[collectd] MySQL plugin reports wrong numbers on mysqld restart

[XANi]

> >> > Do you know how  why it was not DERIVE specified at rrd creation?
> >> Datatype was not DERIVE probably because mysql plugin (as many other)
> >> is much older than actual DERIVE support in collectd :)
> >
> > 100% right: Support for DERIVE was added relatively late – much later
> > than the MySQL plugin. The next major version, version 5.0, will likely
> > convert most of the COUNTER data sources to DERIVE – it's the better
> > default in most cases.
> 
> Can you think of any example where it would be bad/improper to convert?

Derive will ALWAYS lose 1 datapoint when counter (with known size, for
example packet/byte counter on interface) overflows. Not a big problem
on 64 bit counters but 32 bit counters with a lot (tens/hundreds of
megabits) will overflow quite often.
And while that's a bad thing, the alternative in some cases is much
worse, because if for example on of your interface's counters get's
resetted (like VPN interface after reconnect) you get ugly spikes, same
with restarting daemons that are monitored. For example if you run
OpenVPN gateway, with COUNTER graphs are almost completely useless.

-- 
Mariusz Gronczewski (XANi) <xani666 at gmail.com>
GnuPG: 0xEA8ACE64
http://devrandom.pl
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