes/s, the max value exceeds beyond 8Kbytes/s).

Basically this is what you want:

   while (1) {
       wait until it is time to run
       collect a value from the device
       give this value to rrdtool
   }

However, what's happening is:

   while (1) {
       wait until it is time to run
       collect a value from the device
       >>>other stuff happens on your computer<<<
       give this value to rrdtool
   }

Let's do two iterations of this process.  Exagurate the time needed
to do stuff to make a nice example:


   hh:mm:ss   - wait until it is 16:00 hours
   16:00:00   - collect a value from the device
   16:00:01     this process finishes; resulting counter == 10,000
   16:00:01   - some heavy process starts working and blocks your
                computer entirely
   16:01:01   - give this value to rrdtool
   16:01:02     this process finishes
   16;01:02   - wait untile it is 16:05 hours
   16:05:00   - collect a value from the device
   16:05:01     this process finishes; resulting counter == 2,410,000
   16:05:01   - the heavy process doesn't run here
   16:05:01   - give this value to rrdtool
   16:05:01     this process finishes
    

The increase between 16:00 and 16:05 is 2,400,000.  This is comes
down to 8000 per second.  Now have a look at what RRDtool got to
see due to the 'N' time:

   16:01:01   counter value    10,000
   16:05:01   counter value 2,410,000

The differences:

   value:  2,410,000 - 10,000       == 2,400,000
   time:   16:05:01  - 16:01:01     == 240 seconds   ( and *not* 300 !!!)
   rate == delta(value)/delta(time) == 2,400,000/240 == 10,000 per second.

This example isn't complete and it is overly exagurated but I guess
the point is clear:  there is a difference between "now" and "now".

HTH
-- 
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