[rrd-developers] problems graphing past end of RRD

[Alex van den Bogaerdt]

On Mon, Feb 25, 2008 at 07:05:06AM -0800, Thorsten von Eicken wrote:

[snip: data points]

> Define "more data". Getting 599 data points (last data point missing) is 
> more data in my book than getting 22 data points that happen to cover 
> the last 20 seconds.

You aren't presented points. You are presented time and value,
combined.

It's not a data point which is missing. It is a time range which
is missing, and rrdtool is (no, used to be,) trying to fulfill
your request as good as possible. That means as little as possible
of the requested time range should fall outside the available data.

There was nothing to fix, except the inputs to "rrdtool graph" or
"rrdtool fetch".


> > Then ask for an end time which is a whole multiple of 140, and not later
> > than last update time. If your match can be exactly matched, it will.
> > If there are multiple choices left, rrdtool will select the best resolution.
> 
> Actually, this doesn't work for a graph. Suppose the RRD updates are 
> lagging behind by a few minutes. You have to show a graph that ends 
> "now" (+/- some small margin). Even if I round the end time to a 
> multiple of 140 seconds, the calculation will say that the coarser RRA's 
> last data point is > the finer RRA's. The only way to get the finer RRA 
> is to end the graph at or before the last updated data point in the RRA. 

I wrote:
..."and not later than last update time."

You are asking for data that is not yet there. You get as little of
it as possible. And if this means selecting the RRA at 4520 seconds
per bucket, then so be it.


It's clear that I think the decision to alter best-match is wrong.
In my experience, when something is fixed in the wrong place this
will eventually lead to a long series of related problems.


-- 
Alex van den Bogaerdt
http://www.vandenbogaerdt.nl/rrdtool/