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