[collectd] collectd and large MTU

Ulf Zimmermann ulf at atc-onlane.com
Tue Oct 9 00:31:06 CEST 2007

> -----Original Message-----
> From: Florian Forster [mailto:octo at verplant.org]
> Sent: 10/08/2007 14:02
> To: Ulf Zimmermann
> Cc: collectd at verplant.org
> Subject: Re: [collectd] collectd and large MTU
> Hi Ulf,
> On Mon, Oct 08, 2007 at 09:45:42AM -0700, Ulf Zimmermann wrote:
> > Does collectd handle changed MTU on Ethernet? We have some servers
> > where we changed over the weekend the MTU from 1500 to 9000 and now
> > our Ethernet charts do not match with the graphs I got via mrtg on
> > switch side.
> I assume you're talking about the graphs collected with the
> plugin, i. e. `if_octets-*.rrd'?
> Under Linux these values are queried either by reading `/proc/net/dev'
> or by using `getifaddrs', default is to read from `/proc/..'. The
> counters there count the octets that were sent over the link - I don't
> think the MTU should play any role in that at all..
> You can use some one-line-shell-magic (or possibly `iptraf' or
> like that) to check if the counters in /proc work as expected and
> provide the `right' values.. I certainly hope they don't work
> though, since I have no idea whatsoever what the problem in collectd
> might be.
> If the counters seem wrong, I'd ask the author of the network card
> driver if he has an idea what might cause this. If the counters are
> about one sixth of what you'd expect due to the MRTG graphs, he
> assumed an MTU of 1500 somewhere in his code..
> Oh, last but not least: Cisco has had some problems with counters in
> past.. This would mean that the MRTG graphs should look different now,
> though. Have you `old' data to check which graphs look different?
> Regards,
> -octo
> --
> Florian octo Forster
> Hacker in training
> GnuPG: 0x91523C3D
> http://verplant.org/

Problem for me is mostly the traffic on the interfaces is pretty low and
then once in a while larger peaks. But looking at the graphs, the
graphical representation could be correct just the factors are not. Ie.
Mrtg (which only collects at 5 minute interval) shows a throughput of
~40Mbit/sec while my graphs from collectd showed ~4Mbit/sec, or close to
the factor of 6 (1,500 bytes against 9,000 bytes MTU). I am gonna have
to look at it closer later, very busy with other stuff, sigh. Never
enough time.

Regards, Ulf.

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