[collectd] Collectd timeinterval datarecording in RRDTool

[Fabien Wernli]

On Thu, Feb 07, 2013 at 02:58:30PM +0100, Frederic Daguenet wrote:
> Can you show the calculation in simple steps?

You want 4 timespans, so you need to enumerate them in the config, e.g.:

<Plugin rrdtool>
  # first 3 days
  RRATimespan 259200
	# the next 7 days
	RRATimespan 604800
	# the following 30 days
	RRATimespan 2592000
	# "the rest" well, this can't be infinity
	RRATimespan <seconds until retirement>

Now as for the resolution, you can't have the fine grained control you're
asking. rrdtool and rrdcached plugins only allow for one global resolution
parameter which affects all RRATimespans: the RRARows parameter (let's
assume the StepSize is unspecified, which is the sanest).
By default, RRARows is 1200, which means the resolution for our first RRA is
(from the manpage): PDPs = timespan / (stepsize * rrarows) = 259200 / (10 *
1200) = 21.6 (assuming step is 10s), which will get rounded to 22 I guess.
This means you'll end up with 22 steps of 10 seconds being consolidated into
one point, i.e. your resolution for the 3 days will be 220 seconds i.e.
hardly 3.5 minutes. If you want 1 minute resolution for three days, you'll
need to increase RRARows to 1200 * 22 * 10 / 60 = 4400:

  RRARows 4400
</Plugin>

However, you can only have one RRARows parameter, so this will affect all 3
other timespans. Let us see what resolution that will yield for each one of
them:

resolution(3 days) = 259200 / 4400 = 59 seconds
resolution(7 days) = 604800 / 4400 = 2.3 minutes
resolution(30 days) = 2592000 / 4400 =~ 10 minutes
resolution(30 years) = 30*365.24*86400 =~ 2.5 days

Luckily that's close enough to what you asked for. If you need the exact
values you'll have to create the RRDs yourself, and have collectd-rrdtool
feed them (but not create them).

IMHO the rrdtool/rrdcached plugins should mimic the rrdtool configuration
options. I guess the authors wanted to simplify the already complicated
rrd_create configuration. However it would seem this has the opposite effect
to confuse the user even more...

Hope this helps (and hoping my quick numbers are not too far from
correctness ;)

Cheers