[rrd-users] Incorrect numbers returned when monitoring network stats at one second intervals

Simon Hobson linux at thehobsons.co.uk
Thu Jul 26 17:08:06 CEST 2007

Mark Seger wrote:

>>Sampling every second does not occasionally give you an invalid 
>>value as you suggest - the value it gives is 100% valid, just 
>>unexpected ! Just like a lot of 'amateur statistics' manage to come 
>>to invalid conclusions with valid data.
>I guess I have to differ on your conclusion.  When one has a tool 
>that is reporting bytes/sec and it occasionally reports an invalid 
>number like 200MB/sec on a 1G link, they at least owe an explanation 
>to their users why this is the case.

Which tools ? It's unclear from your previous postings what tools you 
are using to produce the figures.

Have you reported to the issue to the package maintainers ?

>Since many people do not monitor at that fine grained of a level - 
>and believe me, they have no idea how much they're losing by not 
>doing so - I suspect very few people even notice.  I guess that's 
>why I have a problem with any data sampled at 1 or even 5 minute 
>intervals - it really doesn't tell me anything about what my system 
>is really doing.

Personally I cannot see what is useful about such fine grained data 
(for most people and most systems). Even on what might normally be 
considered a 'steady' data flow, actual data rates will fluctuate 
wildly at that level of inspection. Very few network topologies are 
deterministic - ethernet certainly is not. Transit delays through 
routers are even less deterministic, not to mention all the other 
circuits a packet must pass through. Oh yes, did I omit to mention 
the task scheduler queue, disk i/o queue, network output queue, ... 
all these things will conspire to give a randomness to your output 
with a lot of variables - even an ntp update will have an effect as 
the task wakes up, sends a packet, waits for a response, and updates 
the status files on disk.

I would reasonably expect the output of almost any real-world system 
to appear pseudo-random !

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