[rrd-users] Re: Easy question about percents

Alex van den Bogaerdt alex at ergens.op.het.net
Thu Dec 12 20:00:49 MET 2002

On Thu, Dec 12, 2002 at 03:29:27PM +0100, Stephan Harren wrote:
> Hi All,
> I need to know how tp create a rrd that does nothing but accept the 
> values that I'm inserting, no calculation, no aggregation. The reason 
> for that is, that I want to put in values between 0 and 100 (percentage) 
> and I don't want rrdtool to average it. I'll keep 5-minute values for 
> maybe one month or so.
> Guess:
> rrdtool create bla.rrd -step 300 DS:percent:GAUGE:600:0:100 
> RRA:LAST:1:1:8640
> Am I right?

That depends.

You didn't specifically say this but I think it's fair to assume
you also want no normalizing to happen.

If so:

*only* update at times equal to a whole multiple of the step size.
In your case this means at time "t" where "t == n * 300".

If you can't update at exactly n*300 (for whatever reason) you have
to create an RRD with a step size of 1.

Also be aware that RRDtool may do some calculations at graph time
as well, if it is necessary to do so.  For instance, if you have
an RRD with step size 1 and an RRA with 1 PDP per CDP (thus resulting
in 31 days time 24 hours times 3600 rows per hour == 2678400 rows)
and decide to display 400 pixels and a time range of 4000 seconds,
RRDtool will use the consolidation function (LAST in your case) on
the data.

My personal opinion on the LAST CF is that it is almost never
useful.  It may seem useful to some, this will be the case where
people don't fully understand what's going on and do the wrong
thing with RRDtool.  LAST will mask those errors but it will
backfire at some point.  In this particular case, updates will
seem to vanish, resulting in questions or even bug reports.
If and only if you're absolutely sure it is the right thing to
do, use LAST.  Make sure to fully understand the concept of using
UTC time for consolidating the data before you decide to use LAST.


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