[rrd-users] rrd at the ragged edge (updates every second)

[Rick Jones]

I've come to rrd via ntop (http://www.ntop.org/) which I'm looking at in
the context of sFlow (http://www.sflow.org/) and receiving sFlow counter
samples as often as once per second per interface.  RRD and its use by
ntop seems rather ideal in that it has the concept of keeping so many
samples at fine resolution, so many at a coarser and so on.  I've (or at
least I think I have) cranked ntop's storage of values into RRD down to
once per second, but the appearance of messages such as:


> Mon Apr 18 15:06:07 2011  **WARNING** RRD:
> rrd_update(/usr/local/var/ntop/rrd/interfaces/sFlow-device.2/sFlow/48/ifOutMulticastPkts.rrd)
> error:
> /usr/local/var/ntop/rrd/interfaces/sFlow-device.2/sFlow/48/ifOutMulticastPkts.rrd:
> illegal attempt to update using time 1303164367 when last update time
> is 1303164367 (minimum one second step)

(Which may have appeared even before the rate was cranked down to every
second, I must confess I'm not quite sure one way or the other) has me
concerned about operations at the limit - in other words, ostensibly I
have values going into the rrd once per second, but suppose that
flutters back and forth across the second boundary?  Is it likely that
is what happened above? That the update happened to hit just before the
second rolled-over?

In this instance I happen to be running 4.0.3 of ntop compiled from source, and:

raj at tardy:~$ dpkg -l | grep rrd
ii  librrd-dev 1.4.3-1ubuntu1 time-series data storage and display system (development)
ii  librrd4    1.4.3-1ubuntu1 time-series data storage and display system (runtime library)
ii  rrdtool    1.4.3-1ubuntu1 time-series data storage and display system (programs)

for the RRD bits.

thanks,

rick jones